Understanding Pathophysiology
SIXTH EDITION
Sue E. Huether, MS, PhD
Professor Emeritus
College of Nursing
University of Utah
Salt Lake City, Utah
Kathryn L. McCance, MS, PhD
Professor Emeritus
College of Nursing
University of Utah
Salt Lake City, Utah
SECTION EDITORS
Valentina L. Brashers, MD
Professor of Nursing and Woodard Clinical Scholar
Attending Physician in Internal Medicine
University of Virginia Health System
Charlottesville, Virginia
Neal S. Rote, PhD
Academic Vice-Chair and Director of Research
Department of Obstetrics and Gynecology
University Hospitals Case Medical Center
William H. Weir, MD, Professor of Reproductive Biology and Pathology
Case Western Reserve University School of Medicine
Cleveland, Ohio
With more than 1000 illustrations
Table of Contents
Cover image
Title page
Health Alerts
Copyright
Contributors
Reviewers
Preface
Organization and Content: What’s New in the Sixth Edition
Features to Promote Learning
Art Program
Teaching/Learning Package
Acknowledgments
Introduction to Pathophysiology
Part One Basic Concepts of Pathophysiology
Unit 1 The Cell
1 Cellular Biology
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Prokaryotes and Eukaryotes
Cellular Functions
Structure and Function of Cellular Components
Cell-to-Cell Adhesions
Cellular Communication and Signal Transduction
Cellular Metabolism
Membrane Transport: Cellular Intake and Output
Cellular Reproduction: the Cell Cycle
Tissues
Did You Understand?
Key Terms
References
2 Genes and Genetic Diseases
DNA, RNA, and Proteins: Heredity at the Molecular Level
Chromosomes
Elements of Formal Genetics
Transmission of Genetic Diseases
Linkage Analysis and Gene Mapping
Multifactorial Inheritance
Did You Understand?
Key Terms
References
3 Epigenetics and Disease
Epigenetic Mechanisms
Epigenetics and Human Development
Genomic Imprinting
Long-Term and Multigenerational Persistence of Epigenetic States Induced by Stochastic and
Environmental Factors
Epigenetics and Cancer
Future Directions
Did You Understand?
Key Terms
References
4 Altered Cellular and Tissue Biology
Cellular Adaptation
Cellular Injury
Manifestations of Cellular Injury: Accumulations
Cellular Death
Aging and Altered Cellular and Tissue Biology
Somatic Death
Did You Understand?
Key Terms
References
5 Fluids and Electrolytes, Acids and Bases
Distribution of Body Fluids and Electrolytes
Alterations in Water Movement
Sodium, Chloride, and Water Balance
Alterations in Sodium, Chloride, and Water Balance
Alterations in Potassium and Other Electrolytes
Acid-Base Balance
Did You Understand?
Key Terms
References
Unit 2 Mechanisms of Self-Defense
6 Innate Immunity: Inflammation and Wound Healing
Human Defense Mechanisms
Innate Immunity
Acute and Chronic Inflammation
Wound Healing
Did You Understand?
Key Terms
References
7 Adaptive Immunity
Third Line of Defense: Adaptive Immunity
Antigens and Immunogens
Antibodies
Immune Response: Collaboration of B Cells and T Cells
Cell-Mediated Immunity
Did You Understand?
Key Terms
References
8 Infection and Defects in Mechanisms of Defense
Infection
Deficiencies in Immunity
Hypersensitivity: Allergy, Autoimmunity, and Alloimmunity
Did You Understand?
Key Terms
References
9 Stress and Disease
Historical Background and General Concepts
The Stress Response
Stress, Personality, Coping, and Illness
Did You Understand?
Key Terms
References
Unit 3 Cellular Proliferation: Cancer
10 Biology of Cancer
Cancer Terminology and Characteristics
The Biology of Cancer Cells
Clinical Manifestations of Cancer
Diagnosis, Characterization, and Treatment of Cancer
Did You Understand?
Key Terms
References
11 Cancer Epidemiology
Genetics, Epigenetics, and Tissue
In Utero and Early Life Conditions
Environmental-Lifestyle Factors
Did You Understand?
In Utero and Early Life Conditions
Key Terms
References
12 Cancer in Children and Adolescents
Incidence, Etiology, and Types of Childhood Cancer
Prognosis
Did You Understand?
Key Terms
References
Part Two Body Systems and Diseases
Unit 4 The Neurologic System
13 Structure and Function of the Neurologic System
Overview and Organization of the Nervous System
Cells of the Nervous System
The Nerve Impulse
The Central Nervous System
The Peripheral Nervous System
The Autonomic Nervous System
Did You Understand?
Key Terms
References
14 Pain, Temperature, Sleep, and Sensory Function
Pain
Temperature Regulation
Sleep
The Special Senses
Somatosensory Function
Geriatric Considerations
Geriatric Considerations
Did You Understand?
Key Terms
References
15 Alterations in Cognitive Systems, Cerebral Hemodynamics, and Motor Function
Alterations in Cognitive Systems
Alterations in Cerebral Hemodynamics
Alterations in Neuromotor Function
Alterations in Complex Motor Performance
Extrapyramidal Motor Syndromes
Did You Understand?
Key Terms
References
16 Disorders of the Central and Peripheral Nervous Systems and Neuromuscular
Junction
Central Nervous System Disorders
Peripheral Nervous System and Neuromuscular Junction Disorders
Tumors of the Central Nervous System
Did You Understand?
Key Terms
References
17 Alterations of Neurologic Function in Children
Development of the Nervous System in Children
References
Structural Malformations
Alterations in Function: Encephalopathies
Cerebrovascular Disease in Children
Childhood Tumors
Did You Understand?
Key Terms
References
Unit 5 The Endocrine System
18 Mechanisms of Hormonal Regulation
Mechanisms of Hormonal Regulation
Structure and Function of the Endocrine Glands
Geriatric Considerations
Did You Understand?
Key Terms
References
19 Alterations of Hormonal Regulation
Mechanisms of Hormonal Alterations
Alterations of the Hypothalamic-Pituitary System
Alterations of Thyroid Function
Alterations of Parathyroid Function
Dysfunction of the Endocrine Pancreas: Diabetes Mellitus
Alterations of Adrenal Function
Did You Understand?
Key Terms
References
Unit 6 The Hematologic System
20 Structure and Function of the Hematologic System
Components of the Hematologic System
Development of Blood Cells
Mechanisms of Hemostasis
Pediatrics & Hematologic Value Changes
Aging & Hematologic Value Changes
Did You Understand?
Key Terms
References
21 Alterations of Hematologic Function
Alterations of Erythrocyte Function
Myeloproliferative Red Cell Disorders
Alterations of Leukocyte Function
Alterations of Lymphoid Function
Alterations of Splenic Function
Hemorrhagic Disorders and Alterations of Platelets and Coagulation
Did You Understand?
Key Terms
References
22 Alterations of Hematologic Function in Children
Disorders of Erythrocytes
Disorders of Coagulation and Platelets
Neoplastic Disorders
Did You Understand?
Key Terms
References
Unit 7 The Cardiovascular and Lymphatic Systems
23 Structure and Function of the Cardiovascular and Lymphatic Systems
The Circulatory System
The Heart
The Systemic Circulation
The Lymphatic System
Did You Understand?
Key Terms
References
24 Alterations of Cardiovascular Function
Diseases of the Veins
Diseases of the Arteries
Disorders of the Heart Wall
Manifestations of Heart Disease
Shock
Did You Understand?
Key Terms
References
25 Alterations of Cardiovascular Function in Children
Congenital Heart Disease
Acquired Cardiovascular Disorders
Did You Understand?
Key Terms
References
Unit 8 The Pulmonary System
26 Structure and Function of the Pulmonary System
Structures of the Pulmonary System
Function of the Pulmonary System
Geriatric Considerations
Did you Understand?
Key Terms
References
27 Alterations of Pulmonary Function
Clinical Manifestations of Pulmonary Alterations
Pulmonary Disorders
Did You Understand?
Key Terms
References
28 Alterations of Pulmonary Function in Children
Disorders of the Upper Airways
Disorders of the Lower Airways
Sudden Infant Death Syndrome (SIDS)
Did You Understand?
Key Terms
References
Unit 9 The Renal and Urologic Systems
29 Structure and Function of the Renal and Urologic Systems
Structures of the Renal System
Renal Blood Flow
Kidney Function
Tests of Renal Function
Pediatric Considerations
Geriatric Considerations
Did You Understand?
Key Terms
References
30 Alterations of Renal and Urinary Tract Function
Urinary Tract Obstruction
Urinary Tract Infection
Glomerular Disorders
Acute Kidney Injury
Chronic Kidney Disease
Did You Understand?
Key Terms
References
31 Alterations of Renal and Urinary Tract Function in Children
Structural Abnormalities
Glomerular Disorders
Nephroblastoma
Bladder Disorders
Urinary Incontinence
Did You Understand?
Key Terms
References
Unit 10 The Reproductive Systems
32 Structure and Function of the Reproductive Systems
Development of the Reproductive Systems
The Female Reproductive System
Structure and Function of the Breast
The Male Reproductive System
Aging & Reproductive Function
Did You Understand?
Key Terms
References
33 Alterations of the Female Reproductive System
Abnormalities of the Female Reproductive Tract
Alterations of Sexual Maturation
Disorders of the Female Reproductive System
References
Disorders of the Female Breast
Did You Understand?
Key Terms
References
34 Alterations of the Male Reproductive System
Alterations of Sexual Maturation
Disorders of the Male Reproductive System
References
Disorders of the Male Breast
Sexually Transmitted Diseases
Did You Understand?
Key Terms
References
Unit 11 The Digestive System
35 Structure and Function of the Digestive System
The Gastrointestinal Tract
Accessory Organs of Digestion
Geriatric Considerations
Did You Understand?
Key Terms
References
36 Alterations of Digestive Function
Disorders of the Gastrointestinal Tract
Disorders of the Accessory Organs of Digestion
Cancer of the Digestive System
Did You Understand?
Key Terms
References
37 Alterations of Digestive Function in Children
Disorders of the Gastrointestinal Tract
Disorders of the Liver
Did You Understand?
Key Terms
References
Unit 12 The Musculoskeletal and Integumentary
Systems
38 Structure and Function of the Musculoskeletal System
Structure and Function of Bones
Structure and Function of Joints
Structure and Function of Skeletal Muscles
Aging & the Musculoskeletal System
Did You Understand?
Key Terms
References
39 Alterations of Musculoskeletal Function
Musculoskeletal Injuries
Disorders of Bones
Disorders of Joints
Disorders of Skeletal Muscle
Musculoskeletal Tumors
Did You Understand?
Key Terms
References
40 Alterations of Musculoskeletal Function in Children
Congenital Defects
Bone Infection
Juvenile Idiopathic Arthritis
Osteochondroses
Scoliosis
Muscular Dystrophy
Musculoskeletal Tumors
Nonaccidental Trauma
Did You Understand?
Key Terms
References
41 Structure, Function, and Disorders of the Integument
Structure and Function of the Skin
Disorders of the Skin
Disorders of the Hair
Disorders of the Nail
Geriatric Considerations
Did You Understand?
Key Terms
References
42 Alterations of the Integument in Children
Acne Vulgaris
Dermatitis
Infections of the Skin
Insect Bites and Parasites
Cutaneous Hemangiomas and Vascular Malformations
Other Skin Disorders
Did You Understand?
Key Terms
References
Glossary
Index
Prefixes and Suffixes Used in Medical Terminology
Word Roots Commonly Used in Medical Terminology
Health Alerts
Gene Therapy, 57
The Percentage of Child Medication–Related Poisoning Deaths Is Increasing, 85
Air Pollution Reported as Largest Single Environmental Health Risk, 87
Low-Level Lead Exposure Harms Children: A Renewed Call for Primary
Prevention, 89
Alcohol: Global Burden, Adolescent Onset, Chronic or Binge Drinking, 92
Unintentional Injury Errors in Health Care and Patient Safety, 93
Hyponatremia and the Elderly, 121
Potassium Intake: Hypertension and Stroke, 122
Risk of HIV Transmission Associated with Sexual Practices, 194
Glucocorticoids, Insulin, Inflammation, and Obesity, 220
Psychosocial Stress and Progression to Coronary Heart Disease, 221
Acute Emotional Stress and Adverse Heart Effects, 226
Partner’s Survival and Spouse’s Hospitalizations and/or Death, 226
Global Cancer Statistics and Risk Factors Associated with Causes of Cancer Death,
273
Increasing Use of Computed Tomography Scans and Risks, 285
Rising Incidence of HPV-Associated Oropharyngeal Cancers, 291
Radiation Risks and Pediatric Computed Tomography (CT): Data from the National
Cancer Institute, 305
Magnetic Fields and Development of Pediatric Cancer, 305
Neuroplasticity, 311
Biomarkers and Neurodegenerative Dementia, 372
Tourette Syndrome, 378
Prevention of Stroke in Women, 403
West Nile Virus, 410
Alcohol-Related Neurodevelopmental Disorder (ARND), 423
Growth Hormone (GH) and Insulin-like Growth Factor (IGF) in Aging, 447
Vitamin D, 450
Immunotherapy for the Prevention and Treatment of Type 1 Diabetes, 474
Incretin Hormones for Type 2 Diabetes Mellitus Therapy, 476
Sticky Platelets, Genetic Variations, and Cardiovascular Complications, 505
A Significant Number of Children Develop and Suffer from Severe Iron Deficiency
Anemia, 555
Myocardial Regeneration, 571
Regression of Myocardial Hypertrophy, 579
The Renin-Angiotensin-Aldosterone System (RAAS) and Cardiovascular Disease,
601
Obesity and Hypertension, 602
New Insights and Guidelines into the Management of Dyslipidemia for the
Prevention of Coronary Artery Disease, 612
Mediterranean Diet, 612
Women and Microvascular Angina, 614
Metabolic Changes in Heart Failure, 634
Gene Therapy for Heart Failure, 635
Central Line–Associated Bloodstream Infection, 645
The Surviving Sepsis Guidelines, 646
Endocarditis Risk, 658
U.S. Childhood Obesity and Its Association with Cardiovascular Disease, 668
Changes in the Chemical Control of Breathing During Sleep, 678
The Microbiome and Asthma, 698
Ventilator-Associated Pneumonia (VAP), 704
Molecular Targets in Lung Cancer Treatment, 711
Exercise-Induced Bronchoconstriction, 724
Newborn Screening for Cystic Fibrosis, 726
The Many Effects of Erythropoietin (Epo), 742
Urinary Tract Infection and Antibiotic Resistance, 754
Childhood Urinary Tract Infections, 775
Nutrition and Premenstrual Syndrome, 810
Vaginal Mesh, 814
Screening with the Papanicolaou (Pap) Test and with the Human Papillomavirus
(HPV) DNA Test: Benefits and Harms from Cervical Cancer Screening (PDQ®),
820
Cervical Cancer Primary Prevention, 823
Breast Cancer Screening Mammography, 834
Paracetamol (Acetaminophen) and Acute Liver Failure, 900
Clostridium difficile and Fecal Microbiome Transplant, 908
Types of Adipose Tissue and Obesity, 925
Childhood Obesity and Nonalcoholic Fatty Liver Disease, 963
Tendon and Ligament Repair, 987
Managing Tendinopathy, 997
Osteoporosis Facts and Figures at a Glance, 1002
Calcium, Vitamin D, and Bone Health, 1005
New Treatments for Osteoporosis, 1006
Musculoskeletal Molecular Imaging, 1015
Psoriasis and Comorbidities, 1063
Melanoma in Non-White People, 1073
Copyright
3251 Riverport Lane
St. Louis, Missouri 63043
UNDERSTANDING PATHOPHYSIOLOGY, SIXTH EDITION ISBN: 978-0-323-
35409-7
Copyright © 2017, Elsevier Inc. All rights reserved.
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Library of Congress Cataloging-in-Publication Data
Names: Huether, Sue E., editor. | McCance, Kathryn L., editor.
Title: Understanding pathophysiology / [edited by] Sue E. Huether, Kathryn L.
McCance ; section editors, Valentina L. Brashers, Neal S. Rote.
Description: Sixth edition. | St. Louis, Missouri : Elsevier, [2017] | Includes
bibliographical references and index.
Identifiers: LCCN 2015037586 | ISBN 9780323354097 (pbk. : alk. paper)
Subjects: | MESH: Pathology—Nurses’ Instruction. | Disease—Nurses’ Instruction. |
Physiology—Nurses’ Instruction.
Classification: LCC RB113 | NLM QZ 4 | DDC 616.07—dc23 LC record available at
http://lccn.loc.gov/2015037586
ABOUT THE COVER
http://lccn.loc.gov/2015037586
Microbiome. This colored scanning electron micrograph of Escherichia coli bacteria (red rods)
was taken from the small intestine of a child. E. coli are part of the normal flora or microbiota of
the human gut and many normal flora are essential for health. The terms microbiota or
microbiome refer to the community of microbes that normally reside on and within the human
body. The microbiome also means the full collection of genes of all the microbes in the
community. DNA-sequencing tools have helped define the microbiome and they outnumber our
own cells by about 10 to 1. These resident microbes are highly skilled and provide crucial
functions—they sense what food is present, if pathogens are lurking, and the inflammatory
state of the gut. Shifts in the bacterial composition of the gut microbiota have been correlated
with intestinal dysfunctions such as inflammatory bowel disease, antibiotic-associated diarrhea
and metabolic dysfunction including obesity. Gut microflora have protective, metabolic, growth,
and immunologic functions because the microbiota interact with both innate and adaptive
immune systems. If the overall interaction is flawed autoimmune or inflammatory diseases may
occur. We acquire our microbiomes from the environment at birth. Our microbial profiles
change with aging because microbial populations shift with changes in the environment. Credit:
STEPHANIE SCHULLER/SCIENCE PHOTO LIBRARY
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Content Development Manager: Laurie Gower
Senior Content Development Specialist: Karen C. Turner
Publishing Services Manager: Jeffrey Patterson
Senior Project Managers: Jeanne Genz and Tracey Schriefer
Designer: Margaret Reid
Printed in the United States of America
Last digit is the print number: 9 8 7 6 5 4 3 2 1
Contributors
Barbara J. Boss RN, PHD, CFNP, CANP
Retired Professor of Nursing
University of Mississippi Medical Center
Jackson, Mississippi
Kristen Lee Carroll MD
Chief of Staff
Medical Staff/Orthopedics
Shriners Hospital for Children
Professor of Orthopedics
University of Utah
Salt Lake City, Utah
Margaret F. Clayton PhD, APRN
Associate Professor and Assistant Dean for the PhD Program
College of Nursing
University of Utah
Salt Lake City, Utah
Christy L. Crowther-Radulewicz RN, MS, CRNP
Nurse Practitioner
Orthopedic Surgery
Anne Arundel Orthopedic Surgeons
Annapolis, Maryland
Susanna G. Cunningham BSN, MA, PhD, RN, FAHA, FAAN
Professor Emeritus
Department of Biobehavioral Nursing
School of Nursing
University of Washington
Seattle, Washington
Sara J. Fidanza MS, RN, CNS-BC, CPNP-BC
Digestive Health Institute
Children’s Hospital Colorado
Clinical Faculty
University of Colorado College of Nursing
Aurora, Colorado
Diane P. Genereux PhD
Assistant Professor
Department of Biology
Westfield State
Westfield, Massachusetts
Todd Cameron Grey MD
Chief Medical Examiner
Office of the Medical Examiner
State of Utah
Salt Lake City, Utah
Robert E. Jones MD, FACP, FACE
Professor of Medicine
Endocrinology Division
University of Utah School of Medicine
Salt Lake City, Utah
Lynn B. Jorde PhD
H.A. and Edna Benning Presidential Professor and Chair
Department of Human Genetics
University of Utah School of Medicine
Salt Lake City, Utah
Lynne M. Kerr MD, PhD
Associate Professor
Department of Pediatrics, Division of Pediatric Neurology
University of Utah Medical Center
Salt Lake City, Utah
Nancy E. Kline PhD, RN, CPNP, FAAN †
Director, Nursing Research, Medicine
Patient Services/Emergency Department
Boston Children’s Hospital
Boston, Massachusetts
Lauri A. Linder PhD, APRN, CPON
Assistant Professor
College of Nursing
University of Utah
Clinical Nurse Specialist
Cancer Transplant Center
Primary Children’s Hospital
Salt Lake City, Utah
Sue Ann McCann MSN, RN, DNC
Programmatic Nurse Specialist
Nursing
Clinical Research Coordinator
Dermatology
University of Pittsburgh Medical Center
Pittsburgh, Pennsylvania
Nancy L. McDaniel MD
Associate Professor of Pediatrics
University of Virginia
Charlottesville, Virginia
Afsoon Moktar PhD, EMBA, CT (ASCP)
Associate Professor
School of Physician Assistant Studies
Massachusetts College of Pharmacy and Health Sciences University
Boston, Massachusetts
Noreen Heer Nicol PhD, RN, FNP, NEA-BC
Associate Professor
College of Nursing
University of Colorado
Denver, Colorado
Nancy Pike PhD, RN, CPNP-AC, FAAN
Assistant Professor
UCLA School of Nursing
Pediatric Nurse Practitioner
Cardiothoracic Surgery
Children’s Hospital Los Angeles
Los Angeles, California
Patricia Ring RN, MSN, PNP, BC
Pediatric Nephrology
Children’s Hospital of Wisconsin
Wauwatosa, Wisconsin
Anna E. Roche MSN, RN, CPNP, CPON
Pediatric Nurse Practitioner
Dana Farber/Boston Children’s Cancer and Blood Disorders Center
Boston, Massachusetts
George W. Rodway PhD, APRN
Associate Clinical Professor
Betty Irene Moore School of Nursing at UC Davis
Sacramento, California
Sharon Sables-Baus PhD, MPA, RN, PCNS-BC
Associate Professor
University of Colorado
College of Nursing and School of Medicine
Department of Pediatrics
Pediatric Nurse Scientist
Children’s Hospital Colorado
Aurora, Colorado
Anna Schwartz PhD, FNP-C, FAAN
Associate Professor
School of Nursing
Northern Arizona University
Flagstaff, Arizona;
Affiliate Associate Professor
Biobehavioral Nursing and Health Systems
University of Washington
Seattle, Washington
Joan Shea MSN, RN, CPON
Staff Nurse III
Hematology/Oncology/Clinical Research
Boston Children’s Hospital
Boston, Massachusetts
Lorey K. Takahashi PhD
Professor of Psychology
Department of Psychology
University of Hawaii at Manoa
Honolulu, Hawaii
David M. Virshup MD
Professor and Director
Program in Cancer and Stem Cell Biology
Duke-NUS Graduate Medical School
Singapore;
Professor of Pediatrics
Duke University School of Medicine
Durham, North Carolina
†Deceased.
Reviewers
Deborah Cipale RN, MSN
Nursing Resource Lab Coordinator
Des Moines Area Community College
Ankeny, Iowa
David J. Derrico RN, MSN
Clinical Assistant Professor
Department of Adult and Elderly Nursing
University of Florida College of Nursing
Gainesville, Florida
Sandra L. Kaminski MS, PA-C
Adjunct Professor
Physician Assistant Program
Pace University
New York, New York
Stephen D. Krau PhD, RN, CNE
Associate Professor
Vanderbilt University School of Nursing
Nashville, Tennessee
Lindsay McCrea PhD, RN, FNP-BC, CWOCN
Professor
Nursing Program Assistant Director
California State University, East Bay
Hayward, California
Afsoon Moktar PhD, EMBA, CT (ASCP)
Associate Professor
School of Physician Assistant Studies
Massachusetts College of Pharmacy and Health Sciences University
Boston, Massachusetts
Kathleen S. Murtaugh RN, MSN, CAN
Assistant Professor of Nursing
Saint Joseph College—St. Elizabeth School of Nursing Cooperative Program
Rensselaer, Indiana
Judith L. Myers MSN, RN
Assistant Professor of Nursing
Grand View University
Nursing Department
Des Moines, Iowa
Holldrid Odreman MScN-Ed, BScN, RN
Professor
Program Coordinator of Nursing
Niagara College Canada
Certified Simulationist
SIMone Ontario Simulation Network
Welland, Ontario, Canada
Jay Schulkin PhD
Director of Research
The American Congress of Obstetricians and Gynecologists
Washington, DC;
Acting Professor
Obstetrics & Gynecology
University of Washington School of Medicine
Seattle, Washington
Crystal R. Sherman DNP, RN, APHN-BC
Associate Professor of Nursing
Shawnee State University
Portsmouth, Ohio
Lorey K. Takahashi PhD
Professor of Psychology
Department of Psychology
University of Hawaii at Manoa
Honolulu, Hawaii
Cheryl A. Tucker MSN, RN, CNE
Senior Lecturer and Undergraduate Theory Coordinator
Baylor University
Louise Herrington School of Nursing
Dallas, Texas
Linda Turchin MSN, CNE
Associate Professor of Nursing
Fairmont State University
Fairmont, West Virginia
Jo A. Voss PhD, RN, CNS
Associate Professor
South Dakota State University
West River Department of Nursing
Rapid City, South Dakota
Kim Webb MN, RN
Part-time Nursing Instructor
Pioneer Technology Center
Ponca City, Oklahoma
Preface
The sixth edition of Understanding Pathophysiology, like other editions, has been
rigorously updated and revised with consideration of the rapid advances in
molecular and cell biology. Many sections have been rewritten or reorganized to
provide a foundation for better understanding of the mechanisms of disease.
Integrated throughout the text are concepts from the basic sciences, including
genetics, epigenetics, gene–environment interaction, immunity, and inflammation.
The text has been written to assist students with the translation of the concepts and
processes of pathophysiology into clinical practice and to promote lifelong
learning.
Although the primary focus of the text is pathophysiology, we continue to include
discussions of the following interconnected topics to highlight their importance for
clinical practice:
• A life-span approach that includes special sections on aging and separate chapters
on children
• Epidemiology and incidence rates showing regional and worldwide differences
that reflect the importance of environmental and lifestyle factors on disease
initiation and progression
• Sex differences that affect epidemiology and pathophysiology
• Molecular biology—mechanisms of normal cell function and how their alteration
leads to disease
• Clinical manifestations, summaries of treatment, and health promotion/risk
reduction
Organization and Content: What’s New in
the Sixth Edition
The book is organized into two parts: Part One, Basic Concepts of Pathophysiology,
and Part Two, Body Systems and Diseases. Two new chapters have been added.
Part One: Basic Concepts of Pathophysiology
Part One introduces basic principles and processes that are important for a
contemporary understanding of the pathophysiology of common diseases. The
concepts include descriptions of cellular communication; forms of cell injury;
genes and genetic disease; epigenetics; fluid and electrolytes and acid and base
balance; immunity and inflammation; mechanisms of infection; stress, coping, and
illness; and tumor biology. A new chapter, Epigenetics and Disease (Chapter 3), has
been added since significant progress is emerging that explains the way heritable
changes in gene expression—phenotype without a change in genotype—are
influenced by several factors, including age, environment/lifestyle, and disease
state.
Significant revisions to Part One also include new or updated information on the
following topics:
• Updated content on cell membranes, cell junctions, intercellular communication,
transport by vesicles, and stem cells (Chapter 1)
• New chapter on epigenetics and disease (Chapter 3)
• Updated content on cellular adaptations, oxidative stress, chemical injury, types of
cell death, and aging (Chapter 4)
• Updates regarding mechanisms of human defense—characteristics of innate and
adaptive immunity (Chapters 6 and 7)
• Updated content on mechanisms of infection, antibiotic-resistant disease, and
alterations in immune defense (Chapter 8)
• Updated content on stress, inflammation, hormones, and disease (Chapter 9)
• Extensive entire chapter revisions and reorganization of tumor biology (Chapter
10)
• Extensive entire chapter revisions and updated epidemiology of cancer (Chapter
11)
Part Two: Body Systems and Diseases
Part Two presents the pathophysiology of the most common alterations according
to body system. To promote readability and comprehension, we have used a logical
sequence and uniform approach in presenting the content of the units and chapters.
Each unit focuses on a specific organ system and contains chapters related to
anatomy and physiology, the pathophysiology of the most common diseases, and
common alterations in children. The anatomy and physiology content is presented
as a review to enhance the learner’s understanding of the structural and functional
changes inherent in pathophysiology. A brief summary of normal aging effects is
included at the end of these review chapters. The general organization of each
disease/disorder discussion includes an introductory paragraph on relevant risk
factors and epidemiology, a significant focus on pathophysiology and clinical
manifestations, and then a brief review of evaluation and treatment.
The information on reproductive pathophysiology is now presented in two
chapters, with a new chapter, Alterations of the Male Reproductive System. Other
significant revisions to Part Two include new and/or updated information on the
following topics:
• Mechanisms of pain transmission, pain syndromes, and categories of sleep
disorders (Chapter 14)
• Alterations in levels of consciousness, seizure disorders, and delirium.
Pathogenesis of degenerative brain diseases, the dementias, movement disorders,
traumatic brain and spinal cord injury, stroke syndromes, headache, and infections
and structural malformations of the CNS (Chapters 15, 16, 17)
• The pathogenesis of type 2 diabetes mellitus (Chapter 19)
• Platelet function and coagulation; anemias, alterations of leukocyte function and
myeloid and lymphoid tumors (Chapters 20 and 21)
• Extensive chapter revisions of alterations of hematologic function in children
(Chapter 22)
• Extensive chapter revisions on structure and function of the cardiovascular and
lymphatic systems (Chapter 23)
• Mechanisms of atherosclerosis, hypertension, coronary artery disease, heart
failure, and shock (Chapter 24)
• Pediatric valvular disorders, heart failure, hypertension, obesity, and heart disease
(Chapter 25)
• Pathophysiology of acute lung injury, asthma, pneumonia, lung cancer, respiratory
distress in the newborn, and cystic fibrosis (Chapters 27 and 28)
• Mechanisms of kidney stone formation, immune processes of glomerulonephritis,
and acute and chronic kidney injury (Chapters 30 and 31)
• Female and male reproductive disorders, female and male reproductive cancers,
breast diseases and mechanisms of breast cancer, prostate cancer, male breast
cancer, and sexually transmitted infections (Chapters 33 and 34)
• Gastroesophageal reflux, nonalcoholic liver disease, inflammatory bowel disease,
viral hepatitis, obesity, gluten-sensitive enteropathy, and necrotizing enterocolitis
(Chapters 36 and 37)
• Bone cells, bone remodeling, joint and tendon diseases, osteoporosis, rheumatoid
arthritis, and osteoarthritis (Chapters 38 and 39)
• Congenital and acquired musculoskeletal disorders, and muscular dystrophies in
children (Chapter 40)
• Psoriasis, discoid lupus erythematosus, and atopic dermatitis (Chapters 41 and 42)
Cancer of the various organ systems was updated for all of the chapters.
Features to Promote Learning
A number of features are incorporated into this text that guide and support learning
and understanding, including:
• Chapter Outlines including page numbers for easy reference
• Quick Check questions strategically placed throughout each chapter to help readers
confirm their understanding of the material; answers are included on the textbook’s
Evolve website
• Health Alerts with concise discussions of the latest research
• Risk Factors boxes for selected diseases
• End-of-chapter Did You Understand? summaries that condense the major concepts
of each chapter into an easy-to-review list format; printable versions of these are
available on the textbook’s Evolve website
• Key Terms set in blue boldface in text and listed, with page numbers, at the end of
each chapter
• Special boxes for Aging and Pediatrics content that highlight discussions of life-
span alterations
Art Program
All of the figures and photographs have been carefully reviewed, revised, or
updated. This edition features approximately 100 new or heavily revised
illustrations and photographs with a total of approximately 1000 images. The
figures are designed to help students visually understand sometimes difficult and
complex material. Hundreds of high-quality photographs show clinical
manifestations, pathologic specimens, and clinical imaging techniques.
Micrographs show normal and abnormal cellular structure. The combination of
illustrations, algorithms, photographs, and use of color for tables and boxes allows
a more precise understanding of essential information.
Teaching/Learning Package
For Students
The free electronic Student Resources on Evolve include review questions and
answers, numerous animations, answers to the Quick Check questions in the book,
printable key points, and bonus case studies with questions and answers. A
comprehensive Glossary for the textbook of more than 600 terms helps students
with the often difficult terminology related to pathophysiology; this is available
both on Evolve and in the electronic version of the textbook. These electronic
resources enhance learning options for students. Go to
http://evolve.elsevier.com/Huether.
The newly rewritten Study Guide includes many different question types, aiming
to help the broad spectrum of student learners. Question types include the following:
• Choose the Correct Words
• Complete These Sentences
• Categorize These Clinical Examples
• Explain the Pictures
• Teach These People about Pathophysiology
• Plus many more…
Answers are found in the back of the Study Guide for easy reference for
students.
For Instructors
The electronic Instructor Resources on Evolve are available free to instructors
with qualified adoptions of the textbook and include the following: TEACH Lesson
Plans with case studies to assist with clinical application; a Test Bank of more than
1200 items; PowerPoint Presentations for each chapter, with integrated images,
audience response questions, and case studies; and an Image Collection of
approximately 1000 key figures from the text. All of these teaching resources are
also available to instructors on the book’s Evolve site. Plus the Evolve Learning
System provides a comprehensive suite of course communication and organization
tools that allow you to upload your class calendar and syllabus, post scores and
announcements, and more. Go to http://evolve.elsevier.com/Huether.
The most exciting part of the learning support package is Pathophysiology
Online, a complete set of online modules that provide thoroughly developed lessons
http://evolve.elsevier.com/Huether
http://evolve.elsevier.com/Huether
on the most important and difficult topics in pathophysiology supplemented with
illustrations, animations, interactive activities, interactive algorithms, self-
assessment reviews, and exams. Instructors can use it to enhance traditional
classroom lecture courses or for distance and online-only courses. Students can use
it as a self-guided study tool.
Acknowledgments
This book would not be possible without the knowledge and expertise of our
contributors, both those who have worked with us through previous editions and the
new members of our team. Their reviews and synthesis of the evidence and clear
concise presentation of information is a strength of the text. We thank them.
Nancy Kline, PhD, RN was a highly respected colleague, researcher, nurse, and
contributor to our textbooks. We dedicate this edition to her memory and the many
contributions she made to nursing research, medicine, patient services, and
children’s health. We will miss her.
The reviewers for this edition provided excellent recommendations for focus of
content and revisions. We appreciate their insightful work.
For more than 30 years Sue Meeks has been the rock of our manuscript
preparation. She is masterful at managing details of the numerous revisions,
maintains the correct formatting, provides helpful recommendations, and manages
the complexity and chaos—all with a wonderful sense of humor. We cannot thank
her enough.
Tina Brashers, MD, and Neal Rote, PhD, continued to serve as section editors and
contributing authors. Tina is a distinguished teacher and has received numerous
awards for her teaching and work with nursing and medical students and faculty. She
is nationally known for her leadership and development in promoting and teaching
interprofessional collaboration. Tina brings innovation and clarity to the subject of
pathophysiology. Her contributions to the online course continue to be intensive and
creative, and a significant learning enhancement for students. Thank you, Tina, for
the outstanding quality of your work. Neal has major expertise, passion, and hard-
to-find precision in the topics of immunity, reproductive biology, and human
defenses. His expertise was well placed to rewrite and update the challenging tumor
biology chapter. Neal has held many appointments, including department chair,
associate dean, and professor in both reproductive biology and pathology. He is a
top-notch researcher and reviewer of grants and has received numerous awards and
recognition for his teaching. Neal has a gift for creating images that bring clarity to
the complex content of immunology. He also completely updated the glossary.
Thank you, Neal, for your persistence in promoting understanding and for your
continuing devotion to students.
Karen Turner was our excellent Senior Content Development Specialist. Always
gracious and efficient, Karen guided us through the hardest times and even the redo
times. Thank you, Karen, especially for another set of “eagle eyes.” Jeanne Genz
retired as the Project Manager during the preparation of this edition and we will
miss her expertise. Always dedicated and an amazing “can do” attitude, we thank
you, Jeanne. Tracey Schriefer picked up the reins without missing a step. Thank you,
Tracey, for such diligence—finding and correcting obscure errors. We also thank
Beth Welch, who has copyedited our last four editions. Kellie White was our
Executive Content Strategist and was responsible for overseeing the entire project.
Very organized and a delightful sense of humor, we thank you Kellie. The internal
layout, selection of colors, and design of the cover highlight the pedagogy and were
done by our Designer, Margaret Reid. Thanks to the team from Graphic World, who
created many new images and managed the cleanup and scanning of artwork
obtained from many resources.
We thank the Department of Dermatology at the University of Utah School of
Medicine, which provided numerous photos of skin lesions. Thank you to our many
colleagues and friends at the University of Utah College of Nursing, School of
Medicine, Eccles Medical Library, and College of Pharmacy for their helpfulness,
suggestions, and critiques.
We extend gratitude to those who contributed to the book supplements. Linda
Felver has created an all new inventive and resourceful Study Guide. Thank you,
Linda, for your very astute edits. Additional thanks to the reviewers of the Study
Guide, Janie Corbitt, Kathleen Murtaugh, and Linda Turchin. A special thanks to
Linda Turchin, Joanna Cain, Stephen Krau, and Melanie Cole for their thorough
approach in preparing the materials for the Evolve website, and to Linda Turchin,
Kim Webb, and Lauren Mussig for the valuable reviews of these resources. Tina
Brashers, Nancy Burruss, Mandi Counters, Joe Gordon, Melissa Geist, Kay Gaehle,
Stephen Krau, Jason Mott, and Kim Webb also updated the interactive online lessons
and activities for Pathophysiology Online.
Special thanks to faculty and nursing students and other health science students
for your questions and suggestions. It is because of you, the future clinicians, that
we are so motivated to put our best efforts into this work.
Sincerely and with great affection we thank our families, especially Mae and
John. Always supportive, you make the work possible!
Sue E. Huether
Kathryn L. McCance
Introduction to Pathophysiology
The word root “patho” is derived from the Greek word pathos, which means
suffering. The Greek word root “logos” means discourse or, more simply, system
of formal study, and “physio” refers to functions of an organism. Altogether,
pathophysiology is the study of the underlying changes in body physiology
(molecular, cellular, and organ systems) that result from disease or injury.
Important, however, is the inextricable component of suffering and the
psychological, spiritual, social, cultural, and economic implications of disease.
The science of pathophysiology seeks to provide an understanding of the
mechanisms of disease and to explain how and why alterations in body structure and
function lead to the signs and symptoms of disease. Understanding pathophysiology
guides healthcare professionals in the planning, selection, and evaluation of
therapies and treatments.
Knowledge of human anatomy and physiology and the interrelationship among
the various cells and organ systems of the body is an essential foundation for the
study of pathophysiology. Review of this subject matter enhances comprehension of
pathophysiologic events and processes. Understanding pathophysiology also entails
the utilization of principles, concepts, and basic knowledge from other fields of
study including pathology, genetics, epigenetics, immunology, and epidemiology. A
number of terms are used to focus the discussion of pathophysiology; they may be
used interchangeably at times, but that does not necessarily indicate that they have
the same meaning. Those terms are reviewed here for the purpose of clarification.
Pathology is the investigation of structural alterations in cells, tissues, and
organs, which can help identify the cause of a particular disease. Pathology differs
from pathogenesis, which is the pattern of tissue changes associated with the
development of disease. Etiology refers to the study of the cause of disease.
Diseases may be caused by infection, heredity, gene–environment interactions,
alterations in immunity, malignancy, malnutrition, degeneration, or trauma.
Diseases that have no identifiable cause are termed idiopathic. Diseases that occur
as a result of medical treatment are termed iatrogenic (for example, some
antibiotics can injure the kidney and cause renal failure). Diseases that are acquired
as a consequence of being in a hospital environment are called nosocomial. An
infection that develops as a result of a person’s immune system being depressed
after receiving cancer treatment during a hospital stay would be defined as a
nosocomial infection.
Diagnosis is the naming or identification of a disease. A diagnosis is made from
an evaluation of the evidence accumulated from the presenting signs and symptoms,
health and medical history, physical examination, laboratory tests, and imaging. A
prognosis is the expected outcome of a disease. Acute disease is the sudden
appearance of signs and symptoms that last only a short time. Chronic disease
develops more slowly and the signs and symptoms last for a long time, perhaps for
a lifetime. Chronic diseases may have a pattern of remission and exacerbation.
Remissions are periods when symptoms disappear or diminish significantly.
Exacerbations are periods when the symptoms become worse or more severe. A
complication is the onset of a disease in a person who is already coping with
another existing disease (for example, a person who has undergone surgery to
remove a diseased appendix may develop the complication of a wound infection or
pneumonia). Sequelae are unwanted outcomes of having a disease or are the result
of trauma, such as paralysis resulting from a stroke or severe scarring resulting
from a burn.
Clinical manifestations are the signs and symptoms or evidence of disease. Signs
are objective alterations that can be observed or measured by another person,
measures of bodily functions such as pulse rate, blood pressure, body temperature,
or white blood cell count. Some signs are local, such as redness or swelling, and
other signs are systemic, such as fever. Symptoms are subjective experiences
reported by the person with disease, such as pain, nausea, or shortness of breath;
and they vary from person to person. The prodromal period of a disease is the time
during which a person experiences vague symptoms such as fatigue or loss of
appetite before the onset of specific signs and symptoms. The term insidious
symptoms describes vague or nonspecific feelings and an awareness that there is a
change within the body. Some diseases have a latent period, a time during which no
symptoms are readily apparent in the affected person, but the disease is nevertheless
present in the body; an example is the incubation phase of an infection or the early
growth phase of a tumor. A syndrome is a group of symptoms that occur together
and may be caused by several interrelated problems or a specific disease; severe
acute respiratory syndrome (SARS), for example, presents with a set of symptoms
that include headache, fever, body aches, an overall feeling of discomfort, and
sometimes dry cough and difficulty breathing. A disorder is an abnormality of
function; this term also can refer to an illness or a particular problem such as a
bleeding disorder.
Epidemiology is the study of tracking patterns or disease occurrence and
transmission among populations and by geographic areas. Incidence of a disease is
the number of new cases occurring in a specific time period. Prevalence of a
disease is the number of existing cases within a population during a specific time
period.
Risk factors, also known as predisposing factors, increase the probability that
disease will occur, but these factors are not the cause of disease. Risk factors include
heredity, age, gender, race, environment, and lifestyle. A precipitating factor is a
condition or event that does cause a pathologic event or disorder. For example,
asthma is precipitated by exposure to an allergen, or angina (pain) is precipitated by
exertion.
Pathophysiology is an exciting field of study that is ever-changing as new
discoveries are made. Understanding pathophysiology empowers healthcare
professionals with the knowledge of how and why disease develops and informs
their decision making to ensure optimal healthcare outcomes. Embedded in the
study of pathophysiology is understanding that suffering is a personal, individual
experience and a major component of disease.
PART ONE
Basic Concepts of Pathophysiology
OUTLINE
Unit 1 The Cell
Unit 2 Mechanisms of Self-Defense
Unit 3 Cellular Proliferation: Cancer
UNIT 1
The Cell
OUTLINE
1 Cellular Biology
2 Genes and Genetic Diseases
3 Epigenetics and Disease
4 Altered Cellular and Tissue Biology
5 Fluids and Electrolytes, Acids and Bases
1
Cellular Biology
Kathryn L. McCance
CHAPTER OUTLINE
Prokaryotes and Eukaryotes, 1
Cellular Functions, 1
Structure and Function of Cellular Components, 2
Nucleus, 2
Cytoplasmic Organelles, 2
Plasma Membranes, 2
Cellular Receptors, 9
Cell-to-Cell Adhesions, 10
Extracellular Matrix, 10
Specialized Cell Junctions, 11
Cellular Communication and Signal Transduction, 12
Cellular Metabolism, 14
Role of Adenosine Triphosphate, 16
Food and Production of Cellular Energy, 16
Oxidative Phosphorylation, 16
Membrane Transport: Cellular Intake and Output, 17
Electrolytes as Solutes, 18
Transport by Vesicle Formation, 21
Movement of Electrical Impulses: Membrane
Potentials, 24
Cellular Reproduction: The Cell Cycle, 25
Phases of Mitosis and Cytokinesis, 26
Rates of Cellular Division, 26
Growth Factors, 26
Tissues, 27
Tissue Formation, 27
Types of Tissues, 27
All body functions depend on the integrity of cells. Therefore an understanding of
cellular biology is increasingly necessary to comprehend disease processes. An
overwhelming amount of information reveals how cells behave as a multicellular
“social” organism. At the heart of it all is cellular communication (cellular
“crosstalk”)—how messages originate and are transmitted, received, interpreted,
and used by the cell. Streamlined conversation between, among, and within cells
maintains cellular function and specialization. Cells must demonstrate a “chemical
fondness” for other cells to maintain the integrity of the entire organism. When they
no longer tolerate this fondness, the conversation breaks down, and cells either
adapt (sometimes altering function) or become vulnerable to isolation, injury, or
disease.
Prokaryotes and Eukaryotes
Living cells generally are divided into eukaryotes and prokaryotes. The cells of
higher animals and plants are eukaryotes, as are the single-celled organisms, fungi,
protozoa, and most algae. Prokaryotes include cyanobacteria (blue-green algae),
bacteria, and rickettsiae. Prokaryotes traditionally were studied as core subjects of
molecular biology. Today, emphasis is on the eukaryotic cell; much of its structure
and function have no counterpart in bacterial cells.
Eukaryotes (eu = good; karyon = nucleus; also spelled eucaryotes) are larger
and have more extensive intracellular anatomy and organization than prokaryotes.
Eukaryotic cells have a characteristic set of membrane-bound intracellular
compartments, called organelles, that includes a well-defined nucleus. The
prokaryotes contain no organelles, and their nuclear material is not encased by a
nuclear membrane. Prokaryotic cells are characterized by lack of a distinct nucleus.
Besides having structural differences, prokaryotic and eukaryotic cells differ in
chemical composition and biochemical activity. The nuclei of prokaryotic cells
carry genetic information in a single circular chromosome, and they lack a class of
proteins called histones, which in eukaryotic cells bind with deoxyribonucleic acid
(DNA) and are involved in the supercoiling of DNA. Eukaryotic cells have several
or many chromosomes. Protein production, or synthesis, in the two classes of cells
also differs because of major structural differences in ribonucleic acid (RNA)–
protein complexes. Other distinctions include differences in mechanisms of
transport across the outer cellular membrane and in enzyme content.
Cellular Functions
Cells become specialized through the process of differentiation, or maturation, so
that some cells eventually perform one kind of function and other cells perform
other functions. Cells with a highly developed function, such as movement, often
lack some other property, such as hormone production, which is more highly
developed in other cells.
The eight chief cellular functions are as follows:
1. Movement. Muscle cells can generate forces that produce motion. Muscles that are
attached to bones produce limb movements, whereas those muscles that enclose
hollow tubes or cavities move or empty contents when they contract (e.g., the
colon).
2. Conductivity. Conduction as a response to a stimulus is manifested by a wave of
excitation, an electrical potential that passes along the surface of the cell to reach its
other parts. Conductivity is the chief function of nerve cells.
3. Metabolic absorption. All cells can take in and use nutrients and other substances
from their surroundings.
4. Secretion. Certain cells, such as mucous gland cells, can synthesize new
substances from substances they absorb and then secrete the new substances to serve
as needed elsewhere.
5. Excretion. All cells can rid themselves of waste products resulting from the
metabolic breakdown of nutrients. Membrane-bound sacs (lysosomes) within cells
contain enzymes that break down, or digest, large molecules, turning them into
waste products that are released from the cell.
6. Respiration. Cells absorb oxygen, which is used to transform nutrients into
energy in the form of adenosine triphosphate (ATP). Cellular respiration, or
oxidation, occurs in organelles called mitochondria.
7. Reproduction. Tissue growth occurs as cells enlarge and reproduce themselves.
Even without growth, tissue maintenance requires that new cells be produced to
replace cells that are lost normally through cellular death. Not all cells are capable
of continuous division (see Chapter 4).
8. Communication. Communication is vital for cells to survive as a society of cells.
Appropriate communication allows the maintenance of a dynamic steady state.
Structure and Function of Cellular
Components
Figure 1-1, A, shows a “typical” eukaryotic cell, which consists of three
components: an outer membrane called the plasma membrane, or plasmalemma; a
fluid “filling” called cytoplasm (Figure 1-1, B); and the “organs” of the cell—the
membrane-bound intracellular organelles, among them the nucleus.
FIGURE 1-1 Typical Components of a Eukaryotic Cell and Structure of the Cytoplasm. A, Artist’s
interpretation of cell structure. Note the many mitochondria known as the “power plants of the
cell.” B, Color-enhanced electron micrograph of a cell. The cell is crowded. Note, too, the
innumerable dots bordering the endoplasmic reticulum. These are ribosomes, the cell’s
“protein factories.” (B, from Patton KT, Thibodeau GA: Anatomy & physiology, ed 8, St Louis, 2013, Mosby.)
Nucleus
The nucleus, which is surrounded by the cytoplasm and generally is located in the
center of the cell, is the largest membrane-bound organelle. Two pliable membranes
compose the nuclear envelope (Figure 1-2, A). The nuclear envelope is pockmarked
with pits, called nuclear pores, which allow chemical messages to exit and enter the
nucleus (see Figure 1-2). The outer membrane is continuous with membranes of the
endoplasmic reticulum (see Figure 1-1). The nucleus contains the nucleolus (a small
dense structure composed largely of ribonucleic acid), most of the cellular DNA,
and the DNA-binding proteins (i.e., the histones) that regulate its activity. The DNA
“chain” in eukaryotic cells is so long that it is easily broken. Therefore the histones
that bind to DNA cause DNA to fold into chromosomes (Figure 1-2, C), which
decreases the risk of breakage and is essential for cell division in eukaryotes.
FIGURE 1-2 The Nucleus. The nucleus is composed of a double membrane, called a nuclear
envelope, that encloses the fluid-filled interior, called nucleoplasm. The chromosomes are
suspended in the nucleoplasm (illustrated here much larger than actual size to show the tightly
packed DNA strands). Swelling at one or more points of the chromosome, shown in A, occurs at
a nucleolus where genes are being copied into RNA. The nuclear envelope is studded with
pores. B, The pores are visible as dimples in this freeze-etch of a nuclear envelope. C, Histone-
folding DNA in chromosomes. (B, from Raven PH, Johnson GB: Biology, St Louis, 1992, Mosby.)
The primary functions of the nucleus are cell division and control of genetic
information. Other functions include the replication and repair of DNA and the
transcription of the information stored in DNA. Genetic information is transcribed
into ribonucleic acid (RNA), which can be processed into messenger, transport, and
ribosomal RNAs and introduced into the cytoplasm, where it directs cellular
activities. Most of the processing of RNA occurs in the nucleolus. (The roles of
DNA and RNA in protein synthesis are discussed in Chapter 2.)
Cytoplasmic Organelles
Cytoplasm is an aqueous solution (cytosol) that fills the cytoplasmic matrix—the
space between the nuclear envelope and the plasma membrane. The cytosol
represents about half the volume of a eukaryotic cell. It contains thousands of
enzymes involved in intermediate metabolism and is crowded with ribosomes
making proteins (see Figure 1-1, B). Newly synthesized proteins remain in the
cytosol if they lack a signal for transport to a cell organelle.1 The organelles
suspended in the cytoplasm are enclosed in biologic membranes, so they can
simultaneously carry out functions requiring different biochemical environments.
Many of these functions are directed by coded messages carried from the nucleus by
RNA. The functions include synthesis of proteins and hormones and their transport
out of the cell, isolation and elimination of waste products from the cell,
performance of metabolic processes, breakdown and disposal of cellular debris and
foreign proteins (antigens), and maintenance of cellular structure and motility. The
cytosol is a storage unit for fat, carbohydrates, and secretory vesicles. Table 1-1
lists the principal cytoplasmic organelles.
Quick Check 1-1
1. Why is the process of differentiation essential to specialization? Give an example.
2. Describe at least two cellular functions.
TABLE 1-1
Principal Cytoplasmic Organelles
Organelle Characteristics and Description
Ribosomes RNA-protein complexes (nucleoproteins) synthesized in nucleolus and secreted into cytoplasm. Provide sites for cellular protein synthesis.
Endoplasmic
reticulum
Network of tubular channels (cisternae) that extend throughout outer nuclear membrane. Specializes in synthesis and transport of protein and
lipid components of most organelles.
Golgi
complex
Network of smooth membranes and vesicles located near nucleus. Responsible for processing and packaging proteins onto secretory vesicles
that break away from the complex and migrate to various intracellular and extracellular destinations, including plasma membrane. Best-
known vesicles are those that have coats largely made of the protein clathrin. Proteins in the complex bind to the cytoskeleton, generating
tension that helps organelle function and keep its stretched shape intact.
Lysosomes Saclike structures that originate from Golgi complex and contain enzymes for digesting most cellular substances to their basic form, such as
amino acids, fatty acids, and carbohydrates (sugars). Cellular injury leads to release of lysosomal enzymes that cause cellular self-destruction.
Peroxisomes Similar to lysosomes but contain several oxidative enzymes (e.g., catalase, urate oxidase) that produce hydrogen peroxide; reactions detoxify
various wastes.
Mitochondria Contain metabolic machinery needed for cellular energy metabolism. Enzymes of respiratory chain (electron-transport chain), found in inner
membrane of mitochondria, generate most of cell’s ATP (oxidative phosphorylation). Have a role in osmotic regulation, pH control, calcium
homeostasis, and cell signaling.
Cytoskeleton “Bone and muscle” of cell. Composed of a network of protein filaments, including microtubules and actin filaments (microfilaments); forms
cell extensions (microvilli, cilia, flagella).
Caveolae Tiny indentations (caves) that can capture extracellular material and shuttle it inside the cell or across the cell.
Vaults Cytoplasmic ribonucleoproteins shaped like octagonal barrels. Thought to act as “trucks,” shuttling molecules from nucleus to elsewhere in
cell.
Plasma Membranes
Every cell is contained within a membrane with gates, channels, and pumps.
Membranes surround the cell or enclose an intracellular organelle and are
exceedingly important to normal physiologic function because they control the
composition of the space, or compartment, they enclose. Membranes can allow or
exclude various molecules and, because of selective transport systems, they can
move molecules in or out of the space (Figure 1-3). By controlling the movement of
substances from one compartment to another, membranes exert a powerful
influence on metabolic pathways. Directional transport is facilitated by polarized
domains, distinct apical and basolateral domains. Cell polarity, the direction of
cellular transport, maintains normal cell and tissue structure for numerous functions
(for example, movement of nutrients in and out of the cell) and becomes altered
with diseases (Figure 1-4). The plasma membrane also has an important role in cell-
to-cell recognition. Other functions of the plasma membrane include cellular
mobility and the maintenance of cellular shape (Table 1-2).
FIGURE 1-3 Functions of Plasma Membrane Proteins. The plasma membrane proteins
illustrated here show a variety of functions performed by the different types of plasma
membranes. (From Raven PH, Johnson GB: Understanding biology, ed 3, Dubuque, Iowa, 1995, Brown.)
FIGURE 1-4 Cell Polarity of Epithelial Cells. Schematic of cell polarity (cell direction) of epithelial
cells. Shown are the directions of the basal side and the apical side. Organelles and
cytoskeleton are also arranged directionally to enable, for example, intestinal cell secretion and
absorption. (Adapted from Life science web textbook, The University of Tokyo.)
TABLE 1-2
Plasma Membrane Functions
Cellular
Mechanism
Membrane Functions
Structure Usually thicker than membranes of intracellular organelles
Containment of cellular organelles
Maintenance of relationship with cytoskeleton, endoplasmic reticulum, and other organelles
Maintenance of fluid and electrolyte balance
Outer surfaces of plasma membranes in many cells are not smooth but are dimpled with cavelike indentations called caveolae; they are also
studded with cilia or even smaller cylindrical projections called microvilli; both are capable of movement
Protection Barrier to toxic molecules and macromolecules (proteins, nucleic acids, polysaccharides)
Barrier to foreign organisms and cells
Activation
of cell
Hormones (regulation of cellular activity)
Mitogens (cellular division; see Chapter 2)
Antigens (antibody synthesis; see Chapter 6)
Growth factors (proliferation and differentiation; see Chapter 10)
Storage Storage site for many receptors
Transport
Diffusion and exchange diffusion
Endocytosis (pinocytosis, phagocytosis)
Exocytosis (secretion)
Active transport
Cell-to-cell
interaction
Communication and attachment at junctional complexes
Symbiotic nutritive relationships
Release of enzymes and antibodies to extracellular environment
Relationships with extracellular matrix
Modified from King DW, Fenoglio CM, Lefkowitch JH: General pathology: principles and dynamics,
Philadelphia, 1983, Lea & Febiger.
Membrane Composition
The basic structure of cell membranes is the lipid bilayer, composed of two
apposing leaflets and proteins that span the bilayer or interact with the lipids on
either side of the two leaflets (Figure 1-5). Lipid research is growing and principles
of membrane organization are being overhauled.2 In short, the main constituents of
cell membranes are lipids and proteins. Historically, the plasma membrane was
described as a fluid lipid bilayer (fluid mosaic model) composed of a uniform lipid
distribution with inserted moving proteins. It now appears that the lipid bilayer is a
much more complex structure where lipids and proteins are not uniformly
distributed but can separate into discrete units called microdomains, differing in
their protein and lipid compositions.3 Different membranes have varying
percentages of lipids and proteins. Intracellular membranes may have a higher
percentage of proteins than do plasma membranes, presumably because most
enzymatic activity occurs within organelles. The membrane organization is
achieved through noncovalent bonds that allow different physical states called
phases. The lipid bilayer can be structured in three main phases: solid gel phase,
fluid liquid-crystalline phase, and liquid-ordered phase (Figure 1-5, B). These
phases can change under physiologic factors such as temperature and pressure
fluctuations. Carbohydrates are mainly associated with plasma membranes, in which
they are chemically combined with lipids, forming glycolipids, and with proteins,
forming glycoproteins (see Figure 1-5).
FIGURE 1-5 Lipid Bilayer Membranes. A, Concepts of biologic membranes have markedly
changed in the last two decades, from the classic fluid mosaic model to the current model that
lipids and proteins are not evenly distributed but can isolate into microdomains, differing in their
protein and lipid composition. B, An example of a microdomain is lipid rafts (yellow). Rafts are
dynamic domain structures composed of cholesterol, sphingolipids, and membrane proteins
important in different cellular processes. Various models exist to clarify the functions of
domains. The three major phases of lipid bilayer organization include a solid gel phase (e.g.,
with low temperatures), a liquid-ordered phase (high temperatures), and a fluid liquid-crystalline
(or liquid-disordered) phase. Some membrane-associated proteins are integrated into the lipid
bilayer; other proteins are loosely attached to the outer and inner surfaces of the membrane.
Transmembrane proteins protrude through the entire outer and inner surfaces of the
membrane, and they can be attracted to microdomains through specific interactions with lipids.
Interaction of the membrane proteins with distinct lipids depends on the hydrophobic thickness
of the membrane, the lateral pressures of the membrane (mechanical force may shift protein
channels from an open to closed state), the polarity or electrical charges at the lipid-protein
interface, and the presence on the protein side of amino acid side chains. Important for
pathophysiology is the proposal that protein-lipid interactions can be critical for correct
insertion, folding, and orientation of membrane proteins. For example, diseases related to lipids
that interfere with protein folding are becoming more prevalent. C, The cell membrane is not
static but is always moving. Observed for the first time from measurements taken at the
National Institute of Standards and Technology (NIST) and France’s Institut Laue-Langevin
(ILL). (Adapted from Bagatolli LA et al: Prog Lipid Res 49[4]:378-389, 2010; Contreras FX et al: Cold Spring Harb Perspect Biol 3[6]:pii a004705,
2011; Cooper GM: The cell—a molecular approach, ed 2, Sunderland (MA): Sinauer Associates, 2000; Defamie N, Mesnil M: Biochim Biophys Acta
1818(8):1866-1869, 2012; W oodka AC et al: Phys Rev Lett 9(5):058102, 2012.)
The outer surface of the plasma membrane in many types of cells, especially
endothelial cells and adipocytes, is not smooth but dimpled with flask-shaped
invaginations known as caveolae (“tiny caves”). Caveolae serve as a storage site for
many receptors, provide a route for transport into the cell, and act as the initiator
for relaying signals from several extracellular chemical messengers into the cell’s
interior (see p. 24).
Lipids.
Each lipid molecule is said to be polar, or amphipathic, which means that one part
is hydrophobic (uncharged, or “water hating”) and another part is hydrophilic
(charged, or “water loving”) (Figure 1-6). The membrane spontaneously organizes
itself into two layers because of these two incompatible solubilities. The
hydrophobic region (hydrophobic tail) of each lipid molecule is protected from
water, whereas the hydrophilic region (hydrophilic head) is immersed in it. The
bilayer serves as a barrier to the diffusion of water and hydrophilic substances,
while allowing lipid-soluble molecules, such as oxygen (O2) and carbon dioxide
(CO2), to diffuse through the membrane readily.
FIGURE 1-6 Structure of a Phospholipid Molecule. A, Each phospholipid molecule consists of a
phosphate functional group and two fatty acid chains attached to a glycerol molecule. B, The
fatty acid chains and glycerol form nonpolar, hydrophobic “tails,” and the phosphate functional
group forms the polar, hydrophilic “head” of the phospholipid molecule. When placed in water,
the hydrophobic tails of the molecule face inward, away from the water, and the hydrophilic
head faces outward, toward the water. (From Raven PH, Johnson GB: Understanding biology, ed 3, Dubuque, Iowa, 1995,
Brown.)
A major component of the plasma membrane is a bilayer of lipid molecules—
glycerophospholipids, sphingolipids, and sterols (for example, cholesterol). The
most abundant lipids are phospholipids. Phospholipids have a phosphate-containing
hydrophilic head connected to a hydrophobic tail. Phospholipids and glycolipids
form self-sealing lipid bilayers. Lipids along with protein assemblies act as
“molecular glue” for the structural integrity of the membrane. Investigators are
studying the concept of lipid rafts. Membrane lipid rafts (MLRs) appear to be
structurally and functionally distinct regions of the plasma membrane4,5 and consist
of cholesterol and sphingolipid-dependent microdomains that form a network of
lipid-lipid, protein-protein, and protein-lipid interactions (Figures 1-5, B, and 1-7)
Although discrepancies between experimental results exist, two main types of MLRs
are hypothesized: those that contain the cholesterol-binding protein caveolin (see p.
24) and those that do not.4 Researchers hypothesized there are lipid rafts that have
several functions, including (1) providing cellular polarity and organization of
signaling trafficking; (2) acting as platforms for extracellular matrix (ECM)
adhesion and intracellular cytoskeletal tethering to the plasma membrane through
cellular adhesion molecules (CAMs, see p. 8); (3) enabling signaling across the
membrane, which can rearrange cytoskeletal architecture and regulate cell growth,
migration, and other functions; and (4) allowing entry of viruses, bacteria, toxins,
and nanoparticles.4
FIGURE 1-7 Lipid Rafts. The plasma membrane is composed of many lipids, including
sphingomyelin (SM) and cholesterol, shown here as a small raft in the external leaflet. GS,
Glycosphingolipid; PC, phosphatidylcholine; PE, phosphatidylethanolamine; PS,
phosphatidylserine. (From Pollard TD, Ernshaw W C: Cell biology, St Louis, 2004, Saunders Elsevier.)
Proteins.
A protein is made from a chain of amino acids known as polypeptides. There are
20 types of amino acids in proteins and each type of protein has a unique sequence
of amino acids. Proteins are the major workhorses of the cell. After translation (the
synthesis of protein from RNA, see Chapter 2) of a protein, posttranslational
modifications (PTMs) are the methods used to diversify the limited numbers of
proteins generated. These modifications alter the activity and functions of proteins
and have become very important in understanding diseases. Researchers have
known for decades that pathogens interfere with the host’s PTMs.6 New approaches
are being used to understand changes in proteins—a field called proteomics is the
study of the proteome, or entire set of proteins expressed by a genome from
synthesis, translocation, and modification (e.g., folding), and the analysis of the
roles of proteomes in a staggering number of diseases.
Membrane proteins associate with the lipid bilayer in different ways (Figure 1-8),
including (1) transmembrane proteins that extend across the bilayer and exposed
to an aqueous environment on both sides of the membrane (see Figure 1-8, A); (2)
proteins located almost entirely in the cytosol and associated with the cytosolic half
of the lipid bilayer by an α helix exposed on the surface of the protein (see Figure 1-
8, B); (3) proteins that exist outside the bilayer, on one side or the other, and attached
to the membrane by one or more covalently attached lipid groups (see Figure 1-8,
C); and (4) proteins bound indirectly to one or the other bilayer membrane face and
held in place by their interactions with other proteins (see Figure 1-8, D).1
FIGURE 1-8 Proteins Attach to the Plasma Membrane in Different Ways. A, Transmembrane
proteins extend through the membrane as a single α helix, as multiple α helices, or as a rolled
up barrel-like sheet called a β barrel. B, Some membrane proteins are anchored to the cytosolic
side of the lipid bilayer by an amphipathic α helix. C, Some proteins are linked on either side of
the membrane by a covalently attached lipid molecule. D, Proteins are attached by weak
noncovalent interactions with other membrane proteins. All are integral membrane proteins
except. (D, adapted from Alberts B: Essential cell biology, ed 4, New York, 2014, Garland.)
Proteins directly attached to the membrane bilayer can be removed by dissolving
the bilayer with detergents called integral membrane proteins. The remaining
proteins that can be removed by gentler procedures that interfere with protein-
protein interactions but do not dissolve the bilayer are known as peripheral
membrane proteins.
Proteins exist in densely folded molecular configurations rather than straight
chains; so most hydrophilic units are at the surface of the molecule and most
hydrophobic units are inside. Membrane proteins, like other proteins, are
synthesized by the ribosome and then make their way, called trafficking, to different
membrane locations of a cell.7 Trafficking places unique demands on membrane
proteins for folding, translocation, and stability.7 Thus, much research is now being
done to understand misfolded proteins (for example, as a cause of disease; Box 1-
1).
Box 1-1
Endoplasmic Reticulum, Protein Folding, and
ER Stress
Protein folding in the endoplasmic reticulum (ER) is critical for us. As the biologic
workhorses, proteins perform vital functions in every cell. To do these tasks
proteins must fold into complex three-dimensional structures (see figure). Most
secreted proteins fold and are modified in an error-free manner, but ER or cell
stress, mutations, or random (stochastic) errors during protein synthesis can
decrease the folding amount or the rate of folding. Pathophysiologic processes,
such as viral infections, environmental toxins, and mutant protein expression, can
perturb the sensitive ER environment. Natural processes also can perturb the
environment, such as the large protein-synthesizing load placed on the ER. These
perturbations cause the accumulation of immature and abnormal proteins in cells,
leading to ER stress. Fortunately, the ER is loaded with protective ways to help
folding; for example, protein chaperones facilitate folding and prevent the
formation of off-pathway types. Because specialized cells produce large amounts
of secreted proteins, the movement or flux through the ER is tremendous.
Therefore misfolded proteins not repaired in the ER are observed in some diseases
and can initiate apoptosis or cell death. It has recently been shown that the
endoplasmic reticulum mediates intracellular signaling pathways in response to the
accumulation of unfolded or misfolded proteins; collectively, the pathways are
known as the unfolded-protein response (UPR). Investigators are studying UPR-
associated inflammation and how the UPR is coupled to inflammation in health and
disease. Specific diseases include Alzheimer disease, Parkinson disease, prion
disease, amyotrophic lateral sclerosis, and diabetes mellitus. Additionally being
studied is ER stress and how it may accelerate age-related dysfunction.
Protein Folding. Each protein exists as an unfolded polypeptide (left) or a random coil after the
process of translation from a sequence of mRNA to a linear string of amino acids. From amino
acids interacting with each other they produce a three-dimensional structure called the folded
protein (right) that is its native state.
Data from Brodsky J, Skach WR: Curr Opin Cell Biol 23:464-475, 2011; Jäger R et al: Biol Cell 104(5):259-
270,2012; Ron D, Walter P: Nat Rev Mol Cell Biol 8:519-529, 2007.
Although membrane structure is determined by the lipid bilayer, membrane
functions are determined largely by proteins. Proteins act as (1) recognition and
binding units (receptors) for substances moving into and out of the cell; (2) pores
or transport channels for various electrically charged particles, called ions or
electrolytes, and specific carriers for amino acids and monosaccharides; (3)
specific enzymes that drive active pumps to promote concentration of certain ions,
particularly potassium (K+), within the cell while keeping concentrations of other
ions (for example, sodium, Na+), less than concentrations found in the extracellular
environment; (4) cell surface markers, such as glycoproteins (proteins attached to
carbohydrates), that identify a cell to its neighbor; (5) cell adhesion molecules
(CAMs), or proteins that allow cells to hook together and form attachments of the
cytoskeleton for maintaining cellular shape; and (6) catalysts of chemical reactions
(for example, conversion of lactose to glucose; see Figure 1-3). Membrane proteins
are key components of energy transduction, converting chemical energy into
electrical energy, or electrical energy into either mechanical energy or synthesis of
ATP.7 Investigators are studying ATP enzymes and the changes in shape of biologic
membranes, particularly mitochondrial membranes, and their relationship to aging
and disease.8-10
In animal cells, the plasma membrane is stabilized by a meshwork of proteins
attached to the underside of the membrane called the cell cortex. Human red blood
cells have a cell cortex that maintains their flattened biconcave shape.1
Protein regulation in a cell: protein homeostasis.
The cellular protein pool is in constant change or flux. The number of copies of a
protein in a cell depends on how quickly it is made and how long it survives or is
broken down. This adaptable system of protein homeostasis is defined by the
“proteostasis” network comprised of ribosomes (makers); chaperones (helpers);
and two protein breakdown systems or proteolytic systems—lysosomes and the
ubiquitin-proteasome system (UPS). These systems regulate protein homeostasis
under a large variety of conditions, including variations in nutrient supply, the
existence of oxidative stress or cellular differentiation, changes in temperature, and
the presence of heavy metal ions and other sources of stress.11 Malfunction or
failure of the proteostasis network is associated with human disease12 (Figure 1-9).
FIGURE 1-9 Protein Homeostasis System and Outcomes. A main role of the protein
homeostasis network (proteostasis) is to minimize protein misfolding and protein aggregation.
The network includes ribosome-mediated protein synthesis, chaperone (folding helpers in the
ER) and enzyme mediated folding, breakdown systems of lysosome and proteasome-mediated
protein degradation, and vesicular trafficking. The network integrates biologic pathways that
balance folding, trafficking, and protein degradation depicted by arrows b, d, e, f, g, h, and i. ER,
Endoplasmic reticulum. (Adapted from Lindquist SL, Kelly JW : Cold Spring Harb Perspect Biol 3[12]:pii: a004507, 2011.)
Carbohydrates.
The short chains of sugars or carbohydrates (oligosaccharides) contained within the
plasma membrane are generally bound to membrane proteins (glycoproteins) and
lipids (glycolipids). Long polysaccharide chains attached to membrane proteins are
called proteoglycans. All of the carbohydrate on the glycoproteins, proteoglycans,
and glycolipids is located on the outside of the plasma membrane and the
carbohydrate coating is called the glycocalyx. The glycocalyx helps protect the cell
from mechanical damage.1 Additionally, the layer of carbohydrate gives the cell a
slimy surface that assists the mobility of other cells, like leukocytes, to squeeze
through the narrow spaces.1 The functions of carbohydrates are more than
protection and lubrication and include specific cell-cell recognition and adhesion.
Intercellular recognition is an important function of membrane oligosaccharides;
for example, the transmembrane proteins called lectins, which bind to a particular
oligosaccharide, recognize neutrophils at the site of bacterial infection. This
recognition allows the neutrophil to adhere to the blood vessel wall and migrate
from the blood into the infected tissue to help eliminate the invading bacteria.1
Cellular Receptors
Cellular receptors are protein molecules on the plasma membrane, in the
cytoplasm, or in the nucleus that can recognize and bind with specific smaller
molecules called ligands (from the Latin ligare, “to bind”) (Figure 1-10). The
region of a protein that associates with a ligand is called its binding site. Hormones,
for example, are ligands. Recognition and binding depend on the chemical
configuration of the receptor and its smaller ligand, which must fit together
somewhat like pieces of a jigsaw puzzle (see Chapter 18). Binding selectively to a
protein receptor with high affinity to a ligand depends on formation of weak,
noncovalent interactions—hydrogen bonds, electrostatic attractions, and van der
Waals attractions—and favorable hydrophobic forces.1 Numerous receptors are
found in most cells, and ligand binding to receptors activates or inhibits the
receptor’s associated signaling or biochemical pathway (see p. 12).
FIGURE 1-10 Cellular Receptors. (A) 1, Plasma membrane receptor for a ligand (here, a
hormone molecule) on the surface of an integral protein. A neurotransmitter can exert its effect
on a postsynaptic cell by means of two fundamentally different types of receptor proteins: 2,
channel-linked receptors, and 3, non–channel-linked receptors. Channel-linked receptors are
also known as ligand-gated channels. (B) Example of ligand-receptor interaction. Insulin-like
growth factor 1 (IGF-1) is a ligand and binds to the insulin-like growth factor 1 receptor (IGF-1R).
With binding at the cell membrane the intracellular signaling pathway is activated, causing
translation of new proteins to act as intracellular communicators. This pathway is important for
cancer growth. Researchers are developing pharmacologic strategies to reduce signaling at
and downstream of the insulin-like growth factor 1 receptor (IGF-1R), hoping this will lead to
compounds useful in cancer treatment.
Plasma membrane receptors protrude from or are exposed at the external
surface of the membrane and are important for cellular uptake of ligands (see
Figure 1-10). The ligands that bind with membrane receptors include hormones,
neurotransmitters, antigens, complement components, lipoproteins, infectious
agents, drugs, and metabolites. Many new discoveries concerning the specific
interactions of cellular receptors with their respective ligands have provided a basis
for understanding disease.
Although the chemical nature of ligands and their receptors differs, receptors are
classified based on their location and function. Cellular type determines overall
cellular function, but plasma membrane receptors determine which ligands a cell
will bind with and how the cell will respond to the binding. Specific processes also
control intracellular mechanisms.
Receptors for different drugs are found on the plasma membrane, in the
cytoplasm, and in the nucleus. Membrane receptors have been found for certain
anesthetics, opiates, endorphins, enkephalins, antibiotics, cancer chemotherapeutic
agents, digitalis, and other drugs. Membrane receptors for endorphins, which are
opiate-like peptides isolated from the pituitary gland, are found in large quantities in
pain pathways of the nervous system (see Chapters 13 and 14). With binding to the
receptor, the endorphins (or drugs such as morphine) change the cell’s permeability
to ions, increase the concentration of molecules that regulate intracellular protein
synthesis, and initiate molecular events that modulate pain perception.
Receptors for infectious microorganisms, or antigen receptors, bind bacteria,
viruses, and parasites to the cell membrane. Antigen receptors on white blood cells
(lymphocytes, monocytes, macrophages, granulocytes) recognize and bind with
antigenic microorganisms and activate the immune and inflammatory responses
(see Chapter 6).
Cell-to-Cell Adhesions
Cells are small and squishy, not like bricks. They are enclosed only by a flimsy
membrane, yet the cell depends on the integrity of this membrane for its survival.
How can cells be connected strongly, with their membranes intact, to form a muscle
that can lift this textbook? Plasma membranes not only serve as the outer boundaries
of all cells but also allow groups of cells to be held together robustly, in cell-to-cell
adhesions, to form tissues and organs. Once arranged, cells are linked by three
different means: (1) cell adhesion molecules in the cell’s plasma membrane (see p.
8), (2) the extracellular matrix, and (3) specialized cell junctions.
Extracellular Matrix
Cells can be united by attachment to one another or through the extracellular matrix
(including the basement membrane), which the cells secrete around themselves.
The extracellular matrix is an intricate meshwork of fibrous proteins embedded in
a watery, gel-like substance composed of complex carbohydrates (Figure 1-11). The
matrix is similar to glue; however, it provides a pathway for diffusion of nutrients,
wastes, and other water-soluble substances between the blood and tissue cells.
Interwoven within the matrix are three groups of macromolecules: (1) fibrous
structural proteins, including collagen and elastin; (2) adhesive glycoproteins, such
as fibronectin; and (3) proteoglycans and hyaluronic acid.
1. Collagen forms cablelike fibers or sheets that provide tensile strength or
resistance to longitudinal stress. Collagen breakdown, such as occurs in
osteoarthritis, destroys the fibrils that give cartilage its tensile strength.
2. Elastin is a rubber-like protein fiber most abundant in tissues that must be capable
of stretching and recoiling, such as found in the lungs.
3. Fibronectin, a large glycoprotein, promotes cell adhesion and cell anchorage.
Reduced amounts have been found in certain types of cancerous cells; this allows
cancer cells to travel, or metastasize, to other parts of the body. All of these
macromolecules occur in intercellular junctions and cell surfaces and may assemble
into two different components: interstitial matrix and basement membrane (BM)
(see Figure 1-11).
FIGURE 1-11 Extracellular Matrix. A, Tissues are not just cells but also extracellular space. The
extracellular space is an intricate network of macromolecules called the extracellular matrix
(ECM). The macromolecules that constitute the ECM are secreted locally (by mostly fibroblasts)
and assembled into a meshwork in close association with the surface of the cell that produced
them. Two main classes of macromolecules include proteoglycans, which are bound to
polysaccharide chains called glycosaminoglycans, and fibrous proteins (e.g., collagen, elastin,
fibronectin, and laminin), which have structural and adhesive properties. Together the
proteoglycan molecules form a gel-like ground substance in which the fibrous proteins are
embedded. The gel permits rapid diffusion of nutrients, metabolites, and hormones between the
blood and the tissue cells. Matrix proteins modulate cell-matrix interactions, including normal
tissue remodeling (which can become abnormal, for example, with chronic inflammation).
Disruptions of this balance result in serious diseases such as arthritis, tumor growth, and other
pathologic conditions. B, Scanning electron micrograph of a chick embryo where a portion of
the epithelium has been removed, exposing the curtain-like extracellular matrix. (A, adapted from
Kumar V et al: Robbins and Cotran pathologic basis of disease, ed 9, Philadelphia, 2015, Saunders; B, © Robert L Trelstad; from Gartner LP, Hiatt
JL: Color textbook of histology, ed 3, St Louis, 2006, Saunders/Elsevier.)
The basement membrane is a thin, tough layer of extracellular matrix
(connective tissue) underlying the epithelium of many organs and is also called the
basal lamina (see Figure 1-11, B).
The extracellular matrix is secreted by fibroblasts (“fiber formers”) (Figure 1-
12), local cells that are present in the matrix. The matrix and the cells within it are
known collectively as connective tissue because they interconnect cells to form
tissues and organs. Human connective tissues are enormously varied. They can be
hard and dense, like bone; flexible, like tendons or the dermis of the skin; resilient
and shock absorbing, like cartilage; or soft and transparent, similar to the jelly-like
substance that fills the eye. In all these examples, the majority of the tissue is
composed of extracellular matrix, and the cells that produce the matrix are scattered
within it like raisins in a pudding (see Figure 1-12).
FIGURE 1-12 Fibroblasts in Connective Tissue. This micrograph shows tissue from the cornea
of a rat. The extracellular matrix surrounds the fibroblasts (F). (From Nishida T et al: The extracellular matrix of
animal connective tissues, Invest Ophthalmol Vis Sci 29:1887-1880, 1998.)
The matrix is not just passive scaffolding for cellular attachment but also helps
regulate the function of the cells with which it interacts. The matrix helps regulate
such important functions as cell growth and differentiation.
Specialized Cell Junctions
Cells in direct physical contact with neighboring cells are often interconnected at
specialized plasma membrane regions called cell junctions. Cell junctions are
classified by their function: (1) some hold cells together and form a tight seal (tight
junctions); (2) some provide strong mechanical attachments (adherens junctions,
desmosomes, hemidesmosomes); (3) some provide a special type of chemical
communication (for example, inorganic ions and small water-soluble molecules to
move from the cytosol of one cell to the cytosol of another cell), such as those
causing an electrical wave (gap junctions); and (4) some maintain apico-basal
polarity of individual epithelial cells (tight junctions) (Figure 1-13). Overall, cell
junctions make the epithelium leak-proof and mediate mechanical attachment of one
cell to another, allow communicating tunnels and maintaining cell polarity.
FIGURE 1-13 Junctional Complex. A, Schematic drawing of a belt desmosome between
epithelial cells. This junction, also called the zonula adherens, encircles each of the interacting
cells. The spot desmosomes and hemidesmosomes, like the belt desmosomes, are adhering
junctions. This tight junction is an impermeable junction that holds cells together but seals them
in such a way that molecules cannot leak between them. The gap junction, as a communicating
junction, mediates the passage of small molecules from one interacting cell to the other. B,
Connexons. The connexin gap junction proteins have four transmembrane domains and they
play a vital role in maintaining cell and tissue function and homeostasis. Cells connected by gap
junctions are considered ionically (electrically) and metabolically coupled. Gap junctions
coordinate the activities of adjacent cells; for example, they are important for synchronizing
contractions of heart muscle cells through ionic coupling and for permitting action potentials to
spread rapidly from cell to cell in neural tissues. The reason gap junctions occur in tissues that
are not electrically active is unknown. Although most gap junctions are associated with
junctional complexes, they sometimes exist as independent structures. C, Electron micrograph
of desmosomes. (A and C from Raven PH, Johnson GB: Biology, St Louis, 1992, Mosby; B, adapted from Gartner LP, Hiatt JL: Color
textbook of histology, ed 3, St Louis, 2006, Saunders Elsevier; Sherwood L: Learning, ed 8, Belmont, Calif, 2013, Brooks/Cole CENGAGE.)
Cell junctions can be classified as symmetric and asymmetric. Symmetric
junctions include tight junctions, the belt desmosome (zonula adherens),
desmosomes (macula adherens), and gap junctions (also called intercellular channel
or communicating junctions).13 An asymmetric junction is the hemidesmosome (see
Figure 1-13). Together they form the junctional complex. Desmosomes unite cells
either by forming continuous bands or belts of epithelial sheets or by developing
button-like points of contact. Desmosomes also act as a system of braces to maintain
structural stability. Tight junctions are barriers to diffusion, prevent the movement
of substances through transport proteins in the plasma membrane, and prevent the
leakage of small molecules between the plasma membranes of adjacent cells. Gap
junctions are clusters of communicating tunnels or connexons that allow small ions
and molecules to pass directly from the inside of one cell to the inside of another.
Connexons are hemichannels that extend outward from each of the adjacent plasma
membranes (Figure 1-13, C).
Multiple factors regulate gap junction intercellular communication, including
voltage across the junction, intracellular pH, intracellular Ca++ concentration, and
protein phosphorylation. The most abundant human connexin is connexin 43
(Cx43).14 Investigators recently showed that loss of Cx43 expression in colorectal
tumors is correlated with a shorter cancer-free survival rate.15 This study is the first
evidence that Cx43 acts as a tumor suppressor for colorectal cancer (enhances
apoptosis) and therefore may be an important prognostic marker and target for
therapy.15 Investigators also recently reported that glycyrrhizic acid (GA), a
glycoside of licorice root extracts, may be a strong chemopreventive agent against
carcinogens; induced colon cancer in rats and Cx43 is one target.16 Too much GA
often in humans may lead to hypokalemia and hypertension.17
The junctional complex is a highly permeable part of the plasma membrane. Its
permeability is controlled by a process called gating. Increased levels of
cytoplasmic calcium cause decreased permeability at the junctional complex. Gating
enables uninjured cells to protect themselves from injured neighbors. Calcium is
released from injured cells.
Cellular Communication and Signal
Transduction
Cells need to communicate with each other to maintain a stable internal
environment, or homeostasis; to regulate their growth and division; to oversee
their development and organization into tissues; and to coordinate their functions.
Cells communicate by using hundreds of kinds of signal molecules, for example,
insulin (see Figure 1-10, B). Cells communicate in three main ways: (1) they display
plasma membrane–bound signaling molecules (receptors) that affect the cell itself
and other cells in direct physical contact (Figure 1-14, A); (2) they affect receptor
proteins inside the target cell and the signal molecule has to enter the cell to bind to
them (Figure 1-14, B); and (3) they form protein channels (gap junctions) that
directly coordinate the activities of adjacent cells (Figure 1-14, C). Alterations in
cellular communication affect disease onset and progression. In fact, if a cell cannot
perform gap junctional intercellular communication, normal growth control and
cell differentiation is compromised, thereby favoring cancerous tumor development
(see Chapter 10). (Communication through gap junctions was discussed earlier, and
contact signaling by plasma membrane–bound molecules is discussed on this page
and on p. 15.) Secreted chemical signals involve communication locally and at a
distance. Primary modes of intercellular signaling are contact-dependent, paracrine,
hormonal, neurohormonal, and neurotransmitter. Autocrine stimulation occurs
when the secreting cell targets itself (Figure 1-15).
FIGURE 1-14 Cellular Communication. Three primary ways cells communicate with one
another. (B adapted from Alberts B et al: Molecular biology of the cell, ed 5, New York, 2008, Garland.)
FIGURE 1-15 Primary Modes of Chemical Signaling. Five forms of signaling mediated by
secreted molecules. Hormones, paracrines, neurotransmitters, and neurohormones are all
intercellular messengers that accomplish communication between cells. Autocrines bind to
receptors on the same cell. Not all neurotransmitters act in the strictly synaptic mode shown;
some act in a contact-dependent mode as local chemical mediators that influence multiple
target cells in the area.
Contact-dependent signaling requires cells to be in close membrane-membrane
contact. In paracrine signaling, cells secrete local chemical mediators that are
quickly taken up, destroyed, or immobilized. Paracrine signaling usually involves
different cell types; however, cells also can produce signals to which they alone
respond, called autocrine signaling (see Figure 1-15). For example, cancer cells
use this form of signaling to stimulate their survival and proliferation. The
mediators act only on nearby cells. Hormonal signaling involves specialized
endocrine cells that secrete chemicals called hormones; hormones are released by
one set of cells and travel through the bloodstream to produce a response in other
sets of cells (see Chapter 18). In neurohormonal signaling hormones are released
into the blood by neurosecretory neurons. Like endocrine cells, neurosecretory
neurons release blood-borne chemical messengers, whereas ordinary neurons
secrete short-range neurotransmitters into a small discrete space (i.e., synapse).
Neurons communicate directly with the cells they innervate by releasing chemicals
or neurotransmitters at specialized junctions called chemical synapses; the
neurotransmitter diffuses across the synaptic cleft and acts on the postsynaptic target
cell (see Figure 1-15). Many of these same signaling molecules are receptors used
in hormonal, neurohormonal, and paracrine signaling. Important differences lie in
the speed and selectivity with which the signals are delivered to their targets.1
Plasma membrane receptors belong to one of three classes that are defined by the
signaling (transduction) mechanism used. Table 1-3 summarizes these classes of
receptors. Cells respond to external stimuli by activating a variety of signal
transduction pathways, which are communication pathways, or signaling cascades
(Figure 1-16, C). Signals are passed between cells when a particular type of
molecule is produced by one cell—the signaling cell—and received by another—the
target cell—by means of a receptor protein that recognizes and responds
specifically to the signal molecule (Figure 1-16, A and B). In turn, the signaling
molecules activate a pathway of intracellular protein kinases that results in various
responses, such as grow and reproduce, die, survive, or differentiate (Figure 1-16,
D). If deprived of appropriate signals, most cells undergo a form of cell suicide
known as programmed cell death, or apoptosis (see p. 104).
TABLE 1-3
Classes of Plasma Membrane Receptors
Type of
Receptor
Description
Ion
channel
coupled
Also called transmitter-gated ion channels; involve rapid synaptic signaling between electrically excitable cells. Channels open and close briefly
in response to neurotransmitters, changing ion permeability of plasma membrane of postsynaptic cell.
Enzyme
coupled
Once activated by ligands, function directly as enzymes or associate with enzymes.
G-protein
coupled
Indirectly activate or inactivate plasma membrane enzyme or ion channel; interaction mediated by GTP-binding regulatory protein (G-
protein). May also interact with inositol phospholipids, which are significant in cell signaling, and with molecules involved in inositol-
phospholipid transduction pathway.
FIGURE 1-16 Schematic of a Signal Transduction Pathway. Like a telephone receiver that
converts an electrical signal into a sound signal, a cell converts an extracellular signal, A, into
an intracellular signal, B. C, An extracellular signal molecule (ligand) bonds to a receptor protein
located on the plasma membrane, where it is transduced into an intracellular signal. This
process initiates a signaling cascade that relays the signal into the cell interior, amplifying and
distributing it during transit. Amplification is often achieved by stimulating enzymes. Steps in the
cascade can be modulated by other events in the cell. D, Different cell behaviors rely on multiple
extracellular signals.
Cellular Metabolism
All of the chemical tasks of maintaining essential cellular functions are referred to
as cellular metabolism. The energy-using process of metabolism is called
anabolism (ana = upward), and the energy-releasing process is known as
catabolism (kata = downward). Metabolism provides the cell with the energy it
needs to produce cellular structures.
Dietary proteins, fats, and starches (i.e., carbohydrates) are hydrolyzed in the
intestinal tract into amino acids, fatty acids, and glucose, respectively. These
constituents are then absorbed, circulated, and incorporated into the cell, where they
may be used for various vital cellular processes, including the production of ATP.
The process by which ATP is produced is one example of a series of reactions
called a metabolic pathway. A metabolic pathway involves several steps whose end
products are not always detectable. A key feature of cellular metabolism is the
directing of biochemical reactions by protein catalysts or enzymes. Each enzyme
has a high affinity for a substrate, a specific substance converted to a product of the
reaction.
Role of Adenosine Triphosphate
Best known about ATP is its role as a universal “fuel” inside living cells. This fuel
or energy drives biologic reactions necessary for cells to function. For a cell to
function, it must be able to extract and use the chemical energy in organic
molecules. When 1 mole (mol) of glucose metabolically breaks down in the
presence of oxygen into carbon dioxide and water, 686 kilocalories (kcal) of
chemical energy are released. The chemical energy lost by one molecule is
transferred to the chemical structure of another molecule by an energy-carrying or
energy-transferring molecule, such as ATP. The energy stored in ATP can be used in
various energy-requiring reactions and in the process is generally converted to
adenosine diphosphate (ADP) and inorganic phosphate (Pi). The energy available as
a result of this reaction is about 7 kcal/mol of ATP. The cell uses ATP for muscle
contraction and active transport of molecules across cellular membranes. ATP not
only stores energy but also transfers it from one molecule to another. Energy stored
by carbohydrate, lipid, and protein is catabolized and transferred to ATP.
Emerging understandings are the role of ATP outside cells—as a messenger. In
animal studies, using the newly developed ATP probe, ATP has been measured in
pericellular spaces. New research is clarifying the role of ATP as an extracellular
messenger and its role in many physiologic processes, including inflammation.18-20
Food and Production of Cellular Energy
Catabolism of the proteins, lipids, and polysaccharides found in food can be divided
into the following three phases (Figure 1-17):
Phase 1: Digestion. Large molecules are broken down into smaller subunits:
proteins into amino acids, polysaccharides into simple sugars (i.e.,
monosaccharides), and fats into fatty acids and glycerol. These processes occur
outside the cell and are activated by secreted enzymes.
Phase 2: Glycolysis and oxidation. The most important part of phase 2 is
glycolysis, the splitting of glucose. Glycolysis produces two molecules of ATP
per glucose molecule through oxidation, or the removal and transfer of a pair of
electrons. The total process is called oxidative cellular metabolism and involves
ten biochemical reactions (Figure 1-18).
Phase 3: Citric acid cycle (Krebs cycle, tricarboxylic acid cycle). Most of the ATP
is generated during this final phase, which begins with the citric acid cycle and
ends with oxidative phosphorylation. About two thirds of the total oxidation of
carbon compounds in most cells is accomplished during this phase. The major
end products are carbon dioxide (CO2) and two dinucleotides—reduced
nicotinamide adenine dinucleotide (NADH) and the reduced form of flavin
adenine dinucleotide (FADH2)—both of which transfer their electrons into the
electron-transport chain.
FIGURE 1-17 Three Phases of Catabolism, Which Lead from Food to Waste Products. These
reactions produce adenosine triphosphate (ATP), which is used to power other processes in the
cell.
FIGURE 1-18 Glycolysis. Sugars are important for fuel or energy and they are oxidized in small
steps to carbon dioxide (CO2) and water. Glycolysis is the process for oxidizing sugars or
glucose. Breakdown of glucose. A, Anaerobic catabolism, to lactic acid and little ATP. B, Aerobic
catabolism, to carbon dioxide, water, and lots of ATP. (From Herlihy B: The human body in health and illness, ed 5, St
Louis, 2015, Saunders.)
Oxidative Phosphorylation
Oxidative phosphorylation occurs in the mitochondria and is the mechanism by
which the energy produced from carbohydrates, fats, and proteins is transferred to
ATP. During the breakdown (catabolism) of foods, many reactions involve the
removal of electrons from various intermediates. These reactions generally require
a coenzyme (a nonprotein carrier molecule), such as nicotinamide adenine
dinucleotide (NAD), to transfer the electrons and thus are called transfer reactions.
Molecules of NAD and flavin adenine dinucleotide (FAD) transfer electrons they
have gained from the oxidation of substrates to molecular oxygen, O2. The
electrons from reduced NAD and FAD, NADH and FADH2, respectively, are
transferred to the electron-transport chain on the inner surfaces of the
mitochondria with the release of hydrogen ions. Some carrier molecules are
brightly colored, iron-containing proteins known as cytochromes that accept a pair
of electrons. These electrons eventually combine with molecular oxygen.
If oxygen is not available to the electron-transport chain, ATP will not be formed
by the mitochondria. Instead, an anaerobic (without oxygen) metabolic pathway
synthesizes ATP. This process, called substrate phosphorylation or anaerobic
glycolysis, is linked to the breakdown (glycolysis) of carbohydrate (see Figure 1-
18). Because glycolysis occurs in the cytoplasm of the cell, it provides energy for
cells that lack mitochondria. The reactions in anaerobic glycolysis involve the
conversion of glucose to pyruvic acid (pyruvate) with the simultaneous production
of ATP. With the glycolysis of one molecule of glucose, two ATP molecules and
two molecules of pyruvate are liberated. If oxygen is present, the two molecules of
pyruvate move into the mitochondria, where they enter the citric acid cycle (Figure
1-19).
FIGURE 1-19 What Happens to Pyruvate, the Product of Glycolysis? In the presence of oxygen,
pyruvate is oxidized to acetyl coenzyme A (Acetyl CoA) and enters the citric acid cycle. In the
absence of oxygen, pyruvate instead is reduced, accepting the electrons extracted during
glycolysis and carried by reduced nicotinamide adenine dinucleotide (NADH). When pyruvate is
reduced directly, as it is in muscles, the product is lactic acid. When CO2 is first removed from
pyruvate and the remainder is reduced, as it is in yeasts, the resulting product is ethanol.
If oxygen is absent, pyruvate is converted to lactic acid, which is released into the
extracellular fluid. The conversion of pyruvic acid to lactic acid is reversible;
therefore once oxygen is restored, lactic acid is quickly converted back to either
pyruvic acid or glucose. The anaerobic generation of ATP from glucose through
glycolysis is not as efficient as the aerobic generation process. Adding an oxygen-
requiring stage to the catabolic process (phase 3; see Figure 1-17) provides cells
with a much more powerful method for extracting energy from food molecules.
Membrane Transport: Cellular Intake and
Output
Cell survival and growth depend on the constant exchange of molecules with their
environment. Cells continually import nutrients, fluids, and chemical messengers
from the extracellular environment and expel metabolites, or the products of
metabolism, and end products of lysosomal digestion. Cells also must regulate ions
in their cytosol and organelles. Simple diffusion across the lipid bilayer of the
plasma membrane occurs for such important molecules as O2 and CO2. However,
the majority of molecular transfer depends on specialized membrane transport
proteins that span the lipid bilayer and provide private conduits for select
molecules.1 Membrane transport proteins occur in many forms and are present in all
cell membranes.1 Transport by membrane transport proteins is sometimes called
mediated transport. Most of these transport proteins allow selective passage (for
example, Na+ but not K+ or K+ but not Na+). Each type of cell membrane has its own
transport proteins that determine which solute can pass into and out of the cell or
organelle.1 The two main classes of membrane transport proteins are transporters
and channels. These transport proteins differ in the type of solute—small particles
of dissolved substances—they transport. A transporter is specific, allowing only
those ions that fit the unique binding sites on the protein (Figure 1-20, A). A
transporter undergoes conformational changes to enable membrane transport. A
channel, when open, forms a pore across the lipid bilayer that allows ions and
selective polar organic molecules to diffuse across the membrane (see Figure 1-20,
B). Transport by a channel depends on the size and electrical charge of the molecule.
Some channels are controlled by a gate mechanism that determines which solute can
move into it. Ion channels are responsible for the electrical excitability of nerve and
muscle cells and play a critical role in the membrane potential.
FIGURE 1-20 Inorganic Ions and Small, Polar Organic Molecules Can Cross a Cell Membrane
Through Either a Transporter or a Channel. (Adapted from Alberts B: Essential cell biology, ed 4, New York, 2014, Garland.)
The mechanisms of membrane transport depend on the characteristics of the
substance to be transported. In passive transport, water and small, electrically
uncharged molecules move easily through pores in the plasma membrane’s lipid
bilayer (see Figure 1-20). This process occurs naturally through any semipermeable
barrier. Molecules will easily flow “downhill” from a region of high concentration
to a region of low concentration; this movement is called passive because it does
not require expenditure of energy or a driving force. It is driven by osmosis,
hydrostatic pressure, and diffusion, all of which depend on the laws of physics and
do not require life.
Other molecules are too large to pass through pores or are ligands bound to
receptors on the cell’s plasma membrane. Some of these molecules are moved into
and out of the cell by active transport, which requires life, biologic activity, and
the cell’s expenditure of metabolic energy (see Figure 1-20). Unlike passive
transport, active transport occurs across only living membranes that have to drive
the flow “uphill” by coupling it to an energy source (see p. 21). Movement of a
solute against its concentration gradient occurs by special types of transporters
called pumps (see Figure 1-20). These transporter pumps must harness an energy
source to power the transport process. Energy can come from ATP hydrolysis, a
transmembrane ion gradient, or sunlight (Figure 1-21). The best-known energy
source is the Na+-K+–dependent adenosine triphosphatase (ATPase) pump (see
Figure 1-26). It continuously regulates the cell’s volume by controlling leaks
through pores or protein channels and maintaining the ionic concentration gradients
needed for cellular excitation and membrane conductivity (see p. 24). The
maintenance of intracellular K+ concentrations is required also for enzyme activity,
including enzymes involved in protein synthesis (see Figure 1-21). Large molecules
(macromolecules), along with fluids, are transported by endocytosis (taking in) and
exocytosis (expelling) (see p. 21). Receptor-macromolecule complexes enter the
cell by means of receptor-mediated endocytosis (see p. 24).
FIGURE 1-21 Pumps Carry Out Active Transport in Three Ways. 1, Coupled pumps link the
uphill transport of one solute to the downhill transport of another solute. 2, ATP-driven pumps
drive uphill transport from hydrolysis of ATP. 3, Light-driven pumps are mostly found in bacteria
and use energy from sunlight to drive uphill transport. (Adapted from Alberts B: Essential cell biology, ed 4, New York,
2014, Garland.)
Mediated transport systems can move solute molecules singly or two at a time.
Two molecules can be moved simultaneously in one direction (a process called
symport; for example, sodium-glucose in the digestive tract) or in opposite
directions (called antiport; for example, the sodium-potassium pump in all cells),
or a single molecule can be moved in one direction (called uniport; for example,
glucose) (Figure 1-22).
FIGURE 1-22 Mediated Transport. Illustration shows simultaneous movement of a single solute
molecule in one direction (Uniport), of two different solute molecules in one direction (Symport),
and of two different solute molecules in opposite directions (Antiport).
Electrolytes as Solutes
Body fluids are composed of electrolytes, which are electrically charged and
dissociate into constituent ions when placed in solution, and nonelectrolytes, such as
glucose, urea, and creatinine, which do not dissociate. Electrolytes account for
approximately 95% of the solute molecules in body water. Electrolytes exhibit
polarity by orienting themselves toward the positive or negative pole. Ions with a
positive charge are known as cations and migrate toward the negative pole, or
cathode, if an electrical current is passed through the electrolyte solution. Anions
carry a negative charge and migrate toward the positive pole, or anode, in the
presence of electrical current. Anions and cations are located in both the
intracellular fluid (ICF) and the extracellular fluid (ECF) compartments, although
their concentration depends on their location. (Fluid and electrolyte balance between
body compartments is discussed in Chapter 5.) For example, sodium (Na+) is the
predominant extracellular cation, and potassium (K+) is the principal intracellular
cation. The difference in ICF and ECF concentrations of these ions is important to
the transmission of electrical impulses across the plasma membranes of nerve and
muscle cells.
Electrolytes are measured in milliequivalents per liter (mEq/L) or milligrams per
deciliter (mg/dl). The term milliequivalent indicates the chemical-combining
activity of an ion, which depends on the electrical charge, or valence, of its ions. In
abbreviations, valence is indicated by the number of plus or minus signs. One
milliequivalent of any cation can combine chemically with 1 mEq of any anion: one
monovalent anion will combine with one monovalent cation. Divalent ions combine
more strongly than monovalent ions. To maintain electrochemical balance, one
divalent ion will combine with two monovalent ions (e.g., Ca++ + 2Cl− ⇌ CaCl2).
Passive Transport: Diffusion, Filtration, and Osmosis
Diffusion.
Diffusion is the movement of a solute molecule from an area of greater solute
concentration to an area of lesser solute concentration. This difference in
concentration is known as a concentration gradient. Although particles in a
solution move randomly in any direction, if the concentration of particles in one
part of the solution is greater than that in another part, the particles distribute
themselves evenly throughout the solution. According to the same principle, if the
concentration of particles is greater on one side of a permeable membrane than on
the other side, the particles diffuse spontaneously from the area of greater
concentration to the area of lesser concentration until equilibrium is reached. The
higher the concentration on one side, the greater the diffusion rate.
The diffusion rate is influenced by differences of electrical potential across the
membrane (see p. 24). Because the pores in the lipid bilayer are often lined with
Ca++, other cations (e.g., Na+ and K+) diffuse slowly because they are repelled by
positive charges in the pores.
The rate of diffusion of a substance depends also on its size (diffusion
coefficient) and its lipid solubility (Figure 1-23). Usually, the smaller the molecule
and the more soluble it is in oil, the more hydrophobic or nonpolar it is and the
more rapidly it will diffuse across the bilayer. Oxygen, carbon dioxide, and steroid
hormones (for example, androgens and estrogens) are all nonpolar molecules.
Water-soluble substances, such as glucose and inorganic ions, diffuse very slowly,
whereas uncharged lipophilic (“lipid-loving”) molecules, such as fatty acids and
steroids, diffuse rapidly. Ions and other polar molecules generally diffuse across
cellular membranes more slowly than lipid-soluble substances.
FIGURE 1-23 Passive Diffusion of Solute Molecules Across the Plasma Membrane. Oxygen,
nitrogen, water, urea, glycerol, and carbon dioxide can diffuse readily down the concentration
gradient. Macromolecules are too large to diffuse through pores in the plasma membrane. Ions
may be repelled if the pores contain substances with identical charges. If the pores are lined
with cations, for example, other cations will have difficulty diffusing because the positive
charges will repel one another. Diffusion can still occur, but it occurs more slowly.
Water readily diffuses through biologic membranes because water molecules are
small and uncharged. The dipolar structure of water allows it to rapidly cross the
regions of the bilayer containing the lipid head groups. The lipid head groups
constitute the two outer regions of the lipid bilayer.
Filtration: hydrostatic pressure.
Filtration is the movement of water and solutes through a membrane because of a
greater pushing pressure (force) on one side of the membrane than on the other
side. Hydrostatic pressure is the mechanical force of water pushing against
cellular membranes (Figure 1-24, A). In the vascular system, hydrostatic pressure is
the blood pressure generated in vessels when the heart contracts. Blood reaching the
capillary bed has a hydrostatic pressure of 25 to 30 mm Hg, which is sufficient
force to push water across the thin capillary membranes into the interstitial space.
Hydrostatic pressure is partially balanced by osmotic pressure, whereby water
moving out of the capillaries is partially balanced by osmotic forces that tend to pull
water into the capillaries (Figure 1-24, B). Water that is not osmotically attracted
back into the capillaries moves into the lymph system (see the discussion of Starling
forces in Chapter 5).
FIGURE 1-24 Hydrostatic Pressure and Oncotic Pressure in Plasma. 1, Hydrostatic pressure in
plasma. 2, Oncotic pressure exerted by proteins in the plasma usually tends to pull water into
the circulatory system. 3, Individuals with low protein levels (e.g., starvation) are unable to
maintain a normal oncotic pressure; therefore water is not reabsorbed into the circulation and,
instead, causes body edema.
Osmosis.
Osmosis is the movement of water “down” a concentration gradient—that is, across
a semipermeable membrane from a region of higher water concentration to one of
lower concentration. For osmosis to occur, (1) the membrane must be more
permeable to water than to solutes and (2) the concentration of solutes on one side
of the membrane must be greater than that on the other side so that water moves
more easily. Osmosis is directly related to both hydrostatic pressure and solute
concentration but not to particle size or weight. For example, particles of the plasma
protein albumin are small but are more concentrated in body fluids than the larger
and heavier particles of globulin. Therefore albumin exerts a greater osmotic force
than does globulin.
Osmolality controls the distribution and movement of water between body
compartments. The terms osmolality and osmolarity often are used interchangeably
in reference to osmotic activity, but they define different measurements. Osmolality
measures the number of milliosmoles per kilogram (mOsm/kg) of water, or the
concentration of molecules per weight of water. Osmolarity measures the number
of milliosmoles per liter of solution, or the concentration of molecules per volume
of solution.
In solutions that contain only dissociable substances, such as sodium and
chloride, the difference between the two measurements is negligible. When
considering all the different solutes in plasma (e.g., proteins, glucose, lipids),
however, the difference between osmolality and osmolarity becomes more
significant. Less of plasma’s weight is water, and the overall concentration of
particles is therefore greater. The osmolality will be greater than the osmolarity
because of the smaller proportion of water. Osmolality is thus preferred in human
clinical assessment.
The normal osmolality of body fluids is 280 to 294 mOsm/kg. The osmolalities
of intracellular and extracellular fluids tend to equalize, providing a measure of
body fluid concentration and thus the body’s hydration status. Hydration is affected
also by hydrostatic pressure because the movement of water by osmosis can be
opposed by an equal amount of hydrostatic pressure. The amount of hydrostatic
pressure required to oppose the osmotic movement of water is called the osmotic
pressure of the solution. Factors that determine osmotic pressure are the type and
thickness of the plasma membrane, the size of the molecules, the concentration of
molecules or the concentration gradient, and the solubility of molecules within the
membrane.
Effective osmolality is sustained osmotic activity and depends on the
concentration of solutes remaining on one side of a permeable membrane. If the
solutes penetrate the membrane and equilibrate with the solution on the other side of
the membrane, the osmotic effect will be diminished or lost.
Plasma proteins influence osmolality because they have a negative charge (see
Figure 1-24, B). The principle involved is known as Gibbs-Donnan equilibrium; it
occurs when the fluid in one compartment contains small, diffusible ions, such as
Na+ and chloride (Cl−), together with large, nondiffusible, charged particles, such as
plasma proteins. Because the body tends to maintain an electrical equilibrium, the
nondiffusible protein molecules cause asymmetry in the distribution of small ions.
Anions such as Cl− are thus driven out of the cell or plasma, and cations such as Na+
are attracted to the cell. The protein-containing compartment maintains a state of
electroneutrality, but the osmolality is higher. The overall osmotic effect of
colloids, such as plasma proteins, is called the oncotic pressure, or colloid osmotic
pressure.
Tonicity describes the effective osmolality of a solution. (The terms osmolality
and tonicity may be used interchangeably.) Solutions have relative degrees of
tonicity. An isotonic solution (or isosmotic solution) has the same osmolality or
concentration of particles (285 mOsm) as the ICF or ECF. A hypotonic solution has
a lower concentration and is thus more dilute than body fluids (Figure 1-25). A
hypertonic solution has a concentration of more than 285 to 294 mOsm/kg. The
concept of tonicity is important when correcting water and solute imbalances by
administering different types of replacement solutions (see Figure 1-25) (see
Chapter 5).
Quick Check 1-2
1. What does glycolysis produce?
2. Define membrane transport proteins.
3. What are the differences between passive and active transport?
4. Why do water and small, electrically charged molecules move easily through
pores in the plasma membrane?
FIGURE 1-25 Tonicity. Tonicity is important, especially for red blood cell function. A, Isotonic
solution. B, Hypotonic solution. C, Hypertonic solution. (From W augh A, Grant A: Ross and Wilson anatomy and
physiology in health and illness, ed 12, London, 2012, Churchill Livingstone.)
Active Transport of Na+ and K+
The active transport system for Na+ and K+ is found in virtually all mammalian
cells. The Na+-K+–antiport system (i.e., Na+ moving out of the cell and K+ moving
into the cell) uses the direct energy of ATP to transport these cations. The
transporter protein is ATPase, which requires Na+, K+, and magnesium (Mg++) ions.
The concentration of ATPase in plasma membranes is directly related to Na+-K+–
transport activity. Approximately 60% to 70% of the ATP synthesized by cells,
especially muscle and nerve cells, is used to maintain the Na+-K+–transport system.
Excitable tissues have a high concentration of Na+-K+ ATPase, as do other tissues
that transport significant amounts of Na+. For every ATP molecule hydrolyzed, three
molecules of Na+ are transported out of the cell, whereas only two molecules of K+
move into the cell. The process leads to an electrical potential and is called
electrogenic, with the inside of the cell more negative than the outside. Although the
exact mechanism for this transport is uncertain, it is possible that ATPase induces
the transporter protein to undergo several conformational changes, causing Na+ and
K+ to move short distances (Figure 1-26). The conformational change lowers the
affinity for Na+ and K+ to the ATPase transporter, resulting in the release of the
cations after transport.
FIGURE 1-26 Active Transport and the Sodium-Potassium Pump. 1, Three Na+ ions bind to
sodium-binding sites on the carrier’s inner face. 2, At the same time, an energy-containing
adenosine triphosphate (ATP) molecule produced by the cell’s mitochondria binds to the carrier.
The ATP dissociates, transferring its stored energy to the carrier. 3 and 4, The carrier then
changes shape, releases the three Na+ ions to the outside of the cell, and attracts two
potassium (K+) ions to its potassium-binding sites. 5, The carrier then returns to its original
shape, releasing the two K+ ions and the remnant of the ATP molecule to the inside of the cell.
The carrier is now ready for another pumping cycle.
Table 1-4 summarizes the major mechanisms of transport through pores and
protein transporters in the plasma membranes. Many disease states are caused or
manifested by loss of these membrane transport systems.
TABLE 1-4
Major Transport Systems in Mammalian Cells
Substance Transported Mechanism of Transport* Tissues
Carbohydrates
Glucose Passive: protein channel
Active: symport with Na+
Most tissues
Fructose Active: symport with Na+ Small intestines and renal tubular cells
Passive Intestines and liver
Amino Acids
Amino acid specific transporters Coupled channels Intestines, kidney, and liver
All amino acids except proline Active: symport with Na+ Liver
Specific amino acids Active: group translocation Small intestine
Passive
Other Organic Molecules
Cholic acid, deoxycholic acid, and taurocholic acid Active: symport with Na+ Intestines
Organic anions (e.g., malate, α-ketoglutarate,
glutamate)
Antiport with counter–organic anion Mitochondria of liver cells
ATP-ADP Antiport transport of nucleotides; can be active Mitochondria of liver cells
Inorganic Ions
Na+ Passive Distal renal tubular cells
Na+/H+ Active antiport, proton pump Proximal renal tubular cells and small
intestines
Na+/K+ Active: ATP driven, protein channel Plasma membrane of most cells
Ca++ Active: ATP driven, antiport with Na+ All cells, antiporter in red cells
H+/K+ Active Parietal cells of gastric cells secreting H+
(perhaps other anions) Mediated: antiport (anion transporter–band 3
protein)
Erythrocytes and many other cells
Water Osmosis passive All tissues
*NOTE: The known transport systems are listed here; others have been proposed. Most transport systems
have been studied in only a few tissues and their sites of activity may be more limited than indicated.
ADP, Adenosine diphosphate; ATP, adenosine triphosphate.
Data from Alberts B et al: Molecular biology of the cell, ed 4, New York, 2001, Wiley; Alberts B et al:
Essential cell biology, ed 4, New York, 2014, Garland, Devlin TM, editor: Textbook of biochemistry: with
clinical correlations, ed 3, New York, 1992, Wiley; Raven PH, Johnson GB: Understanding biology, ed 3,
Dubuque, IA, 1995, Brown.
Transport by Vesicle Formation
Endocytosis and Exocytosis
The active transport mechanisms by which the cells move large proteins,
polynucleotides, or polysaccharides (macromolecules) across the plasma
membrane are very different from those that mediate small solute and ion transport.
Transport of macromolecules involves the sequential formation and fusion of
membrane-bound vesicles.
In endocytosis, a section of the plasma membrane enfolds substances from
outside the cell, invaginates (folds inward), and separates from the plasma
membrane, forming a vesicle that moves into the cell (Figure 1-27, A). Two types of
endocytosis are designated based on the size of the vesicle formed. Pinocytosis
(cell drinking) involves the ingestion of fluids, bits of the plasma membrane, and
solute molecules through formation of small vesicles; and phagocytosis (cell
eating) involves the ingestion of large particles, such as bacteria, through formation
of large vesicles (vacuoles).
FIGURE 1-27 Endocytosis and Exocytosis. A, Endocytosis and fusion with lysosome and
exocytosis. B, Electron micrograph of exocytosis. (B from Raven PH, Johnson GB: Biology, ed 5, New York, 1999,
McGraw-Hill.)
Because most cells continually ingest fluid and solutes by pinocytosis, the terms
pinocytosis and endocytosis often are used interchangeably. In pinocytosis, the
vesicle containing fluids, solutes, or both fuses with a lysosome, and lysosomal
enzymes digest the vesicle’s contents for use by the cell. Vesicles that bud from
membranes have a particular protein coat on their cytosolic surface and are called
coated vesicles. The best studied are those that have an outer coat of bristlelike
structures—the protein clathrin. Pinocytosis occurs mainly by the clathrin-coated
pits and vesicles (Figure 1-28). After the coated pits pinch off from the plasma
membrane, they quickly shed their coats and fuse with an endosome. An endosome
is a vesicle pinched off from the plasma membrane from which its contents can be
recycled to the plasma membrane or sent to lysosomes for digestion. In
phagocytosis, the large molecular substances are engulfed by the plasma membrane
and enter the cell so that they can be isolated and destroyed by lysosomal enzymes
(see Chapter 6). Substances that are not degraded by lysosomes are isolated in
residual bodies and released by exocytosis. Both pinocytosis and phagocytosis
require metabolic energy and often involve binding of the substance with plasma
membrane receptors before membrane invagination and fusion with lysosomes in
the cell. New data are revealing that endocytosis has an even larger and more
important role than previously known (Box 1-2).
FIGURE 1-28 Ligand Internalization by Means of Receptor-Mediated Endocytosis. A, The ligand
attaches to its surface receptor (through the bristle coat or clathrin coat) and, through receptor-
mediated endocytosis, enters the cell. The ingested material fuses with a lysosome and is
processed by hydrolytic lysosomal enzymes. Processed molecules can then be transferred to
other cellular components. B, Electron micrograph of a coated pit showing different sizes of
filaments of the cytoskeleton (×82,000). (B from Erlandsen SL, Magney JE: Color atlas of histology, St Louis, 1992, Mosby.)
Box 1-2
The New Endocytic Matrix
An explosion of new data is disclosing a much more involved role for endocytosis
than just a simple way to internalize nutrients and membrane-associated molecules.
These new data show that endocytosis not only is a master organizer of signaling
pathways but also has a major role in managing signals in time and space.
Endocytosis appears to control signaling; therefore it determines the net output of
biochemical pathways. This occurs because endocytosis modulates the presence of
receptors and their ligands as well as effectors at the plasma membrane or at
intermediate stations of the endocytic route. The overall processes and anatomy of
these new functions are sometimes called the “endocytic matrix.” All of these
functions ultimately have a large impact on almost every cellular process,
including the nucleus.
In eukaryotic cells, secretion of macromolecules almost always occurs by
exocytosis (see Figure 1-27). Exocytosis has two main functions: (1) replacement of
portions of the plasma membrane that have been removed by endocytosis and (2)
release of molecules synthesized by the cells into the extracellular matrix.
Receptor-Mediated Endocytosis
The internalization process, called receptor-mediated endocytosis (ligand
internalization), is rapid and enables the cell to ingest large amounts of receptor-
macromolecule complexes in clathrin-coated vesicles without ingesting large
volumes of extracellular fluid (see Figure 1-28). The cellular uptake of cholesterol,
for example, depends on receptor-mediated endocytosis. Additionally, many
essential metabolites (for example, vitamin B12 and iron) depend on receptor-
mediated endocytosis and, unfortunately, the influenza flu virus.
Caveolae
The outer surface of the plasma membrane is dimpled with tiny flask-shaped pits
(cavelike) called caveolae. Caveolae are thought to form from membrane
microdomains or lipid rafts. Caveolae are cholesterol- and glycosphingolipid-rich
microdomains where the protein caveolin is thought to be involved in several
processes, including clathrin-independent endocytosis, cellular cholesterol
regulation and transport, and cellular communication. Many proteins, including a
variety of receptors, cluster in these tiny chambers.
Caveolae are not only uptake vehicles but also important sites for signal
transduction, a tedious process in which extracellular chemical messages or signals
are communicated to the cell’s interior for execution. For example, in vitro evidence
now exists that plasma membrane estrogen receptors can localize in caveolae, and
crosstalk with estradiol facilitates several intracellular biologic actions.21
Movement of Electrical Impulses: Membrane
Potentials
All body cells are electrically polarized, with the inside of the cell more negatively
charged than the outside. The difference in electrical charge, or voltage, is known as
the resting membrane potential and is about −70 to −85 millivolts (mV). The
difference in voltage across the plasma membrane results from the differences in
ionic composition of ICF and ECF. Sodium ions are more concentrated in the ECF,
and potassium ions are in greater concentration in the ICF. The concentration
difference is maintained by the active transport of Na+ and K+ (the sodium-
potassium pump), which transports sodium outward and potassium inward (Figure
1-29). Because the resting plasma membrane is more permeable to K+ than to Na+,
K+ diffuses easily from the ICF to the ECF. Because both sodium and potassium are
cations, the net result is an excess of anions inside the cell, resulting in the resting
membrane potential.
FIGURE 1-29 Sodium-Potassium Pump and Propagation of an Action Potential. A,
Concentration difference of sodium (Na+) and potassium (K+) intracellularly and extracellularly.
The direction of active transport by the sodium-potassium pump is also shown. B, The left
diagram represents the polarized state of a neuronal membrane when at rest. The middle and
right diagrams represent changes in sodium and potassium membrane permeabilities with
depolarization and repolarization.
Nerve and muscle cells are excitable and can change their resting membrane
potential in response to electrochemical stimuli. Changes in resting membrane
potential convey messages from cell to cell. When a nerve or muscle cell receives a
stimulus that exceeds the membrane threshold value, a rapid change occurs in the
resting membrane potential, known as the action potential. The action potential
carries signals along the nerve or muscle cell and conveys information from one
cell to another in a domino-like fashion. Nerve impulses are described in Chapter
13. When a resting cell is stimulated through voltage-regulated channels, the cell
membranes become more permeable to sodium, so a net movement of sodium into
the cell occurs and the membrane potential decreases, or moves forward, from a
negative value (in millivolts) to zero. This decrease is known as depolarization.
The depolarized cell is more positively charged, and its polarity is neutralized.
To generate an action potential and the resulting depolarization, the threshold
potential must be reached. Generally this occurs when the cell has depolarized by
15 to 20 millivolts. When the threshold is reached, the cell will continue to
depolarize with no further stimulation. The sodium gates open, and sodium rushes
into the cell, causing the membrane potential to drop to zero and then become
positive (depolarization). The rapid reversal in polarity results in the action
potential.
During repolarization, the negative polarity of the resting membrane potential is
reestablished. As the voltage-gated sodium channels begin to close, voltage-gated
potassium channels open. Membrane permeability to sodium decreases and
potassium permeability increases, so potassium ions leave the cell. The sodium
gates close, and with the loss of potassium the membrane potential becomes more
negative. The Na+, K+ pump then returns the membrane to the resting potential by
pumping potassium back into the cell and sodium out of the cell.
During most of the action potential, the plasma membrane cannot respond to an
additional stimulus. This time is known as the absolute refractory period and is
related to changes in permeability to sodium. During the latter phase of the action
potential, when permeability to potassium increases, a stronger-than-normal
stimulus can evoke an action potential; this time is known as the relative refractory
period.
When the membrane potential is more negative than normal, the cell is in a
hyperpolarized state (less excitable: decreased K+ levels within the cell). A
stronger-than-normal stimulus is then required to reach the threshold potential and
generate an action potential. When the membrane potential is more positive than
normal, the cell is in a hypopolarized state (more excitable than normal: increased
K+ levels within the cell) and a weaker-than-normal stimulus is required to reach the
threshold potential. Changes in the intracellular and extracellular concentrations of
ions or a change in membrane permeability can cause these alterations in membrane
excitability.
Quick Check 1-3
1. Identify examples of molecules transported in one direction (symport) and
opposite directions (antiport).
2. If oxygen is no longer available to make ATP, what happens to the transport of
Na+?
3. Define the differences between pinocytosis, phagocytosis, and receptor-mediated
endocytosis.
Cellular Reproduction: the Cell Cycle
Human cells are subject to wear and tear, and most do not last for the lifetime of the
individual. In most tissues, new cells are created as fast as old cells die. Cellular
reproduction is therefore necessary for the maintenance of life. Reproduction of
gametes (sperm and egg cells) occurs through a process called meiosis, described in
Chapter 2. The reproduction, or division, of other body cells (somatic cells)
involves two sequential phases—mitosis, or nuclear division, and cytokinesis, or
cytoplasmic division. Before a cell can divide, however, it must double its mass and
duplicate all its contents. Separation for division occurs during the growth phase,
called interphase. The alternation between mitosis and interphase in all tissues with
cellular turnover is known as the cell cycle.
The four designated phases of the cell cycle (Figure 1-30) are (1) the S phase (S
= synthesis), in which DNA is synthesized in the cell nucleus; (2) the G2 phase (G =
gap), in which RNA and protein synthesis occurs, namely, the period between the
completion of DNA synthesis and the next phase (M); (3) the M phase (M =
mitosis), which includes both nuclear and cytoplasmic division; and (4) the G1
phase, which is the period between the M phase and the start of DNA synthesis.
FIGURE 1-30 Interphase and the Phases of Mitosis. A, The G1/S checkpoint is to “check” for
cell size, nutrients, growth factors, and DNA damage. See text for resting phases. The G2/M
checkpoint checks for cell size and DNA replication. B, The orderly progression through the
phases of the cell cycle is regulated by cyclins (so called because levels rise and fall) and
cyclin-dependent protein kinases (CDKs) and their inhibitors. When cyclins are complexed with
CDKs, cell cycle events are triggered.
Phases of Mitosis and Cytokinesis
Interphase (the G1, S, and G2 phases) is the longest phase of the cell cycle. During
interphase, the chromatin consists of very long, slender rods jumbled together in the
nucleus. Late in interphase, strands of chromatin (the substance that gives the
nucleus its granular appearance) begin to coil, causing shortening and thickening.
The M phase of the cell cycle, mitosis and cytokinesis, begins with prophase, the
first appearance of chromosomes. As the phase proceeds, each chromosome is seen
as two identical halves called chromatids, which lie together and are attached by a
spindle site called a centromere. (The two chromatids of each chromosome, which
are genetically identical, are sometimes called sister chromatids.) The nuclear
membrane, which surrounds the nucleus, disappears. Spindle fibers are
microtubules formed in the cytoplasm. They radiate from two centrioles located at
opposite poles of the cell and pull the chromosomes to opposite sides of the cell,
beginning metaphase. Next, the centromeres become aligned in the middle of the
spindle, which is called the equatorial plate (or metaphase plate) of the cell. In this
stage, chromosomes are easiest to observe microscopically because they are highly
condensed and arranged in a relatively organized fashion.
Anaphase begins when the centromeres split and the sister chromatids are pulled
apart. The spindle fibers shorten, causing the sister chromatids to be pulled,
centromere first, toward opposite sides of the cell. When the sister chromatids are
separated, each is considered to be a chromosome. Thus the cell has 92
chromosomes during this stage. By the end of anaphase, there are 46 chromosomes
lying at each side of the cell. Barring mitotic errors, each of the 2 groups of 46
chromosomes is identical to the original 46 chromosomes present at the start of the
cell cycle.
During telophase, the final stage, a new nuclear membrane is formed around
each group of 46 chromosomes, the spindle fibers disappear, and the chromosomes
begin to uncoil. Cytokinesis causes the cytoplasm to divide into almost equal parts
during this phase. At the end of telophase, two identical diploid cells, called
daughter cells, have been formed from the original cell.
Rates of Cellular Division
Although the complete cell cycle lasts 12 to 24 hours, about 1 hour is required for
the four stages of mitosis and cytokinesis. All types of cells undergo mitosis during
formation of the embryo, but many adult cells—such as nerve cells, lens cells of the
eye, and muscle cells—lose their ability to replicate and divide. The cells of other
tissues, particularly epithelial cells (e.g., cells of the intestine, lung, or skin), divide
continuously and rapidly, completing the entire cell cycle in less than 10 hours.
The difference between cells that divide slowly and cells that divide rapidly is the
length of time spent in the G1 phase of the cell cycle. Once the S phase begins,
however, progression through mitosis takes a relatively constant amount of time.
The mechanisms that control cell division depend on the integrity of genetic,
epigenetic (heritable changes in genome function that occur without alterations in
the DNA sequence; see Chapter 3), and protein growth factors. Protein growth
factors govern the proliferation of different cell types. Individual cells are members
of a complex cellular society in which survival of the entire organism is key—not
survival or proliferation of just the individual cells. When a need arises for new
cells, as in repair of injured cells, previously nondividing cells must be triggered
rapidly to reenter the cell cycle. With continual wear and tear, the cell birth rate and
the cell death rate must be kept in balance.
Growth Factors
Growth factors, also called cytokines, are peptides (protein fractions) that transmit
signals within and between cells. They have a major role in the regulation of tissue
growth and development (Table 1-5). Having nutrients is not enough for a cell to
proliferate; it must also receive stimulatory chemical signals (growth factors) from
other cells, usually its neighbors or the surrounding supporting tissue called
stroma. These signals act to overcome intracellular braking mechanisms that tend
to restrain cell growth and block progress through the cell cycle (Figure 1-31).
TABLE 1-5
Examples of Growth Factors and Their Actions
Growth Factor Physiologic Actions
Platelet-derived growth factor (PDGF) Stimulates proliferation of connective tissue cells and neuroglial cells
Epidermal growth factor (EGF) Stimulates proliferation of epidermal cells and other cell types
Insulin-like growth factor 1 (IGF-1) Collaborates with PDGF and EGF; stimulates proliferation of fat cells and connective tissue cells
Vascular endothelial growth factor (VEGF) Mediates functions of endothelial cells; proliferation, migration, invasion, survival, and permeability
Insulin-like growth factor 2 (IGF-2) Collaborates with PDGF and EGF; stimulates or inhibits response of most cells to other growth factors;
regulates differentiation of some cell types (e.g., cartilage)
Transforming growth factor-beta (TGF-β;
multiple subtypes)
Stimulates or inhibits response of most cells to other growth factors; regulates differentiation of some cell
types (e.g., cartilage)
Fibroblast growth factor (FGF; multiple
subtypes)
Stimulates proliferation of fibroblasts, endothelial cells, myoblasts, and other multiple subtypes
Interleukin-2 (IL-2) Stimulates proliferation of T lymphocytes
Nerve growth factor (NGF) Promotes axon growth and survival of sympathetic and some sensory and central nervous system (CNS)
neurons
Hematopoietic cell growth factors (IL-3, GM-
CSF, G-CSF, erythropoietin)
Promote proliferation of blood cells
G-CSF, Granulocyte colony-stimulating factor; GM-CSF, granulocyte-macrophage colony-stimulating
factor.
FIGURE 1-31 How Growth Factors Stimulate Cell Proliferation. A, Resting cell. With the absence
of growth factors, the retinoblastoma (Rb) protein is not phosphorylated; thus it holds the gene
regulatory proteins in an inactive state. The gene regulatory proteins are required to stimulate
the transcription of genes needed for cell proliferation. B, Proliferating cell. Growth factors bind
to the cell surface receptors and activate intracellular signaling pathways, leading to activation
of intracellular proteins. These intracellular proteins phosphorylate and thereby inactivate the
Rb protein. The gene regulatory proteins are now free to activate the transcription of genes,
leading to cell proliferation.
An example of a brake that regulates cell proliferation is the retinoblastoma
(Rb) protein, first identified through studies of a rare childhood eye tumor called
retinoblastoma, in which the Rb protein is missing or defective. The Rb protein is
abundant in the nucleus of all vertebrate cells. It binds to gene regulatory proteins,
preventing them from stimulating the transcription of genes required for cell
proliferation (see Figure 1-31). Extracellular signals, such as growth factors,
activate intracellular signaling pathways that inactivate the Rb protein, leading to
cell proliferation.
Different types of cells require different growth factors; for example, platelet-
derived growth factor (PDGF) stimulates the production of connective tissue cells.
Table 1-5 summarizes the most significant growth factors. Evidence shows that
some growth factors also regulate other cellular processes, such as cellular
differentiation. In addition to growth factors that stimulate cellular processes, there
are factors that inhibit these processes; these factors are not well understood. Cells
that are starved of growth factors come to a halt after mitosis and enter the arrested
(resting) (G0) state of the cell cycle (see p. 25 for cell cycle).
1
Tissues
Cells of one or more types are organized into tissues, and different types of tissues
compose organs. Finally, organs are integrated to perform complex functions as
tracts or systems.
All cells are in contact with a network of extracellular macromolecules known as
the extracellular matrix (see p. 10). This matrix not only holds cells and tissues
together but also provides an organized latticework within which cells can migrate
and interact with one another.
Tissue Formation
To form tissues, cells must exhibit intercellular recognition and communication,
adhesion, and memory. Specialized cells sense their environment through signals,
such as growth factors, from other cells. This type of communication ensures that
new cells are produced only when and where they are required. Different cell types
have different adhesion molecules in their plasma membranes, sticking selectively
to other cells of the same type. They can also adhere to extracellular matrix
components. Because cells are tiny and squishy and enclosed by a flimsy membrane,
it is remarkable that they form a strong human being. Strength can occur because of
the extracellular matrix and the strength of the cytoskeleton with cell-cell adhesions
to neighboring cells. Cells have memory because of specialized patterns of gene
expression evoked by signals that acted during embryonic development. Memory
allows cells to autonomously preserve their distinctive character and pass it on to
their progeny.1
Fully specialized or terminally differentiated cells that are lost are regenerated
from proliferating precursor cells. These precursor cells have been derived from a
smaller number of stem cells.1 Stem cells are cells with the potential to develop into
many different cell types during early development and growth. In many tissues,
stem cells serve as an internal repair and maintenance system, dividing indefinitely.
These cells can maintain themselves over very long periods of time, called self-
renewal, and can generate all the differentiated cell types of the tissue or
multipotency. This stem cell–driven tissue renewal is very evident in the epithelial
lining of the intestine, stomach, blood cells, and skin, which is continuously exposed
to environmental factors. A class of extracellular signaling proteins, known as Wnt
signals, sustain tissue renewal and enable tissue to be continuously replenished and
maintained over a lifetime.22 When a stem cell divides, each daughter cell has a
choice: it can remain as a stem cell or it can follow a pathway that results in terminal
differentiation (Figure 1-32).
FIGURE 1-32 Properties of Stem Cell Systems. A, Stem cells have three characteristics: self-
renewal, proliferation, and differentiation into mature cells. Stem cells are housed in niches
consisting of stromal cells that provide factors for their maintenance. Stem cells of the embryo
can give rise to cell precursors that generate all the tissues of the body. This property defines
stem cells as multipotent. Stem cells are difficult to identify anatomically. Their identification is
based on specific cell surface markers (cell surface antigens recognized by specific
monoclonal antibodies) and on the lineage they generate following transplantation. B, Wnt
signaling fuels tissue renewal. (A, from Kierszenbaum A: Histology and cell biology: an introduction to pathology, ed 3, St Louis,
2012, Elsevier. B, from Clevers H, et al: Science 346(3), 2014.)
Types of Tissues
The four basic types of tissues are nerve, epithelial, connective, and muscle. The
structure and function of these four types underlie the structure and function of each
organ system. Neural tissue is composed of highly specialized cells called neurons,
which receive and transmit electrical impulses rapidly across junctions called
synapses (see Figure 13-1). Different types of neurons have special characteristics
that depend on their distribution and function within the nervous system. Epithelial,
connective, and muscle tissues are summarized in Tables 1-6, 1-7, and 1-8,
respectively.
Quick Check 1-4
1. What is the cell cycle?
2. Discuss the five types of intracellular communication.
3. Why is the extracellular matrix important for tissue cells?
TABLE 1-6
Characteristics of Epithelial Tissues
Simple Squamous Epithelium
Structure
Single layer of cells
Location and Function
Lining of blood vessels leads to diffusion and filtration
Lining of pulmonary alveoli (air sacs) leads to separation of blood from fluids in tissues
Bowman’s capsule (kidney), where it filters substances from blood, forming urine
Simple Squamous Epithelial Cell. Photomicrograph of simple squamous epithelial cell in parietal wall of Bowman’s capsule in kidney. (From Erlandsen SL,
Magney JE: Color atlas of histology, St Louis, 1992, Mosby.)
Stratified Squamous Epithelium
Structure
Two or more layers, depending on location, with cells closest to basement membrane tending to be cuboidal
Location and Function
Epidermis of skin and linings of mouth, pharynx, esophagus, and anus provide protection and secretion
Cornified Stratified Squamous Epithelium. Diagram of stratified squamous epithelium of skin. (Copyright Ed Reschke. Used with permission.)
Transitional Epithelium
Structure
Vary in shape from cuboidal to squamous depending on whether basal cells of bladder are columnar or are composed of many layers; when bladder is full
and stretched, the cells flatten and stretch like squamous cells
Location and Function
Linings of urinary bladder and other hollow structures stretch, allowing expansion of the hollow organs
Stratified Squamous Transitional Epithelium. Photomicrograph of stratified squamous transitional epithelium of urinary bladder. (Copyright Ed
Reschke. Used with permission.)
Simple Cuboidal Epithelium
Structure
Simple cuboidal cells; rarely stratified (layered)
Location and Function
Glands (e.g., thyroid, sweat, salivary) and parts of the kidney tubules and outer covering of ovary secrete fluids
Simple Cuboidal Epithelium. Photomicrograph of simple cuboidal epithelium of pancreatic duct. (From Erlandsen SL, Magney JE: Color atlas of
histology, St Louis, 1992, Mosby.)
Simple Columnar Epithelium
Structure
Large amounts of cytoplasm and cellular organelles
Location and Function
Ducts of many glands and lining of digestive tract allow secretion and absorption from stomach to anus
Simple Columnar Epithelium. Photomicrograph of simple columnar epithelium. (Copyright Ed Reschke. Used with permission.)
Ciliated Simple Columnar Epithelium
Structure
Same as simple columnar epithelium but ciliated
Location and Function
Linings of bronchi of lungs, nasal cavity, and oviducts allow secretion, absorption, and propulsion of fluids and particles
Stratified Columnar Epithelium
Structure
Small and rounded basement membrane (columnar cells do not touch basement membrane)
Location and Function
Linings of epiglottis, part of pharynx, anus, and male urethra provide protection
Pseudostratified Ciliated Columnar Epithelium
Structure
All cells in contact with basement membrane
Nuclei found at different levels within cell, giving stratified appearance
Free surface often ciliated
Location and Function
Linings of large ducts of some glands (parotid, salivary), male urethra, respiratory passages, and eustachian tubes of ears transport substances
Pseudostratified Ciliated Columnar Epithelium. Photomicrograph of pseudostratified ciliated columnar epithelium of trachea. (Copyright Robert L.
Calentine. Used with permission.)
TABLE 1-7
Connective Tissues
Loose or Areolar Tissue
Structure
Unorganized; spaces between fibers
Most fibers collagenous, some elastic and reticular
Includes many types of cells (fibroblasts and macrophages most common) and large amount of intercellular fluid
Location and Function
Attaches skin to underlying tissue; holds organs in place by filling spaces between them; supports blood vessels
Intercellular fluid transports nutrients and waste products
Fluid accumulation causes swelling (edema)
Loose Areolar Connective Tissue. (Copyright Ed Reschke. Used with permission.)
Dense Irregular Tissue
Structure
Dense, compact, and areolar tissue, with fewer cells and greater number of closely woven collagenous fibers than in loose tissue
Location and Function
Dermis layer of skin; acts as protective barrier
Dense, Irregular Connective Tissue. (Copyright Ed Reschke. Used with permission.)
Dense, Regular (White Fibrous) Tissue
Structure
Collagenous fibers and some elastic fibers, tightly packed into parallel bundles, with only fibroblast cells
Location and Function
Forms strong tendons of muscle, ligaments of joints, some fibrous membranes, and fascia that surrounds organs and muscles
Dense, Regular (W hite Fibrous) Connective Tissue. (Copyright Phototake. Used with permission.)
Elastic Tissue
Structure
Elastic fibers, some collagenous fibers, fibroblasts
Location and Function
Lends strength and elasticity to walls of arteries, trachea, vocal cords, and other structures
Elastic Connective Tissue. (From Erlandsen SL, Magney JE: Color atlas of histology, St Louis, 1992, Mosby.)
Adipose Tissue
Structure
Fat cells dispersed in loose tissues; each cell containing a large droplet of fat flattens nucleus and forces cytoplasm into a ring around cell’s periphery
Location and Function
Stores fat, which provides padding and protection
Adipose Tissue. A, Fat storage areas—distribution of fat in male and female bodies. B, Photomicrograph of adipose tissue. (A from Thibodeau GA,
Patton KT: Anatomy & physiology, ed 6, St Louis, 2007, Mosby; B copyright Ed Reschke. Used with permission.)
Cartilage (Hyaline, Elastic, Fibrous)
Structure
Collagenous fibers embedded in a firm matrix (chondrin); no blood supply
Location and Function
Gives form, support, and flexibility to joints, trachea, nose, ear, vertebral disks, embryonic skeleton, and many internal structures
Cartilage. A, Hyaline cartilage. B, Elastic cartilage. C, Fibrous cartilage. (A and C copyright Robert L. Calentine; B copyright Ed Reshke. Used with
permission.)
Bone
Structure
Rigid connective tissue consisting of cells, fibers, ground substances, and minerals
Location and Function
Lends skeleton rigidity and strength
Bone. (Copyright Phototake. Used with permission.)
Special Connective Tissues
Plasma
Structure
Fluid
Location and Function
Serves as matrix for blood cells
Macrophages in Tissue, Reticuloendothelial, or Macrophage System
Structure
Scattered macrophages (phagocytes) called Kupffer cells (in liver), alveolar macrophages (in lungs), microglia (in central nervous system)
Location and Function
Facilitate inflammatory response and carry out phagocytosis in loose connective, lymphatic, digestive, medullary (bone marrow), splenic, adrenal, and
pituitary tissues
TABLE 1-8
Muscle Tissues
Skeletal (Striated) Muscle
Structure Characteristics of Cells
Long, cylindrical cells that extend throughout length of muscles
Striated myofibrils (proteins)
Many nuclei on periphery
Location and Function
Attached to bones directly or by tendons and provide voluntary movement of skeleton and maintenance of posture
Skeletal (Striated) Muscle. (From Thibodeau GA, Patton KT: Anatomy & physiology, ed 6, St Louis, 2007, Mosby.)
Cardiac Muscle
Structure Characteristics of Cells
Branching networks throughout muscle tissue
Striated myofibrils
Location and Function
Cells attached end-to-end at intercalated disks with tissue forming walls of heart (myocardium) to provide involuntary pumping action of heart
Cardiac Muscle. (Copyright Ed Reschke. Used with permission.)
Smooth (Visceral) Muscle
Structure Characteristics of Cells
Long spindles that taper to a point
Absence of striated myofibrils
Location and Function
Walls of hollow internal structures, such as digestive tract and blood vessels (viscera), provide voluntary and involuntary contractions that move substances
through hollow structures
Smooth (Visceral) Muscle. (Copyright Phototake. Used with permission.)
Did You Understand?
Cellular Functions
1. Cells become specialized through the process of differentiation or maturation.
2. The eight specialized cellular functions are movement, conductivity, metabolic
absorption, secretion, excretion, respiration, reproduction, and communication.
Structure and Function of Cellular Components
1. The eukaryotic cell consists of three general components: the plasma membrane,
the cytoplasm, and the intracellular organelles.
2. The nucleus is the largest membrane-bound organelle and is found usually in the
cell’s center. The chief functions of the nucleus are cell division and control of
genetic information.
3. Cytoplasm, or the cytoplasmic matrix, is an aqueous solution (cytosol) that fills
the space between the nucleus and the plasma membrane.
4. The organelles are suspended in the cytoplasm and are enclosed in biologic
membranes.
5. The endoplasmic reticulum is a network of tubular channels (cisternae) that
extend throughout the outer nuclear membrane. It specializes in the synthesis and
transport of protein and lipid components of most of the organelles.
6. The Golgi complex is a network of smooth membranes and vesicles located near
the nucleus. The Golgi complex is responsible for processing and packaging
proteins into secretory vesicles that break away from the Golgi complex and
migrate to a variety of intracellular and extracellular destinations, including the
plasma membrane.
7. Lysosomes are saclike structures that originate from the Golgi complex and
contain digestive enzymes. These enzymes are responsible for digesting most
cellular substances to their basic form, such as amino acids, fatty acids, and
carbohydrates (sugars).
8. Cellular injury leads to a release of the lysosomal enzymes, causing cellular self-
digestion.
9. Peroxisomes are similar to lysosomes but contain several enzymes that either
produce or use hydrogen peroxide.
10. Mitochondria contain the metabolic machinery necessary for cellular energy
metabolism. The enzymes of the respiratory chain (electron-transport chain), found
in the inner membrane of the mitochondria, generate most of the cell’s ATP.
11. The cytoskeleton is the “bone and muscle” of the cell. The internal skeleton is
composed of a network of protein filaments, including microtubules and actin
filaments (microfilaments).
12. The plasma membrane encloses the cell and, by controlling the movement of
substances across it, exerts a powerful influence on metabolic pathways. Principles
of membrane structure are being overhauled.
13. Proteins are the major workhorses of the cell. Membrane proteins, like other
proteins, are synthesized by the ribosome and then make their way, called
trafficking, to different locations in the cell. Trafficking places unique demands on
membrane proteins for folding, translocation, and stability. Misfolded proteins are
emerging as an important cause of disease.
14. Protein regulation in a cell is called protein homeostasis and is defined by the
proteostasis network. This network is composed of ribosomes (makers),
chaperones (helpers), and protein breakdown or proteolytic systems. Malfunction of
these systems is associated with disease.
15. Carbohydrates contained within the plasma membrane are generally bound to
membrane proteins (glycoproteins) and lipids (glycolipids).
16. Protein receptors (recognition units) on the plasma membrane enable the cell to
interact with other cells and with extracellular substances.
17. Membrane functions are determined largely by proteins. These functions include
recognition by protein receptors and transport of substances into and out of the cell.
Cell-to-Cell Adhesions
1. Cell-to-cell adhesions are formed on plasma membranes, thereby allowing the
formation of tissues and organs. Cells are held together by three different means:
(1) the extracellular membrane, (2) cell adhesion molecules in the cell’s plasma
membrane, and (3) specialized cell junctions.
2. The extracellular matrix includes three groups of macromolecules: (1) fibrous
structural proteins (collagen and elastin), (2) adhesive glycoproteins, and (3)
proteoglycans and hyaluronic acid. The matrix helps regulate cell growth,
movement, and differentiation.
3. The basement membrane is a tough layer of extracellular matrix underlying the
epithelium of many organs; it is also called the basal lamina.
4. Cell junctions can be classified as symmetric and asymmetric. Symmetric
junctions include tight junctions, the belt desmosome, desmosomes, and gap
junctions. An asymmetric junction is the hemidesmosome.
Cellular Communication and Signal Transduction
1. Cells communicate in three main ways: (1) they form protein channels (gap
junctions); (2) they display receptors that affect intracellular processes or other
cells in direct physical contact; and (3) they use receptor proteins inside the target
cell.
2. Primary modes of intercellular signaling include contact-dependent, paracrine,
hormonal, neurohormonal, and neurotransmitter.
3. Signal transduction involves signals or instructions from extracellular chemical
messengers that are conveyed to the cell’s interior for execution. If deprived of
appropriate signals, cells undergo a form of cell suicide known as programmed cell
death or apoptosis.
Cellular Metabolism
1. The chemical tasks of maintaining essential cellular functions are referred to as
cellular metabolism. Anabolism is the energy-using process of metabolism,
whereas catabolism is the energy-releasing process.
2. Adenosine triphosphate (ATP) functions as an energy-transferring molecule. It is
fuel for cell survival. Energy is stored by molecules of carbohydrate, lipid, and
protein, which, when catabolized, transfers energy to ATP.
3. Oxidative phosphorylation occurs in the mitochondria and is the mechanism by
which the energy produced from carbohydrates, fats, and proteins is transferred to
ATP.
Membrane Transport: Cellular Intake and Output
1. Cell survival and growth depends on the constant exchange of molecules with
their environment. The two main classes of membrane transport proteins are
transporters and channels. The majority of molecular transfer depends on
specialized membrane transport proteins.
2. Water and small, electrically uncharged molecules move through pores in the
plasma membrane’s lipid bilayer in the process called passive transport.
3. Passive transport does not require the expenditure of energy; rather, it is driven
by the physical effect of osmosis, hydrostatic pressure, and diffusion.
4. Larger molecules and molecular complexes are moved into the cell by active
transport, which requires the cell to expend energy (by means of ATP).
5. The largest molecules (macromolecules) and fluids are transported by the
processes of endocytosis (ingestion) and exocytosis (expulsion). Endocytosis, or
vesicle formation, is when the substance to be transported is engulfed by a segment
of the plasma membrane, forming a vesicle that moves into the cell.
6. Pinocytosis is a type of endocytosis in which fluids and solute molecules are
ingested through formation of small vesicles.
7. Phagocytosis is a type of endocytosis in which large particles, such as bacteria,
are ingested through formation of large vesicles, called vacuoles.
8. In receptor-mediated endocytosis, the plasma membrane receptors are clustered,
along with bristlelike structures, in specialized areas called coated pits.
9. Endocytosis occurs when coated pits invaginate, internalizing ligand-receptor
complexes in coated vesicles.
10. Inside the cell, lysosomal enzymes process and digest material ingested by
endocytosis.
11. Two types of solutes exist in body fluids: electrolytes and nonelectrolytes.
Electrolytes are electrically charged and dissociate into constituent ions when
placed in solution. Nonelectrolytes do not dissociate when placed in solution.
12. Diffusion is the passive movement of a solute from an area of higher solute
concentration to an area of lower solute concentration.
13. Filtration is the measurement of water and solutes through a membrane because
of a greater pushing pressure.
14. Hydrostatic pressure is the mechanical force of water pushing against cellular
membranes.
15. Osmosis is the movement of water across a semipermeable membrane from a
region of lower solute concentration to a region of higher solute concentration.
16. The amount of hydrostatic pressure required to oppose the osmotic movement
of water is called the osmotic pressure of solution.
17. The overall osmotic effect of colloids, such as plasma proteins, is called the
oncotic pressure or colloid osmotic pressure.
18. All body cells are electrically polarized, with the inside of the cell more
negatively charged than the outside. The difference in voltage across the plasma
membrane is the resting membrane potential.
19. When an excitable (nerve or muscle) cell receives an electrochemical stimulus,
cations enter the cell and cause a rapid change in the resting membrane potential
known as the action potential. The action potential “moves” along the cell’s plasma
membrane and is transmitted to an adjacent cell. This is how electrochemical signals
convey information from cell to cell.
Cellular Reproduction: The Cell Cycle
1. Cellular reproduction in body tissues involves mitosis (nuclear division) and
cytokinesis (cytoplasmic division).
2. Only mature cells are capable of division. Maturation occurs during a stage of
cellular life called interphase (growth phase).
3. The cell cycle is the reproductive process that begins after interphase in all tissues
with cellular turnover. There are four phases of the cell cycle: (1) the S phase,
during which DNA synthesis takes place in the cell nucleus; (2) the G2 phase, the
period between the completion of DNA synthesis and the next phase (M); (3) the M
phase, which involves both nuclear (mitotic) and cytoplasmic (cytokinetic) division;
and (4) the G1 phase (growth phase), after which the cycle begins again.
4. The M phase (mitosis) involves four stages: prophase, metaphase, anaphase, and
telophase.
5. The mechanisms that control cellular division depend on the integrity of genetic,
epigenetic, and protein growth factors.
Tissues
1. Cells of one or more types are organized into tissues, and different types of
tissues compose organs. Organs are organized to function as tracts or systems.
2. Three key factors that maintain the cellular organization of tissues are (1)
recognition and cell communication, (2) selective cell-to-cell adhesion, and (3)
memory.
3. Fully specialized or terminally differentiated cells that are lost are generated
from proliferating precursor cells and they, in turn, have been derived from a
smaller number of stem cells. Stem cells are cells with the potential to develop into
many different cell types during early development and growth. In many tissues,
stem cells serve as an internal repair and maintenance system dividing indefinitely.
These cells can maintain themselves over very long periods of time, called self-
renewal, and can generate all the differentiated cell types of the tissue or
multipotency.
4. Tissue cells are linked at cell junctions, which are specialized regions on their
plasma membranes. Cell junctions attach adjacent cells and allow small molecules
to pass between them.
5. The four basic types of tissues are epithelial, muscle, nerve, and connective
tissues.
6. Neural tissue is composed of highly specialized cells called neurons that receive
and transmit electrical impulses rapidly across junctions called synapses.
7. Epithelial tissue covers most internal and external surfaces of the body. The
functions of epithelial tissue include protection, absorption, secretion, and
excretion.
8. Connective tissue binds various tissues and organs together, supporting them in
their locations and serving as storage sites for excess nutrients.
9. Muscle tissue is composed of long, thin, highly contractile cells or fibers called
myocytes. Muscle tissue that is attached to bones enables voluntary movement.
Muscle tissue in internal organs enables involuntary movement, such as the
heartbeat.
Key Terms
Absolute refractory period, 25
Action potential, 24
Active transport, 17
Amphipathic, 3
Anabolism, 14
Anaphase, 26
Anion, 19
Antiport, 18
Arrested (resting) (G0) state, 27
Autocrine signaling, 12
Basal lamina, 10
Basement membrane, 10
Binding site, 9
Catabolism, 14
Cation, 19
Caveolae, 24
Cell adhesion molecule (CAM), 8
Cell cortex, 8
Cell cycle, 25
Cell junction, 11
Cell polarity, 2
Cell-to-cell adhesion, 10
Cellular metabolism, 14
Cellular receptor, 9
Centromere, 26
Channel, 17
Chemical synapse, 12
Chromatid, 26
Chromatin, 26
Citric acid cycle (Krebs cycle, tricarboxylic acid cycle), 16
Clathrin, 22
Coated vesicle, 22
Collagen, 10
Concentration gradient, 19
Connective tissue, 10
Connexon, 12
Contact-dependent signaling, 12
Cytokinesis, 25
Cytoplasm, 2
Cytoplasmic matrix, 2
Cytosol, 2
Daughter cell, 26
Depolarization, 24
Desmosome, 12
Differentiation, 1
Diffusion, 19
Digestion, 16
Effective osmolality, 20
Elastin, 10
Electrolyte, 18
Electron-transport chain, 16
Endocytosis, 22
Endosome, 22
Equatorial plate (metaphase plate), 26
ER stress, 8
Eukaryote, 1
Exocytosis, 22
Extracellular matrix, 10
Fibroblast, 10
Fibronectin, 10
Filtration, 19
G1 phase, 26
G2 phase, 25
Gap junction, 12
Gating, 12
Glycocalyx, 9
Glycolipid, 3
Glycolysis, 16
Glycoprotein, 3
Growth factor (cytokine), 26
Homeostasis, 12
Hormonal signaling, 12
Hydrostatic pressure, 19
Hyperpolarized state, 25
Hypopolarized state, 25
Integral membrane protein, 7
Interphase, 25
Ions, 7
Junctional complex, 12
Ligand, 9
Lipid bilayer, 2
M phase, 25
Macromolecule, 10
Mediated transport, 17
Membrane lipid raft (MLR), 5
Membrane transport protein, 17
Metabolic pathway, 16
Metaphase, 26
Mitosis, 25
Multipotency, 27
Neurohormonal signaling, 12
Neurotransmitter, 12
Nuclear envelope, 2
Nuclear pores, 2
Nucleolus, 2
Nucleus, 2
Oncotic pressure (colloid osmotic pressure), 20
Organelle, 2
Osmolality, 19
Osmolarity, 19
Osmosis, 19
Osmotic pressure, 20
Oxidation, 16
Oxidative phosphorylation, 16
Paracrine signaling, 12
Passive transport, 17
Peripheral membrane protein, 7
Phagocytosis, 22
Phospholipid, 5
Pinocytosis, 22
Plasma membrane (plasmalemma), 2
Plasma membrane receptor, 9
Platelet-derived growth factor (PDGF), 27
Polarity, 19
Polypeptide, 5
Posttranslational modification (PTM), 5
Prokaryote, 1
Prophase, 26
Protein, 5
Proteolytic, 9
Proteome, 7
Proteomic, 7
Receptor protein, 12
Receptor-mediated endocytosis (ligand internalization), 24
Relative refractory period, 25
Repolarization, 25
Resting membrane potential, 24
Retinoblastoma (Rb) protein, 26
Self-renewal, 27
S phase, 25
Signal transduction pathway, 12
Signaling cell, 12
Solute, 17
Spindle fiber, 26
Stem cell, 27
Stroma, 26
Substrate, 16
Substrate phosphorylation (anaerobic glycolysis), 16
Symport, 18
Target cell, 12
Telophase, 26
Terminally differentiated, 27
Threshold potential, 24
Tight junction, 12
Tonicity, 20
Transfer reaction, 16
Transmembrane protein, 7
Transporter, 17
Unfolded-protein response, 8
Uniport, 18
Valence, 19
Wnt signals, 27
References
1. Alberts B. Essential cell biology. ed 4. Garland: New York; 2014.
2. Simons K, Sampaio JL. Membrane organization and lipid rafts. Cold Spring
Harb Perspect Biol. 2011;3(10):a004697.
3. Contreras FX, et al. Specificity of intramembrane protein-lipid interactions.
Cold Spring Harb Perspec Biol. 2011;3(6) [pii a004705].
4. Head BP, et al. Interaction of membrane/lipid rafts with the cytoskeleton:
impact on signaling and function: membrane/lipid rafts, mediators of
cytoskeletal arrangement and cell signaling. Biochim Biophys Acta.
2014;1838(2):532–545.
5. Karnovsky MJ, et al. The concept of lipid domains in membranes. J Cell
Biol. 1982;94:1–6.
6. Ribert D, Cossart P. Pathogen-mediated postranslational modification: a re-
emerging field. Cell. 2010;143:694–702.
7. Vinothkumar KR, Henderson R. Structure of membrane proteins. Q Rev
Biophysics. 2010;43(1):65–158.
8. Cogliati S, et al. Mitochondrial cristae shape determines respiratory chain
supercomplexes assembly and respiratory efficiency. Cell.
2013;155(1):160–171.
9. Daum B, et al. Age-dependent dissociation of ATP synthase dimers and loss
of inner-membrane cristae in mitochondria. Proc Natl Acad Sci U S A.
2013;110(38):15301–15306.
10. Friedman JR, Nunnari J. Mitochondrial form and function. Nature.
2014;505:335–343.
11. Amm I, et al. Protein quality control and elimination of protein waste: the
role of the ubiquitin-proteosome system. Biochim Biophys Acta.
2014;1843:182–196.
12. Lindquist SL, Kelly JW. Chemical and biological approaches for adapting
proteostasis to ameliorate protein misfolding and aggregation diseases:
progress and prognosis. Cold Spring Harb Perspect Biol. 2011;3(12).
13. Kierszenbaum AL, Tres LT. Histology and cell biology: an introduction to
pathology. ed 3. Elsevier: St Louis; 2011.
14. Xu Q, et al. Gating of connexin 43 gap junctions by a cytoplasmic loop
calmodulin binding domain. Am J Physiol Cell Physiol.
2012;302(10):C1548–C1556.
15. Sirnes S, et al. Connexin43 acts as a colorectal tumor suppressor and
predicts disease outcome. Int J Cancer. 2012;131(3):570–581.
16. Khan R, et al. Glycyrrhizic acid suppresses the development of precancerous
lesions via regulating the hyperproliferation, inflammation, angiogenesis
ad apoptosis in the colon of Wistar rats. PLoS One. 2013;8(2):e56020.
17. Zhang MZ, et al. Inhibition of 11β hydroxysteroid dehydrogenase type II
selectively blocks the tumor COX-2 pathway and suppresses colon
carcinogenesis in mice and humans. J Clin Invest. 2009;119:876–885.
18. Burnstock G. Physiology and pathophysiology in purinergic
neurotransmission. Physiol Rev. 2007;87(2):659–797.
19. Falzoni S, et al. Detecting adenosine triphosphate in the pericellular space.
Interface Focus. 2013;3(3):20120101.
20. Nurse CA, Piskuric NA. Signal processing at mammalian carotid body
chemoreceptors. Semin Cell Dev Biol. 2012;24(1):22–30.
21. Chaudhri RA, et al. Role of ERα36 in membrane-associated signaling by
estrogen. Steroids. 2014;81:74–80.
22. Clevers H, et al. Stem cell signaling. An integral program for tissue renewal
and regeneration: Wnt signaling and stem cell control. Science.
2014;346(6205):1248012.
2
Genes and Genetic Diseases
Lynn B. Jorde
CHAPTER OUTLINE
DNA, RNA, and Proteins: Heredity at the Molecular Level, 38
Definitions, 38
From Genes to Proteins, 39
Chromosomes, 42
Chromosome Aberrations and Associated
Diseases, 42
Elements of Formal Genetics, 49
Phenotype and Genotype, 49
Dominance and Recessiveness, 49
Transmission of Genetic Diseases, 49
Autosomal Dominant Inheritance, 50
Autosomal Recessive Inheritance, 52
X-Linked Inheritance, 54
Linkage Analysis and Gene Mapping, 56
Classic Pedigree Analysis, 56
Complete Human Gene Map: Prospects and
Benefits, 57
Multifactorial Inheritance, 57
Genetics occupies a central position in the entire study of biology. An understanding
of genetics is essential to study human, animal, plant, or microbial life. Genetics is
the study of biologic inheritance. In the nineteenth century, microscopic studies of
cells led scientists to suspect the nucleus of the cell contained the important
mechanisms of inheritance. Scientists found chromatin, the substance giving the
nucleus a granular appearance, is observable in nondividing cells. Just before the
cell divides, the chromatin condenses to form discrete, dark-staining organelles,
which are called chromosomes. (Cell division is discussed in Chapter 1.) With the
rediscovery of Mendel’s important breeding experiments at the turn of the twentieth
century, it soon became apparent the chromosomes contained genes, the basic units
of inheritance (Figure 2-1).
FIGURE 2-1 Successive Enlargements from a Human to the Genetic Material.
The primary constituent of chromatin is deoxyribonucleic acid (DNA). Genes are
composed of sequences of DNA. By serving as the blueprints of proteins in the
body, genes ultimately influence all aspects of body structure and function. Humans
have approximately 20,000 protein-coding genes and an additional 9000 to 10,000
genes that encode various types of RNA (see below) that are not translated into
proteins. An error in one of these genes often leads to a recognizable genetic
disease.
To date, more than 20,000 genetic traits and diseases have been identified and
cataloged. As infectious diseases continue to be more effectively controlled, the
proportion of beds in pediatric hospitals occupied by children with genetic diseases
has risen. In addition to children, many common diseases primarily affecting adults,
such as hypertension, coronary heart disease, diabetes, and cancer, are now known
to have important genetic components.
Great progress is being made in the diagnosis of genetic diseases and in the
understanding of genetic mechanisms underlying them. With the huge strides being
made in molecular genetics, “gene therapy”—the utilization of normal genes to
correct genetic disease—has begun.
DNA, RNA, and Proteins: Heredity at the
Molecular Level
Definitions
Composition and Structure of DNA
Genes are composed of DNA, which has three basic components: the five-carbon
monosaccharide deoxyribose; a phosphate molecule; and four types of nitrogenous
bases. Two of the bases, cytosine and thymine, are single carbon-nitrogen rings
called pyrimidines. The other two bases, adenine and guanine, are double carbon-
nitrogen rings called purines. The four bases are commonly represented by their
first letters: A (adenine), C (cytosine), T (thymine), and G (guanine).
Watson and Crick demonstrated how these molecules are physically assembled as
DNA, proposing the double-helix model, in which DNA appears like a twisted
ladder with chemical bonds as its rungs (Figure 2-2). The two sides of the ladder
consist of deoxyribose and phosphate molecules, united by strong phosphodiester
bonds. Projecting from each side of the ladder, at regular intervals, are the
nitrogenous bases. The base projecting from one side is bound to the base
projecting from the other by a weak hydrogen bond. Therefore the nitrogenous
bases form the rungs of the ladder; adenine pairs with thymine, and guanine pairs
with cytosine. Each DNA subunit—consisting of one deoxyribose molecule, one
phosphate group, and one base—is called a nucleotide.
FIGURE 2-2 Watson-Crick Model of the DNA Molecule. The DNA structure illustrated here is
based on that published by James Watson (photograph, left) and Francis Crick (photograph,
right) in 1953. Note that each side of the DNA molecule consists of alternating sugar and
phosphate groups. Each sugar group is bonded to the opposing sugar group by a pair of
nitrogenous bases (adenine-thymine or cytosine-guanine). The sequence of these pairs
constitutes a genetic code that determines the structure and function of a cell. (Illustration from Herlihy
B: The human body in health and illness, ed 5, St Louis, 2015, Saunders.)
DNA as the Genetic Code
DNA directs the synthesis of all the body’s proteins. Proteins are composed of one
or more polypeptides (intermediate protein compounds), which in turn consist of
sequences of amino acids. The body contains 20 different types of amino acids; they
are specified by the 4 nitrogenous bases. To specify (code for) 20 different amino
acids with only 4 bases, different combinations of bases, occurring in groups of 3
(triplets), are used. These triplets of bases are known as codons. Each codon
specifies a single amino acid in a corresponding protein. Because there are 64 (4 ×
4 × 4) possible codons but only 20 amino acids, there are many cases in which
several codons correspond to the same amino acid.
The genetic code is universal: all living organisms use precisely the same DNA
codes to specify proteins except for mitochondria, the cytoplasmic organelles in
which cellular respiration takes place (see Chapter 1)—they have their own
extranuclear DNA. Several codons of mitochondrial DNA encode different amino
acids, as compared to the same nuclear DNA codons.
Replication of DNA
DNA replication consists of breaking the weak hydrogen bonds between the bases,
leaving a single strand with each base unpaired (Figure 2-3). The consistent pairing
of adenine with thymine and of guanine with cytosine, known as complementary
base pairing, is the key to accurate replication. The unpaired base attracts a free
nucleotide only if the nucleotide has the proper complementary base. When
replication is complete, a new double-stranded molecule identical to the original is
formed. The single strand is said to be a template, or molecule on which a
complementary molecule is built, and is the basis for synthesizing the new double
strand.
FIGURE 2-3 Replication of DNA. The two chains of the double helix separate and each chain
serves as the template for a new complementary chain. (From Herlihy B: The human body in health and illness, ed 5,
St Louis, 2015, Saunders.)
Several different proteins are involved in DNA replication. The most important
of these proteins is an enzyme known as DNA polymerase. This enzyme travels
along the single DNA strand, adding the correct nucleotides to the free end of the
new strand and checking to ensure that its base is actually complementary to the
template base. This mechanism of DNA proofreading substantially enhances the
accuracy of DNA replication.
Mutation
A mutation is any inherited alteration of genetic material. One type of mutation is
the base pair substitution, in which one base pair replaces another. This
replacement can result in a change in the amino acid sequence. However, because of
the redundancy of the genetic code, many of these mutations do not change the
amino acid sequence and thus have no consequence. Such mutations are called silent
mutations. Base pair substitutions altering amino acids consist of two basic types:
missense mutations, which produce a change (i.e., the “sense”) in a single amino
acid; and nonsense mutations, which produce one of the three stop codons (UAA,
UAG, or UGA) in the messenger RNA (mRNA) (Figure 2-4). Missense mutations
(see Figure 2-4, A) produce a single amino acid change, whereas nonsense
mutations (see Figure 2-4, B) produce a premature stop codon in the mRNA. Stop
codons terminate translation of the polypeptide.
FIGURE 2-4 Base Pair Substitution. Missense mutations (A) produce a single amino acid
change, whereas nonsense mutations (B) produce a stop codon in the mRNA. Stop codons
terminate translation of the polypeptide. (From Jorde L et al: Medical genetics, ed 4, St Louis, 2010, Mosby.)
The frameshift mutation involves the insertion or deletion of one or more base
pairs of the DNA molecule. As Figure 2-5 shows, these mutations change the entire
“reading frame” of the DNA sequence because the deletion or insertion is not a
multiple of three base pairs (the number of base pairs in a codon). Frameshift
mutations can thus greatly alter the amino acid sequence. (In-frame insertions or
deletions, in which a multiple of three bases is inserted or lost, tend to have less
severe disease consequences than do frameshift mutations.)
FIGURE 2-5 Frameshift Mutations. Frameshift mutations result from the addition or deletion of
a number of bases that is not a multiple of 3. This mutation alters all of the codons downstream
from the site of insertion or deletion. (From Jorde L et al: Medical genetics, ed 4, St Louis, 2010, Mosby.)
Agents known as mutagens increase the frequency of mutations. Examples
include radiation and chemicals such as nitrogen mustard, vinyl chloride, alkylating
agents, formaldehyde, and sodium nitrite.
Mutations are rare events. The rate of spontaneous mutations (those occurring
in the absence of exposure to known mutagens) in humans is about 10−4 to 10−7 per
gene per generation. This rate varies from one gene to another. Some DNA
sequences have particularly high mutation rates and are known as mutational hot
spots.
From Genes to Proteins
DNA is formed and replicated in the cell nucleus, but protein synthesis takes place in
the cytoplasm. The DNA code is transported from nucleus to cytoplasm, and
subsequent protein is formed through two basic processes: transcription and
translation. These processes are mediated by ribonucleic acid (RNA), which is
chemically similar to DNA except the sugar molecule is ribose rather than
deoxyribose, and uracil rather than thymine is one of the four bases. The other bases
of RNA, as in DNA, are adenine, cytosine, and guanine. Uracil is structurally
similar to thymine, so it also can pair with adenine. Whereas DNA usually occurs as
a double strand, RNA usually occurs as a single strand.
Transcription
In transcription, RNA is synthesized from a DNA template, forming messenger
RNA (mRNA). RNA polymerase binds to a promoter site, a sequence of DNA that
specifies the beginning of a gene. RNA polymerase then separates a portion of the
DNA, exposing unattached DNA bases. One DNA strand then provides the template
for the sequence of mRNA nucleotides.
The sequence of bases in the mRNA is thus complementary to the template strand,
and except for the presence of uracil instead of thymine, the mRNA sequence is
identical to that of the other DNA strand. Transcription continues until a
termination sequence, codons that act as signals for the termination of protein
synthesis, is reached. Then the RNA polymerase detaches from the DNA, and the
transcribed mRNA is freed to move out of the nucleus and into the cytoplasm
(Figures 2-6 and 2-7).
FIGURE 2-6 General Scheme of Ribonucleic Acid (RNA) Transcription. In transcription of
messenger RNA (mRNA), a DNA molecule “unzips” in the region of the gene to be transcribed.
RNA nucleotides already present in the nucleus temporarily attach themselves to exposed DNA
bases along one strand of the unzipped DNA molecule according to the principle of
complementary pairing. As the RNA nucleotides attach to the exposed DNA, they bind to each
other and form a chainlike RNA strand called a messenger RNA (mRNA) molecule. Notice that
the new mRNA strand is an exact copy of the base sequence on the opposite side of the DNA
molecule. As in all metabolic processes, the formation of mRNA is controlled by an enzyme—in
this case, the enzyme is called RNA polymerase. (From Ignatavicius DD, W orkman LD: Medical-surgical nursing, ed 6, St
Louis, 2010, Saunders.)
FIGURE 2-7 Protein Synthesis. The site of transcription is the nucleus and the site of
translation is the cytoplasm. See the text for details.
Gene Splicing
When the mRNA is first transcribed from the DNA template, it reflects exactly the
base sequence of the DNA. In eukaryotes, many RNA sequences are removed by
nuclear enzymes, and the remaining sequences are spliced together to form the
functional mRNA that migrates to the cytoplasm. The excised sequences are called
introns (intervening sequences), and the sequences that are left to code for proteins
are called exons.
Translation
In translation, RNA directs the synthesis of a polypeptide (see Figure 2-7),
interacting with transfer RNA (tRNA), a cloverleaf-shaped strand of about 80
nucleotides. The tRNA molecule has a site where an amino acid attaches. The three-
nucleotide sequence at the opposite side of the cloverleaf is called the anticodon. It
undergoes complementary base pairing with an appropriate codon in the mRNA,
which specifies the sequence of amino acids through tRNA.
The site of actual protein synthesis is in the ribosome, which consists of
approximately equal parts of protein and ribosomal RNA (rRNA). During
translation, the ribosome first binds to an initiation site on the mRNA sequence and
then binds to its surface, so that base pairing can occur between tRNA and mRNA.
The ribosome then moves along the mRNA sequence, processing each codon and
translating an amino acid by way of the interaction of mRNA and tRNA.
The ribosome provides an enzyme that catalyzes the formation of covalent
peptide bonds between the adjacent amino acids, resulting in a growing polypeptide.
When the ribosome arrives at a termination signal on the mRNA sequence,
translation and polypeptide formation cease; the mRNA, ribosome, and polypeptide
separate from one another; and the polypeptide is released into the cytoplasm to
perform its required function.
Chromosomes
Human cells can be categorized into gametes (sperm and egg cells) and somatic
cells, which include all cells other than gametes. Each somatic cell nucleus has 46
chromosomes in 23 pairs (Figure 2-8). These are diploid cells, and the individual’s
father and mother each donate one chromosome per pair. New somatic cells are
formed through mitosis and cytokinesis. Gametes are haploid cells: they have only
1 member of each chromosome pair, for a total of 23 chromosomes. Haploid cells
are formed from diploid cells by meiosis (Figure 2-9).
FIGURE 2-8 From Molecular Parts to the Whole Somatic Cell.
FIGURE 2-9 Phases of Meiosis and Comparison to Mitosis. (From Jorde LB et al: Medical genetics, ed 4, St Louis,
2010, Mosby.)
In 22 of the 23 chromosome pairs, the 2 members of each pair are virtually
identical in microscopic appearance: thus they are homologous (Figure 2-10, B).
These 22 chromosome pairs are homologous in both males and females and are
termed autosomes. The remaining pair of chromosomes, the sex chromosomes,
consists of two homologous X chromosomes in females and a nonhomologous
pair, X and Y, in males.
FIGURE 2-10 Karyotype of Chromosomes. A, Human karyotype. B, Homologous chromosomes
and sister chromatids. (From Raven PH et al: Biology, ed 8, New York, 2008, McGraw-Hill.)
Figure 2-10, A, illustrates a metaphase spread, which is a photograph of the
chromosomes as they appear in the nucleus of a somatic cell during metaphase.
(Chromosomes are easiest to visualize during this stage of mitosis.) In Figure 2-10,
A, the chromosomes are arranged according to size, with the homologous
chromosomes paired. The 22 autosomes are numbered according to length, with
chromosome 1 being the longest and chromosome 22 the shortest. A karyotype, or
karyogram, is an ordered display of chromosomes. Some natural variation in
relative chromosome length can be expected from person to person, so it is not
always possible to distinguish each chromosome by its length. Therefore the
position of the centromere (region of DNA responsible for movement of the
replicated chromosomes into the two daughter cells during mitosis and meiosis)
also is used to classify chromosomes (Figures 2-10, B and 2-11).
FIGURE 2-11 Structure of Chromosomes. A, Human chromosomes 2, 5, and 13. Each is
replicated and consists of two chromatids. Chromosome 2 is a metacentric chromosome
because the centromere is close to the middle; chromosome 5 is submetacentric because the
centromere is set off from the middle; chromosome 13 is acrocentric because the centromere
is at or very near the end. B, During mitosis, the centromere divides and the chromosomes
move to opposite poles of the cell. At the time of centromere division, the chromatids are
designated as chromosomes.
The chromosomes in Figure 2-10 were stained with Giemsa stain, resulting in
distinctive chromosome bands. These form various patterns in the different
chromosomes so that each chromosome can be distinguished easily. Using banding
techniques, researchers can number chromosomes and study individual variations.
Missing or duplicated portions of chromosomes, which often result in serious
diseases, also are readily identified. More recently, techniques have been devised
permitting each chromosome to be visualized with a different color.
Chromosome Aberrations and Associated Diseases
Chromosome abnormalities are the leading known cause of intellectual disability
and miscarriage. Estimates indicate that a major chromosome aberration occurs in
at least 1 in 12 conceptions. Most of these fetuses do not survive to term; about 50%
of all recovered first-trimester spontaneous abortuses have major chromosome
aberrations.1 The number of live births affected by these abnormalities is, however,
significant; approximately 1 in 150 has a major diagnosable chromosome
abnormality.1
Polyploidy
Cells with a multiple of the normal number of chromosomes are euploid cells
(Greek eu = good or true). Because normal gametes are haploid and most normal
somatic cells are diploid, they are both euploid forms. When a euploid cell has
more than the diploid number of chromosomes, it is said to be a polyploid cell.
Several types of body tissues, including some liver, bronchial, and epithelial tissues,
are normally polyploid. A zygote that has three copies of each chromosome, rather
than the usual two, has a form of polyploidy called triploidy. Nearly all triploid
fetuses are spontaneously aborted or stillborn. The prevalence of triploidy among
live births is approximately 1 in 10,000. Tetraploidy, a condition in which euploid
cells have 92 chromosomes, has been found primarily in early abortuses, although
occasionally affected infants have been born alive. Like triploid infants, however,
they do not survive. Triploidy and tetraploidy are relatively common conditions,
accounting for approximately 10% of all known miscarriages.2
Aneuploidy
A cell that does not contain a multiple of 23 chromosomes is an aneuploid cell. A
cell containing three copies of one chromosome is said to be trisomic (a condition
termed trisomy) and is aneuploid. Monosomy, the presence of only one copy of a
given chromosome in a diploid cell, is the other common form of aneuploidy.
Among the autosomes, monosomy of any chromosome is lethal, but newborns with
trisomy of chromosomes 13, 18, 21, or X can survive. This difference illustrates an
important principle: in general, loss of chromosome material has more serious
consequences than duplication of chromosome material.
Aneuploidy of the sex chromosomes is less serious than that of the autosomes.
Very little genetic material—only about 40 genes—is located on the Y chromosome.
For the X chromosome, inactivation of extra chromosomes (see p. 54) largely
diminishes their effect. A zygote bearing no X chromosome, however, will not
survive.
Aneuploidy is usually the result of nondisjunction, an error in which
homologous chromosomes or sister chromatids fail to separate normally during
meiosis or mitosis (Figure 2-12). Nondisjunction produces some gametes that have
two copies of a given chromosome and others that have no copies of the
chromosome. When such gametes unite with normal haploid gametes, the resulting
zygote is monosomic or trisomic for that chromosome. Occasionally, a cell can be
monosomic or trisomic for more than one chromosome.
FIGURE 2-12 Nondisjunction. Nondisjunction causes aneuploidy when chromosomes or sister
chromatids fail to divide properly. (From Jorde LB et al: Medical genetics, ed 4, St Louis, 2010, Mosby.)
Autosomal aneuploidy.
Trisomy can occur for any chromosome, but fetuses with other trisomies of
chromosomes (other than 13, 18, 21, or X) do not survive to term. Trisomy 16, for
example, is the most common trisomy among abortuses, but it is not seen in live
births.3
Partial trisomy, in which only an extra portion of a chromosome is present in
each cell, can occur also. The consequences of partial trisomies are not as severe as
those of complete trisomies. Trisomies may occur in only some cells of the body.
Individuals thus affected are said to be chromosomal mosaics, meaning that the
body has two or more different cell lines, each of which has a different karyotype.
Mosaics are often formed by early mitotic nondisjunction occurring in one
embryonic cell but not in others.
The best-known example of aneuploidy in an autosome is trisomy of
chromosome 21, which causes Down syndrome (named after J. Langdon Down,
who first described the syndrome in 1866). Down syndrome is seen in
approximately 1 in 800 to 1 in 1000 live births;4 its principal features are shown and
outlined in Figure 2-13 and Table 2-1.
FIGURE 2-13 Child with Down Syndrome. (Courtesy Drs. A. Olney and M. MacDonald, University of Nebraska Medical Center,
Omaha, Neb.)
TABLE 2-1
Characteristics of Various Chromosome Disorders
Disease/Disorder Features
Down Syndrome
Trisomy of Chromosome 21
IQ Usually ranges from 20 to 70 (intellectual disability)
Male/female
findings
Virtually all males are sterile; some females can reproduce
Face Distinctive: low nasal bridge, epicanthal folds, protruding tongue, low-set ears
Musculoskeletal
system
Poor muscle tone (hypotonia), short stature
Systemic disorders Congenital heart disease (one third to half of cases), reduced ability to fight respiratory tract infections, increased susceptibility to
leukemia—overall reduced survival rate; by age 40 years usually develop symptoms similar to those of Alzheimer disease
Mortality About 75% of fetuses with Down syndrome abort spontaneously or are stillborn; 20% of infants die before age 10 years; those who
live beyond 10 years have life expectancy of about 60 years
Causative factors 97% caused by nondisjunction during formation of one of parent’s gametes or during early embryonic development; 3% result from
translocations; in 95% of cases, nondisjunction occurs when mother’s egg cell is formed; remainder involve paternal nondisjunction;
1% are mosaics—these have a large number of normal cells, and effects of trisomic cells are attenuated and symptoms are generally less
severe
Turner Syndrome
(45,X) Monosomy of X Chromosome
IQ Not considered to be intellectually disabled, although some impairment of spatial and mathematical reasoning ability is found
Male/female
findings
Found only in females
Musculoskeletal
system
Short stature common, characteristic webbing of neck, widely spaced nipples, reduced carrying angle at elbow
Systemic disorders Coarctation (narrowing) of aorta, edema of feet in newborns, usually sterile and have gonadal streaks rather than ovaries; streaks are
sometimes susceptible to cancer
Mortality About 15-20% of spontaneous abortions with chromosome abnormalities have this karyotype, most common single-chromosome
aberration; highly lethal during gestation, only about 0.5% of these conceptions survive to term
Causative factors 75% inherit X chromosome from mother, thus caused by meiotic error in father; frequency low compared with other sex chromosome
aneuploidies (1 : 5000 newborn females); 50% have simple monosomy of X chromosome; remainder have more complex
abnormalities; combinations of 45, X cells with XX or XY cells common
Klinefelter Syndrome
(47,XXY) XXY Condition
IQ Moderate degree of mental impairment may be present
Male/female
findings
Have a male appearance but usually sterile; 50% develop female-like breasts (gynecomastia); occurs in 1 : 1000 male births
Face Voice somewhat high pitched
Systemic disorders Sparse body hair, sterile, small testicles
Causative factors 50% of cases the result of nondisjunction of X chromosomes in mother, frequency rises with increasing maternal age; also involves
XXY and XXXY karyotypes with degree of physical and mental impairment increasing with each added X chromosome; mosaicism
fairly common with most prevalent combination of XXY and XY cells
The risk of having a child with Down syndrome increases greatly with maternal
age. As Figure 2-14 demonstrates, women younger than 30 years have a risk
ranging from about 1 in 1000 births to 1 in 2000 births. The risk begins to rise
substantially after 35 years of age, and reaches 3% to 5% for women older than 45
years. This dramatic increase in risk is caused by the age of maternal egg cells,
which are held in an arrested state of prophase I from the time they are formed in
the female embryo until they are shed in ovulation. Thus an egg cell formed by a
45-year-old woman is itself 45 years old. This long suspended state may allow
defects to accumulate in the cellular proteins responsible for meiosis, leading to
nondisjunction. The risk of Down syndrome, as well as other trisomies, does not
increase with paternal age.4
FIGURE 2-14 Down Syndrome Increases with Maternal Age. Rate is per 1000 live births related
to maternal age.
Sex chromosome aneuploidy.
The incidence of sex chromosome aneuploidies is fairly high. Among live births,
about 1 in 500 males and 1 in 900 females have a form of sex chromosome
aneuploidy.5 Because these conditions are generally less severe than autosomal
aneuploidies, all forms except complete absence of any X chromosome material
allow at least some individuals to survive.
One of the most common sex chromosome aneuploidies, affecting about 1 in
1000 newborn females, is trisomy X. Instead of two X chromosomes, these females
have three X chromosomes in each cell. Most of these females have no overt
physical abnormalities, although sterility, menstrual irregularity, or intellectual
disability is sometimes seen. Some females have four X chromosomes, and they are
more often intellectually disabled. Those with five or more X chromosomes
generally have more severe intellectual disability and various physical defects.
A condition that leads to somewhat more serious problems is the presence of a
single X chromosome and no homologous X or Y chromosome, so that the
individual has a total of 45 chromosomes. The karyotype is usually designated 45,X,
and it causes a set of symptoms known as Turner syndrome (Figure 2-15; see Table
2-1). Individuals with at least two X chromosomes and one Y chromosome in each
cell (47,XXY karyotype) have a disorder known as Klinefelter syndrome (Figure 2-
16; see Table 2-1).
FIGURE 2-15 Turner Syndrome. A, A sex chromosome is missing, and the person’s
chromosomes are 45,X. Characteristic signs are short stature, female genitalia, webbed neck,
shieldlike chest with underdeveloped breasts and widely spaced nipples, and imperfectly
developed ovaries. B, As this karyotype shows, Turner syndrome results from monosomy of sex
chromosomes (genotype XO). (From Patton KT, Thibodeau GA: Anatomy & physiology, ed 8, St Louis, 2013, Mosby. Courtesy
Nancy S. W exler, PhD, Columbia University.)
FIGURE 2-16 Klinefelter Syndrome. This young man exhibits many characteristics of Klinefelter
syndrome: small testes, some development of the breasts, sparse body hair, and long limbs.
This syndrome results from the presence of two or more X chromosomes with one Y
chromosome (genotypes XXY or XXXY, for example). (From Patton KT, Thibodeau GA: Anatomy & physiology, ed 9, St
Louis, 2016, Mosby. Courtesy Nancy S. W exler, PhD, Columbia University.)
Abnormalities of Chromosome Structure
In addition to the loss or gain of whole chromosomes, parts of chromosomes can
be lost or duplicated as gametes are formed, and the arrangement of genes on
chromosomes can be altered. Unlike aneuploidy and polyploidy, these changes
sometimes have no serious consequences for an individual’s health. Some of them
can even remain entirely unnoticed, especially when very small pieces of
chromosomes are involved. Nevertheless, abnormalities of chromosome structure
can also produce serious disease in individuals or their offspring.
During meiosis and mitosis, chromosomes usually maintain their structural
integrity, but chromosome breakage occasionally occurs. Mechanisms exist to
“heal” these breaks and usually repair them perfectly with no damage to the
daughter cell. However, some breaks remain or heal in a way that alters the
chromosome’s structure. The risk of chromosome breakage increases with
exposure to harmful agents called clastogens (e.g., ionizing radiation, viral
infections, or some types of chemicals).
Deletions.
Broken chromosomes and lost DNA cause deletions (Figure 2-17). Usually, a
gamete with a deletion unites with a normal gamete to form a zygote. The zygote
thus has one chromosome with the normal complement of genes and one with some
missing genes. Because many genes can be lost in a deletion, serious consequences
result even though one normal chromosome is present. The most often cited
example of a disease caused by a chromosomal deletion is the cri du chat
syndrome. The term literally means “cry of the cat” and describes the characteristic
cry of the affected child. Other symptoms include low birth weight, severe
intellectual disability, microcephaly (smaller than normal head size), and heart
defects. The disease is caused by a deletion of part of the short arm of chromosome
5.
FIGURE 2-17 Abnormalities of Chromosome Structure. A, Deletion occurs when a
chromosome segment is lost. B, Normal crossing over. C, The generation of duplication and
deletion through unequal crossing over.
Duplications.
A deficiency of genetic material is more harmful than an excess, so duplications
usually have less serious consequences than deletions. For example, a deletion of a
region of chromosome 5 causes cri du chat syndrome, but a duplication of the same
region causes mental retardation but less serious physical defects.
Inversions.
An inversion occurs when two breaks take place on a chromosome, followed by the
reinsertion of the missing fragment at its original site but in inverted order.
Therefore a chromosome symbolized as ABCDEFG might become ABEDCFG
after an inversion.
Unlike deletions and duplications, no loss or gain of genetic material occurs, so
inversions are “balanced” alterations of chromosome structure, and they often have
no apparent physical effect. Some genes are influenced by neighboring genes,
however, and this position effect, a change in a gene’s expression caused by its
position, sometimes results in physical defects in these persons. Inversions can
cause serious problems in the offspring of individuals carrying the inversion
because the inversion can lead to duplications and deletions in the chromosomes
transmitted to the offspring.
Translocations.
The interchange of genetic material between nonhomologous chromosomes is
called translocation. A reciprocal translocation occurs when breaks take place in
two different chromosomes and the material is exchanged (Figure 2-18, A). As with
inversions, the carrier of a reciprocal translocation is usually normal, but his or her
offspring can have duplications and deletions.
FIGURE 2-18 Normal and Abnormal Chromosome Translocation. A, Normal chromosomes and
reciprocal translocation. B, Pairing at meiosis. C, Consequences of translocation in gametes;
unbalanced gametes result in zygotes that are partially trisomic and partially monosomic and
consequently develop abnormally.
A second and clinically more important type of translocation is Robertsonian
translocation. In this disorder, the long arms of two nonhomologous
chromosomes fuse at the centromere, forming a single chromosome. Robertsonian
translocations are confined to chromosomes 13, 14, 15, 21, and 22 because the short
arms of these chromosomes are very small and contain no essential genetic
material. The short arms are usually lost during subsequent cell divisions. Because
the carriers of Robertsonian translocations lose no important genetic material, they
are unaffected although they have only 45 chromosomes in each cell. Their
offspring, however, may have serious monosomies or trisomies. For example, a
common Robertsonian translocation involves the fusion of the long arms of
chromosomes 21 and 14. An offspring who inherits a gamete carrying the fused
chromosome can receive an extra copy of the long arm of chromosome 21 and
develop Down syndrome. Robertsonian translocations are responsible for
approximately 3% to 5% of Down syndrome cases. Parents who carry a
Robertsonian translocation involving chromosome 21 have an increased risk for
producing multiple offspring with Down syndrome.
Fragile sites.
A number of areas on chromosomes develop distinctive breaks and gaps
(observable microscopically) when the cells are cultured. Most of these fragile sites
do not appear to be related to disease. However, one fragile site, located on the long
arm of the X chromosome, is associated with fragile X syndrome. The most
important feature of this syndrome is intellectual disability. With a relatively high
population prevalence (affecting approximately 1 in 4000 males and 1 in 8000
females), fragile X syndrome is the second most common genetic cause of
intellectual disability (after Down syndrome).
In fragile X syndrome, females who inherit the mutation do not necessarily
express the disease condition, but they can pass it on to descendants who do express
it. Ordinarily, a male who inherits a disease gene on the X chromosome expresses
the condition, because he has only one X chromosome. An uncommon feature of
this disease is that about one third of carrier females are affected, although less
severely than males. Unaffected transmitting males have been shown to have more
than about 50 repeated DNA sequences near the beginning of the fragile X gene.
These trinucleotide sequences, which consist of CGG sequences duplicated many
times, cause fragile X syndrome when the number of copies exceeds 200.6 The
number of these repeats can increase from generation to generation. More than 20
other genetic diseases, including Huntington disease and myotonic dystrophy, also
are caused by this mechanism.7
Quick Check 2-1
1. What is the major composition of DNA?
2. Define the terms mutation, autosomes, and sex chromosomes.
3. What is the significance of mRNA?
4. What is the significance of chromosomal translocation?
Elements of Formal Genetics
The mechanisms by which an individual’s set of paired chromosomes produces
traits are the principles of genetic inheritance. Mendel’s work with garden peas first
defined these principles. Later geneticists have refined Mendel’s work to explain
patterns of inheritance for traits and diseases that appear in families.
Analysis of traits that occur with defined, predictable patterns has helped
geneticists to assemble the pieces of the human gene map. Current research focuses
on determining the RNA or protein products of each gene and understanding the
way they contribute to disease. Eventually, diseases and defects caused by single
genes can be traced and therapies to prevent and treat such diseases can be
developed.
Traits caused by single genes are called mendelian traits (after Gregor Mendel).
Each gene occupies a position along a chromosome known as a locus. The genes at
a particular locus can have different forms (i.e., they can be composed of different
nucleotide sequences) called alleles. A locus that has two or more alleles that each
occur with an appreciable frequency in a population is said to be polymorphic (or a
polymorphism).
Because humans are diploid organisms, each chromosome is represented twice,
with one member of the chromosome pair contributed by the father and one by the
mother. At a given locus, an individual has one allele whose origin is paternal and
one whose origin is maternal. When the two alleles are identical, the individual is
homozygous at that locus. When the alleles are not identical, the individual is
heterozygous at that locus.
Phenotype and Genotype
The composition of genes at a given locus is known as the genotype. The outward
appearance of an individual, which is the result of both genotype and environment,
is the phenotype. For example, an infant who is born with an inability to metabolize
the amino acid phenylalanine has the single-gene disorder known as
phenylketonuria (PKU) and thus has the PKU genotype. If the condition is left
untreated, abnormal metabolites of phenylalanine will begin to accumulate in the
infant’s brain and irreversible intellectual disability will occur. Intellectual disability
is thus one aspect of the PKU phenotype. By imposing dietary restrictions to exclude
food that contains phenylalanine, however, intellectual disability can be prevented.
Foods high in phenylalanine include proteins found in milk, dairy products, meat,
fish, chicken, eggs, beans, and nuts. Although the child still has the PKU genotype, a
modification of the environment (in this case, the child’s diet) produces an
outwardly normal phenotype.
Dominance and Recessiveness
In many loci, the effects of one allele mask those of another when the two are found
together in a heterozygote. The allele whose effects are observable is said to be
dominant. The allele whose effects are hidden is said to be recessive (from the
Latin root for “hiding”). Traditionally, for loci having two alleles, the dominant
allele is denoted by an uppercase letter and the recessive allele is denoted by a
lowercase letter. When one allele is dominant over another, the heterozygote
genotype Aa has the same phenotype as the dominant homozygote AA. For the
recessive allele to be expressed, it must exist in the homozygote form, aa. When the
heterozygote is distinguishable from both homozygotes, the locus is said to exhibit
codominance.
A carrier is an individual who has a disease gene but is phenotypically normal.
Many genes for a recessive disease occur in heterozygotes who carry one copy of
the gene but do not express the disease. When recessive genes are lethal in the
homozygous state, they are eliminated from the population when they occur in
homozygotes. By “hiding” in carriers, however, recessive genes for diseases are
passed on to the next generation.
Transmission of Genetic Diseases
The pattern in which a genetic disease is inherited through generations is termed the
mode of inheritance. Knowing the mode of inheritance can reveal much about the
disease-causing gene itself, and members of families with the disease can be given
reliable genetic counseling.
Gregor Mendel systematically studied modes of inheritance and formulated two
basic laws of inheritance. His principle of segregation states that homologous
genes separate from one another during reproduction and that each reproductive
cell carries only one copy of a homologous gene. Mendel’s second law, the
principle of independent assortment, states that the hereditary transmission of one
gene does not affect the transmission of another. Mendel discovered these laws in
the mid-nineteenth century by performing breeding experiments with garden peas,
even though he had no knowledge of chromosomes. Early twentieth century
geneticists found that chromosomal behavior essentially corresponds to Mendel’s
laws, which now form the basis for the chromosome theory of inheritance.
The known single-gene diseases can be classified into four major modes of
inheritance: autosomal dominant, autosomal recessive, X-linked dominant, and X-
linked recessive. The first two types involve genes known to occur on the 22 pairs
of autosomes. The last two types occur on the X chromosome; very few disease-
causing genes occur on the Y chromosome.
The pedigree chart summarizes family relationships and shows which members
of a family are affected by a genetic disease (Figure 2-19). Generally, the pedigree
begins with one individual in the family, the proband. This individual is usually the
first person in the family diagnosed or seen in a clinic.
FIGURE 2-19 Symbols Commonly Used in Pedigrees. (From Jorde LB et al: Medical genetics, ed 4, St Louis, 2010,
Mosby.)
Autosomal Dominant Inheritance
Characteristics of Pedigrees
Diseases caused by autosomal dominant genes are rare, with the most common
occurring in fewer than 1 in 500 individuals. Therefore it is uncommon for two
individuals who are both affected by the same autosomal dominant disease to
produce offspring together. Figure 2-20, A, illustrates this unusual pattern. Affected
offspring are usually produced by the union of a normal parent with an affected
heterozygous parent. The Punnett square in Figure 2-20, B, illustrates this mating.
The affected parent can pass either a disease-causing allele or a normal allele to the
next generation. On average, half the children will be heterozygous and will express
the disease, and half will be normal.
FIGURE 2-20 Punnett Square and Autosomal Dominant Traits. A, Punnett square for the mating
of two individuals with an autosomal dominant gene. Here both parents are affected by the trait.
B, Punnett square for the mating of a normal individual with a carrier for an autosomal dominant
gene.
The pedigree in Figure 2-21 shows the transmission of an autosomal dominant
allele. Several important characteristics of this pedigree support the conclusion that
the trait is caused by an autosomal dominant gene:
1. The two sexes exhibit the trait in approximately equal proportions; males and
females are equally likely to transmit the trait to their offspring.
2. No generations are skipped. If an individual has the trait, one parent must also
have it. If neither parent has the trait, none of the children have it (with the exception
of new mutations, as discussed later).
3. Affected heterozygous individuals transmit the trait to approximately half their
children, and because gamete transmission is subject to chance fluctuations, all or
none of the children of an affected parent may have the trait. When large numbers of
matings of this type are studied, however, the proportion of affected children
closely approaches one half.
FIGURE 2-21 Pedigree Illustrating the Inheritance Pattern of Postaxial Polydactyly, an
Autosomal Dominant Disorder. Affected individuals are represented by shading. (From Jorde LB et al:
Medical genetics, ed 4, St Louis, 2010, Mosby.)
Recurrence Risks
Parents at risk for producing children with a genetic disease nearly always ask the
question, “What is the chance that our child will have this disease?” The probability
that an individual will develop a genetic disease is termed the recurrence risk.
When one parent is affected by an autosomal dominant disease (and is a
heterozygote) and the other is unaffected, the recurrence risk for each child is one
half.
An important principle is that each birth is an independent event, much like a coin
toss. Thus, even though parents may have already had a child with the disease, their
recurrence risk remains one half. Even if they have produced several children, all
affected (or all unaffected) by the disease, the law of independence dictates the
probability their next child will have the disease is still one half. Parents’
misunderstanding of this principle is a common problem encountered in genetic
counseling.
If a child is born with an autosomal dominant disease and there is no history of
the disease in the family, the child is probably the product of a new mutation. The
gene transmitted by one of the parents has thus undergone a mutation from a normal
to a disease-causing allele. The alleles at this locus in most of the parent’s other
germ cells are still normal. In this situation the recurrence risk for the parent’s
subsequent offspring is not greater than that of the general population. The
offspring of the affected child, however, will have a recurrence risk of one half.
Because these diseases often reduce the potential for reproduction, many autosomal
dominant diseases result from new mutations.
Occasionally, two or more offspring have symptoms of an autosomal dominant
disease when there is no family history of the disease. Because mutation is a rare
event, it is unlikely that this disease would be a result of multiple mutations in the
same family. The mechanism most likely responsible is termed germline mosaicism.
During the embryonic development of one of the parents, a mutation occurred that
affected all or part of the germline. Few or none of the somatic cells of the embryo
were affected. Thus the parent carries the mutation in his or her germline but does
not actually express the disease. As a result, the unaffected parent can transmit the
mutation to multiple offspring. This phenomenon, although relatively rare, can have
significant effects on recurrence risks.8
Delayed Age of Onset
One of the best-known autosomal dominant diseases is Huntington disease, a
neurologic disorder whose main features are progressive dementia and
increasingly uncontrollable limb movements (chorea; discussed further in Chapter
15). A key feature of this disease is its delayed age of onset: symptoms usually are
not seen until 40 years of age or later. Thus those who develop the disease often
have borne children before they are aware they have the disease-causing mutation. If
the disease was present at birth, nearly all affected persons would die before
reaching reproductive age and the occurrence of the disease-causing allele in the
population would be much lower. An individual whose parent has the disease has a
50% chance of developing it during middle age. He or she is thus confronted with a
torturous question: Should I have children, knowing that there is a 50 : 50 chance
that I may have this disease-causing gene and will pass it to half of my children? A
DNA test can now be used to determine whether an individual has inherited the
trinucleotide repeat mutation that causes Huntington disease.
Penetrance and Expressivity
The penetrance of a trait is the percentage of individuals with a specific genotype
who also exhibit the expected phenotype. Incomplete penetrance means individuals
who have the disease-causing genotype may not exhibit the disease phenotype at all,
even though the genotype and the associated disease may be transmitted to the next
generation. A pedigree illustrating the transmission of an autosomal dominant
mutation with incomplete penetrance is provided in Figure 2-22. Retinoblastoma,
the most common malignant eye tumor affecting children, typically exhibits
incomplete penetrance. About 10% of the individuals who are obligate carriers of
the disease-causing mutation (i.e., those who have an affected parent and affected
children and therefore must themselves carry the mutation) do not have the disease.
The penetrance of the disease-causing genotype is then said to be 90%.
FIGURE 2-22 Pedigree for Retinoblastoma Showing Incomplete Penetrance. Female with
marked arrow in line II must be heterozygous, but she does not express the trait.
The gene responsible for retinoblastoma is a tumor-suppressor gene: the normal
function of its protein product is to regulate the cell cycle so cells do not divide
uncontrollably. When the protein is altered because of a genetic mutation, its tumor-
suppressing capacity is lost and a tumor can form9 (see Chapters 10 and 17).
Expressivity is the extent of variation in phenotype associated with a particular
genotype. If the expressivity of a disease is variable, penetrance may be complete
but the severity of the disease can vary greatly. A good example of variable
expressivity in an autosomal dominant disease is neurofibromatosis type 1, or von
Recklinghausen disease. As in retinoblastoma, the mutations that cause
neurofibromatosis type 1 occur in a tumor-suppressor gene.10 The expression of
this disease varies from a few harmless café-au-lait (light brown) spots on the skin
to numerous neurofibromas, scoliosis, seizures, gliomas, neuromas, malignant
peripheral nerve sheath tumors, hypertension, and learning disorders (Figure 2-23).
FIGURE 2-23 Neurofibromatosis. Tumors. The most common is sessile or pedunculated. Early
tumors are soft, dome-shaped papules or nodules that have a distinctive violaceous hue. Most
are benign. (From Habif et al: Skin disease: diagnosis and treatment, ed 2, St Louis, 2005, Mosby.)
Several factors cause variable expressivity. Genes at other loci sometimes modify
the expression of a disease-causing gene. Environmental factors also can influence
expression of a disease-causing gene. Finally, different mutations at a locus can
cause variation in severity. For example, a mutation that alters only one amino acid
of the factor VIII gene usually produces a mild form of hemophilia A, whereas a
“stop” codon (premature termination of translation) usually produces a more severe
form of this blood coagulation disorder.
Epigenetics and Genomic Imprinting
Although this chapter focuses on DNA sequence variation and its consequence for
disease, there is increasing evidence that the same DNA sequence can produce
dramatically different phenotypes because of chemical modifications altering the
expression of genes (these modifications are collectively termed epigenetic,
Chapter 3). An important example of such a modification is DNA methylation, the
attachment of a methyl group to a cytosine base followed by a guanine base in the
DNA sequence (Figure 2-24). These sequences, which are common near many
genes, are termed CpG islands. When the CpG islands located near a gene become
heavily methylated, the gene is less likely to be transcribed into mRNA. In other
words, the gene becomes transcriptionally inactive. One study showed that identical
(monozygotic) twins accumulate different methylation patterns in the DNA
sequences of their somatic cells as they age, causing increasing numbers of
phenotypic differences.11 Intriguingly, twins with more differences in their lifestyles
(e.g., smoking versus nonsmoking) accumulated larger numbers of differences in
their methylation patterns. The twins, despite having identical DNA sequences,
become more and more different as a result of epigenetic changes, which in turn
affect the expression of genes (see Figure 3-5).
FIGURE 2-24 Epigenetic Modifications. Because DNA is a long molecule, it needs packaging to
fit in the tiny nucleus. Packaging involves coiling of the DNA in a “left-handed” spiral around
spools, made of four pairs of proteins individually known as histones and collectively termed the
histone octamer. The entire spool is called a nucleosome (also see Figure 1-2). Nucleosomes
are organized into chromatin, the repeating building blocks of a chromosome. Histone
modifications are correlated with methylation, are reversible, and occur at multiple sites.
Methylation occurs at the 5 position of cytosine and provides a “footprint” or signature as a
unique epigenetic alteration (red). When genes are expressed, chromatin is open or active;
however, when chromatin is condensed because of methylation and histone modification,
genes are inactivated.
Epigenetic alteration of gene activity can have important disease consequences.
For example, a major cause of one form of inherited colon cancer (termed
hereditary nonpolyposis colorectal cancer [HNPCC]) is the methylation of a gene
whose protein product repairs damaged DNA. When this gene becomes inactive,
damaged DNA accumulates, eventually resulting in colon tumors. Epigenetic
changes are also discussed in Chapters 3, 10 and 11.
Approximately 100 human genes are thought to be methylated differently,
depending on which parent transmits the gene. This epigenetic modification,
characterized by methylation and other changes, is termed genomic imprinting. For
each of these genes, one of the parents imprints the gene (inactivates it) when it is
transmitted to the offspring. An example is the insulin-like growth factor 2 gene
(IGF2) on chromosome 11, which is transmitted by both parents, but the copy
inherited from the mother is normally methylated and inactivated (imprinted). Thus
only one copy of IGF2 is active in normal individuals. However, the maternal
imprint is occasionally lost, resulting in two active copies of IGF2. This causes
excess fetal growth and contributes to a condition known as Beckwith-Weidemann
syndrome (see p. 65).
A second example of genomic imprinting is a deletion of part of the long arm of
chromosome 15 (15q11-q13), which, when inherited from the father, causes the
offspring to manifest a disease known as Prader-Willi syndrome (short stature,
obesity, hypogonadism). When the same deletion is inherited from the mother, the
offspring develop Angelman syndrome (intellectual disability, seizures, ataxic gait).
The two different phenotypes reflect the fact that different genes are normally active
in the maternally and paternally transmitted copies of this region of chromosome 15
(see p. 65).
Autosomal Recessive Inheritance
Characteristics of Pedigrees
Like autosomal dominant diseases, diseases caused by autosomal recessive genes
are rare in populations, although there can be numerous carriers. The most
common lethal recessive disease in white children, cystic fibrosis, occurs in about 1
in 2500 births. Approximately 1 in 25 whites carries a copy of a mutation that causes
cystic fibrosis (see Chapter 28). Carriers are phenotypically unaffected. Some
autosomal recessive diseases are characterized by delayed age of onset, incomplete
penetrance, and variable expressivity.
Figure 2-25 shows a pedigree for cystic fibrosis. The gene responsible for cystic
fibrosis encodes a chloride ion channel in some epithelial cells. Defective transport
of chloride ions leads to a salt imbalance that results in secretions of abnormally
thick, dehydrated mucus. Some digestive organs, particularly the pancreas, become
obstructed, causing malnutrition, and the lungs become clogged with mucus,
making them highly susceptible to bacterial infections. Death from lung disease or
heart failure occurs before 40 years of age in about half of persons with cystic
fibrosis.
FIGURE 2-25 Pedigree for Cystic Fibrosis. Cystic fibrosis is an autosomal recessive disorder.
The double bar denotes a consanguineous mating. Because cystic fibrosis is relatively common
in European populations, most cases do not involve consanguinity.
The important criteria for discerning autosomal recessive inheritance include the
following:
1. Males and females are affected in equal proportions.
2. Consanguinity (marriage between related individuals) is sometimes present,
especially for rare recessive diseases.
3. The disease may be seen in siblings of affected individuals but usually not in their
parents.
4. On average, one fourth of the offspring of carrier parents will be affected.
Recurrence Risks
In most cases of recessive disease, both of the parents of affected individuals are
heterozygous carriers. On average, one fourth of their offspring will be normal
homozygotes, half will be phenotypically normal carrier heterozygotes, and one
fourth will be homozygotes with the disease (Figure 2-26). Thus the recurrence risk
for the offspring of carrier parents is 25%. However, in any given family, there are
chance fluctuations.
FIGURE 2-26 Punnett Square for the Mating of Heterozygous Carriers Typical of Most Cases of
Recessive Disease.
If two parents have a recessive disease, they each must be homozygous for the
disease. Therefore all their children also must be affected. This distinguishes
recessive from dominant inheritance because two parents both affected by a
dominant gene are nearly always both heterozygotes and thus one fourth of their
children will be unaffected.
Because carrier parents usually are unaware that they both carry the same
recessive allele, they often produce an affected child before becoming aware of
their condition. Carrier detection tests can identify heterozygotes by analyzing the
DNA sequence to reveal a mutation. Some recessive diseases for which carrier
detection tests are routinely used include phenylketonuria (PKU), sickle cell disease,
cystic fibrosis, Tay-Sachs disease, hemochromatosis, and galactosemia.
Consanguinity
Consanguinity and inbreeding are related concepts. Consanguinity refers to the
mating of two related individuals, and the offspring of such matings are said to be
inbred. Consanguinity is sometimes an important characteristic of pedigrees for
recessive diseases because relatives share a certain proportion of genes received
from a common ancestor. The proportion of shared genes depends on the closeness
of their biologic relationship. Consanguineous matings produce a significant
increase in recessive disorders and are seen most often in pedigrees for rare
recessive disorders.
X-Linked Inheritance
Some genetic conditions are caused by mutations in genes located on the sex
chromosomes, and this mode of inheritance is termed sex linked. Only a few
diseases are known to be inherited as X-linked dominant or Y chromosome traits,
so only the more common X-linked recessive diseases are discussed here.
Because females receive two X chromosomes, one from the father and one from
the mother, they can be homozygous for a disease allele at a given locus,
homozygous for the normal allele at the locus, or heterozygous. Males, having only
one X chromosome, are hemizygous for genes on this chromosome. If a male
inherits a recessive disease gene on the X chromosome, he will be affected by the
disease because the Y chromosome does not carry a normal allele to counteract the
effects of the disease gene. Because a single copy of an X-linked recessive gene will
cause disease in a male, whereas two copies are required for disease expression in
females, more males are affected by X-linked recessive diseases than are females.
X Inactivation
In the late 1950s Mary Lyon proposed that one X chromosome in the somatic cells
of females is permanently inactivated, a process termed X inactivation.12,13 This
proposal, the Lyon hypothesis, explains why most gene products coded by the X
chromosome are present in equal amounts in males and females, even though males
have only one X chromosome and females have two X chromosomes. This
phenomenon is called dosage compensation. The inactivated X chromosomes are
observable in many interphase cells as highly condensed intranuclear chromatin
bodies, termed Barr bodies (after Barr and Bertram, who discovered them in the
late 1940s). Normal females have one Barr body in each somatic cell, whereas
normal males have no Barr bodies.
X inactivation occurs very early in embryonic development—approximately 7 to
14 days after fertilization. In each somatic cell, one of the two X chromosomes is
inactivated. In some cells, the inactivated X chromosome is the one contributed by
the father; in other cells it is the one contributed by the mother. Once the X
chromosome has been inactivated in a cell, all the descendants of that cell have the
same chromosome inactivated (Figure 2-27). Thus inactivation is said to be random
but fixed.
FIGURE 2-27 The X Inactivation Process. The maternal (m) and paternal (p) X chromosomes
are both active in the zygote and in early embryonic cells. X inactivation then takes place,
resulting in cells having either an active paternal X or an active maternal X. Females are thus X
chromosome mosaics, as shown in the tissue sample at the bottom of the page. (From Jorde LB et al:
Medical genetics, ed 4, St Louis, 2010, Mosby.)
Some individuals do not have the normal number of X chromosomes in their
somatic cells. For example, males with Klinefelter syndrome typically have two X
chromosomes and one Y chromosome. These males do have one Barr body in each
cell. Females whose cell nuclei have three X chromosomes have two Barr bodies in
each cell, and females whose cell nuclei have four X chromosomes have three Barr
bodies in each cell. Females with Turner syndrome have only one X chromosome
and no Barr bodies. Thus the number of Barr bodies is always one less than the
number of X chromosomes in the cell. All but one X chromosome are always
inactivated.
Persons with abnormal numbers of X chromosomes, such as those with Turner
syndrome or Klinefelter syndrome, are not physically normal. This situation
presents a puzzle because they presumably have only one active X chromosome, the
same as individuals with normal numbers of chromosomes. This is probably
because the distal tips of the short and long arms of the X chromosome, as well as
several other regions on the chromosome arm, are not inactivated. Thus X
inactivation is also known to be incomplete.
The inactivated X chromosome DNA is heavily methylated. Inactive X
chromosomes can be at least partially reactivated in vitro by administering 5-
azacytidine, a demethylating agent.
Sex Determination
The process of sexual differentiation, in which the embryonic gonads become either
testes or ovaries, begins during the sixth week of gestation. A key principle of
mammalian sex determination is that one copy of the Y chromosome is sufficient to
initiate the process of gonadal differentiation that produces a male fetus. The
number of X chromosomes does not alter this process. For example, an individual
with two X chromosomes and one Y chromosome in each cell is still phenotypically
a male. Thus the Y chromosome contains a gene that begins the process of male
gonadal development.
This gene, termed SRY (for “sex-determining region on the Y”), has been located
on the short arm of the Y chromosome.14 The SRY gene lies just outside the
pseudoautosomal region (Figure 2-28), which pairs with the distal tip of the short
arm of the X chromosome during meiosis and exchanges genetic material with it
(crossover), just as autosomes do. The DNA sequences of these regions on the X
and Y chromosomes are highly similar. The rest of the X and Y chromosomes,
however, do not exchange material and are not similar in DNA sequence.
FIGURE 2-28 Distal Short Arms of the X and Y Chromosomes Exchange Material During
Meiosis in the Male. The region of the Y chromosome in which this crossover occurs is called
the pseudoautosomal region. The SRY gene, which triggers the process leading to male
gonadal differentiation, is located just outside the pseudoautosomal region. Occasionally, the
crossover occurs on the centromeric side of the SRY gene, causing it to lie on an X
chromosome instead of a Y chromosome. An offspring receiving this X chromosome will be an
XX male, and an offspring receiving the Y chromosome will be an XY female.
Other genes that contribute to male differentiation are located on other
chromosomes. Thus SRY triggers the action of genes on other chromosomes. This
concept is supported by the fact that the SRY protein product is similar to other
proteins known to regulate gene expression.
Occasionally, the crossover between X and Y occurs closer to the centromere
than it should, placing the SRY gene on the X chromosome after crossover. This
variation can result in offspring with an apparently normal XX karyotype but a male
phenotype. Such XX males are seen in about 1 in 20,000 live births and resemble
males with Klinefelter syndrome. Conversely, it is possible to inherit a Y
chromosome that has lost the SRY gene (the result of either a crossover error or a
deletion of the gene). This situation produces an XY female. Such females have
gonadal streaks rather than ovaries and have poorly developed secondary sex
characteristics.
Quick Check 2-2
1. Why is the influence of environment significant to phenotype?
2. Discuss the differences between a dominant and a recessive allele.
3. Why are the concepts of variable expressivity, incomplete penetrance, and
delayed age of onset so important in relation to genetic diseases?
4. What is the recurrence risk for autosomal dominant inheritance and recessive
inheritance?
Characteristics of Pedigrees
X-linked pedigrees show distinctive modes of inheritance. The most striking
characteristic is that females seldom are affected. To express an X-linked recessive
trait fully, a female must be homozygous: either both her parents are affected, or her
father is affected and her mother is a carrier. Such matings are rare.
The following are important principles of X-linked recessive inheritance:
1. The trait is seen much more often in males than in females.
2. Because a father can give a son only a Y chromosome, the trait is never
transmitted from father to son.
3. The gene can be transmitted through a series of carrier females, causing the
appearance of one or more “skipped generations.”
4. The gene is passed from an affected father to all his daughters, who, as
phenotypically normal carriers, transmit it to approximately half their sons, who are
affected.
A relatively common X-linked recessive disorder is Duchenne muscular
dystrophy (DMD), which affects approximately 1 in 3500 males. As its name
suggests, this disorder is characterized by progressive muscle degeneration.
Affected individuals usually are unable to walk by age 10 or 12 years. The disease
affects the heart and respiratory muscles, and death caused by respiratory or cardiac
failure usually occurs before 20 years of age. Identification of the disease-causing
gene (on the short arm of the X chromosome) has greatly increased our
understanding of the disorder.15 The DMD gene is the largest gene ever found in
humans, spanning more than 2 million DNA bases. It encodes a previously
undiscovered muscle protein, termed dystrophin. Extensive study of dystrophin
indicates that it plays an essential role in maintaining the structural integrity of
muscle cells: it may also help to regulate the activity of membrane proteins. When
dystrophin is absent, as in DMD, the cell cannot survive, and muscle deterioration
ensues. Most cases of DMD are caused by frameshift deletions of portions of the
DMD gene and thus involve alterations of the amino acids encoded by the DNA
following the deletion.
Recurrence Risks
The most common mating type involving X-linked recessive genes is the
combination of a carrier female and a normal male (Figure 2-29, A). On average,
the carrier mother will transmit the disease-causing allele to half her sons (who are
affected) and half her daughters (who are carriers).
FIGURE 2-29 Punnett Square and X-Linked Recessive Traits. A, Punnett square for the mating
of a normal male (XHY) and a female carrier of an X-linked recessive gene (XHXh). B, Punnett
square for the mating of a normal female (XHXH) with a male affected by an X-linked recessive
disease (XhY). C, Punnett square for the mating of a female who carries an X-linked recessive
gene (XHXh) with a male who is affected with the disease caused by the gene (XhY).
The other common mating type is an affected father and a normal mother (see
Figure 2-29, B). In this situation, all the sons will be normal because the father can
transmit only his Y chromosome to them. Because all the daughters must receive the
father’s X chromosome, they will all be heterozygous carriers. Because the sons
must receive the Y chromosome and the daughters must receive the X chromosome
with the disease gene, these are precise outcomes and not probabilities. None of the
children will be affected.
The final mating pattern, less common than the other two, involves an affected
father and a carrier mother (see Figure 2-29, C). With this pattern, on average, half
the daughters will be heterozygous carriers, and half will be homozygous for the
disease allele and thus affected. Half the sons will be normal, and half will be
affected. Some X-linked recessive diseases, such as DMD, are fatal or incapacitating
before the affected individual reaches reproductive age, and therefore affected
fathers are rare.
Sex-Limited and Sex-Influenced Traits
A sex-limited trait can occur in only one sex, often because of anatomic
differences. Inherited uterine and testicular defects are two obvious examples. A sex-
influenced trait occurs much more often in one sex than the other. For example,
male-pattern baldness occurs in both males and females but is much more common
in males. Autosomal dominant breast cancer, which is much more commonly
expressed in females than males, is another example of a sex-influenced trait.
Linkage Analysis and Gene Mapping
Locating genes on specific regions of chromosomes has been one of the most
important goals of human genetics. The location and identification of a gene can tell
much about the function of the gene, the interaction of the gene with other genes,
and the likelihood that certain individuals will develop a genetic disease.
Classic Pedigree Analysis
Mendel’s second law, the principle of independent assortment, states that an
individual’s genes will be transmitted to the next generation independently of one
another. This law is only partly true, however, because genes located close together
on the same chromosome do tend to be transmitted together to the offspring. Thus
Mendel’s principle of independent assortment holds true for most pairs of genes but
not those that occupy the same region of a chromosome. Such loci demonstrate
linkage and are said to be linked.
During the first meiotic stage, the arms of homologous chromosome pairs
intertwine and sometimes exchange portions of their DNA (Figure 2-30) in a
process known as crossover. During crossover, new combinations of alleles can be
formed. For example, two loci on a chromosome have alleles A and a and alleles B
and b. Alleles A and B are located together on one member of a chromosome pair,
and alleles a and b are located on the other member. The genotype of this individual
is denoted as AB/ab.
FIGURE 2-30 Genetic Results of Crossing Over. A, No crossing over. B, Crossing over with
recombination. C, Double crossing over, resulting in no recombination.
As Figure 2-30, A, shows, the allele pairs AB and ab would be transmitted
together when no crossover occurs. However, when crossover occurs (see Figure 2-
30, B), all four possible pairs of alleles can be transmitted to the offspring: AB, aB,
Ab, and ab. The process of forming such new arrangements of alleles is called
recombination. Crossover does not necessarily lead to recombination, however,
because double crossover between two loci can result in no actual recombination of
the alleles at the loci (see Figure 2-30, C).
Once a close linkage has been established between a disease locus and a “marker”
locus (a DNA sequence that varies among individuals) and once the alleles of the
two loci that are inherited together within a family have been determined, reliable
predictions can be made as to whether a member of a family will develop the
disease. Linkage has been established between several DNA polymorphisms and
each of the two major genes that can cause autosomal dominant breast cancer (about
5% of breast cancer cases are caused by these autosomal dominant genes).
Determining this kind of linkage means that it is possible for offspring of an
individual with autosomal dominant breast cancer to know whether they also carry
the gene and thus could pass it on to their own children. In most cases, specific
disease-causing mutations can be identified, allowing direct detection and diagnosis.
For some genetic diseases, prophylactic treatment is available if the condition can
be diagnosed in time. An example of this is hemochromatosis, a recessive genetic
disease in which excess iron is absorbed, causing degeneration of the heart, liver,
brain, and other vital organs. Individuals at risk for developing the disease can be
determined by testing for a mutation in the hemochromatosis gene and through
clinical tests, and preventive therapy (periodic phlebotomy) can be initiated to
deplete iron stores and ensure a normal life span.
Complete Human Gene Map: Prospects and
Benefits
The major goals of the Human Genome Project were to find the locations of all
human genes (the “gene map”) and to determine the entire human DNA sequence.
These goals have now been accomplished and the genes responsible for more than
4000 mendelian conditions have been identified (Figure 2-31).1,16,17 This has greatly
increased our understanding of the mechanisms that underlie many diseases, such as
retinoblastoma, cystic fibrosis, neurofibromatosis, and Huntington disease. The
project also has led to more accurate diagnosis of these conditions, and in some
cases more effective treatment.
FIGURE 2-31 Example of Diseases: A Gene Map. ADA, Adenosine deaminase; ALD,
adrenoleukodystrophy; PKU, phenylketonuria.
DNA sequencing has become much less expensive and more efficient in recent
years. Consequently, many thousands of individuals have now been completely
sequenced, leading in some cases to the identification of disease-causing genes (see
Health Alert: Gene Therapy).18
Health Alert
Gene Therapy
Thousands of subjects are currently enrolled in more than 1000 gene therapy
protocols. Most of these protocols involve the genetic alteration of cells to combat
various types of cancer. Others involve the treatment of inherited diseases, such as
β-thalassemia, hemophilia B, severe combined immunodeficiency, and retinitis
pigmentosa.
Multifactorial Inheritance
Not all traits are produced by single genes; some traits result from several genes
acting together. These are called polygenic traits. When environmental factors
influence the expression of the trait (as is usually the case), the term multifactorial
inheritance is used. Many multifactorial and polygenic traits tend to follow a
normal distribution in populations (the familiar bell-shaped curve). Figure 2-32
shows how three loci acting together can cause grain color in wheat to vary in a
gradual way from white to red, exemplifying multifactorial inheritance. If both
alleles at each of the three loci are white alleles, the color is pure white. If most
alleles are white but a few are red, the color is somewhat darker; if all are red, the
color is dark red.
FIGURE 2-32 Multifactorial Inheritance. Analysis of mode of inheritance for grain color in
wheat. The trait is controlled by three independently assorted gene loci.
Other examples of multifactorial traits include height and IQ. Although both
height and IQ are determined in part by genes, they are influenced also by
environment. For example, the average height of many human populations has
increased by 5 to 10 cm in the past 100 years because of improvements in nutrition
and health care. Also, IQ scores can be improved by exposing individuals
(especially children) to enriched learning environments. Thus both genes and
environment contribute to variation in these traits.
A number of diseases do not follow the bell-shaped distribution. Instead they
appear to be either present in or absent from an individual. Yet they do not follow
the patterns expected of single-gene diseases. Many of these are probably polygenic
or multifactorial, but a certain threshold of liability must be crossed before the
disease is expressed. Below the threshold the individual appears normal; above it,
the individual is affected by the disease (Figure 2-33).
FIGURE 2-33 Threshold of Liability for Pyloric Stenosis in Males and Females.
A good example of such a threshold trait is pyloric stenosis, a disorder
characterized by a narrowing or obstruction of the pylorus, the area between the
stomach and small intestine. Chronic vomiting, constipation, weight loss, and
electrolyte imbalance can result from the condition, but it is easily corrected by
surgery. The prevalence of pyloric stenosis is about 3 in 1000 live births in whites.
This disorder is much more common in males than females, affecting 1 in 200
males and 1 in 1000 females. The apparent reason for this difference is the threshold
of liability is much lower in males than females, as shown in Figure 2-33. Thus
fewer defective alleles are required to generate the disorder in males. This situation
also means the offspring of affected females are more likely to have pyloric
stenosis because affected females necessarily carry more disease-causing alleles
than do most affected males.
A number of other common diseases are thought to correspond to a threshold
model. They include cleft lip and cleft palate, neural tube defects (anencephaly, spina
bifida), clubfoot (talipes), and some forms of congenital heart disease.
Although recurrence risks can be given with confidence for single-gene diseases
(e.g., 50% for autosomal dominants, 25% for autosomal recessives), it is
considerably more difficult to do so for multifactorial diseases. The number of
genes contributing to the disease is not known, the precise allelic constitution of the
biologic parents is not known, and the extent of environmental effects can vary
from one population to another. For most multifactorial diseases, empirical risks
(i.e., those based on direct observation) have been derived. To determine empirical
risks, a large sample of biologic families in which one child has developed the
disease is examined. The siblings of each child are then surveyed to calculate the
percentage who also develop the disease.
Another difficulty is distinguishing polygenic or multifactorial diseases from
single-gene diseases having incomplete penetrance or variable expressivity. Large
data sets and good epidemiologic data often are necessary to make the distinction.
Box 2-1 lists criteria commonly used to define multifactorial diseases.
Box 2-1
Criteria Used to Define Multifactorial Diseases
1. The recurrence risk becomes higher if more than one family member is affected.
For example, the recurrence risk for neural tube defects in a British family
increases to 10% if two siblings have been born with the disease. By contrast, the
recurrence risk for single-gene diseases remains the same regardless of the
number of siblings affected.
2. If the expression of the disease is more severe, the recurrence risk is higher. This
is consistent with the liability model; a more severe expression indicates that the
individual is at the extreme end of the liability distribution. Relatives of the
affected individual are thus at a higher risk for inheriting disease genes. Cleft lip
or cleft palate is a condition in which this has been shown to be true.
3. Relatives of probands of the less commonly affected are more likely to develop
the disease. As with pyloric stenosis, this occurs because an affected individual of
the less susceptible sex is usually at a more extreme position on the liability
distribution.
4. Generally, if the population frequency of the disease is f, the risk for offspring
and siblings of probands is approximately . This does not usually hold true for
single-gene traits.
5. The recurrence risk for the disease decreases rapidly in more remotely related
relatives. Although the recurrence risk for single-gene diseases decreases by 50%
with each degree of relationship (e.g., an autosomal dominant disease has a 50%
recurrence risk for siblings, 25% for uncle-nephew relationship, 12.5% for first
cousins), the risk for multifactorial inheritance decreases much more quickly.
The genetics of common disorders such as hypertension, heart disease, and
diabetes is complex and often confusing. Nevertheless, the public health impact of
these diseases, together with the evidence for hereditary factors in their etiology,
demands that genetic studies be pursued. Hundreds of genes contributing to
susceptibility for these diseases have been discovered, and the next decade will
undoubtedly witness substantial advancements in our understanding of these
disorders.
Quick Check 2-3
1. Define linkage analysis; cite an example.
2. Why is “threshold of liability” an important consideration in multifactorial
inheritance?
3. Discuss the concept of multifactorial inheritance, and include two examples.
Did You Understand?
DNA, RNA, and Proteins: Heredity at the
Molecular Level
1. Genes, the basic units of inheritance, are composed of deoxyribonucleic acid
(DNA) and are located on chromosomes.
2. DNA is composed of deoxyribose, a phosphate molecule, and four types of
nitrogenous bases. The physical structure of DNA is a double helix.
3. The DNA bases code for amino acids, which in turn make up proteins. The amino
acids are specified by triplet codons of nitrogenous bases.
4. DNA replication is based on complementary base pairing, in which a single
strand of DNA serves as the template for attracting bases that form a new strand of
DNA.
5. DNA polymerase is the primary enzyme involved in replication. It adds bases to
the new DNA strand and performs “proofreading” functions.
6. A mutation is an inherited alteration of genetic material (i.e., DNA).
7. Substances that cause mutations are called mutagens.
8. The mutation rate in humans varies from locus to locus and ranges from 10−4 to
10−7 per gene per generation.
9. Transcription and translation, the two basic processes in which proteins are
specified by DNA, both involve ribonucleic acid (RNA). RNA is chemically similar
to DNA, but it is single stranded, has a ribose sugar molecule, and has uracil rather
than thymine as one of its four nitrogenous bases.
10. Transcription is the process by which DNA specifies a sequence of messenger
RNA (mRNA).
11. Much of the RNA sequence is spliced from the mRNA before the mRNA leaves
the nucleus. The excised sequences are called introns, and those that remain to code
for proteins are called exons.
12. Translation is the process by which RNA directs the synthesis of polypeptides.
This process takes place in the ribosomes, which consist of proteins and ribosomal
RNA (rRNA).
13. During translation, mRNA interacts with transfer RNA (tRNA), a molecule that
has an attachment site for a specific amino acid.
Chromosomes
1. Human cells consist of diploid somatic cells (body cells) and haploid gametes
(sperm and egg cells).
2. Humans have 23 pairs of chromosomes. Twenty-two of these pairs are
autosomes. The remaining pair consists of the sex chromosomes. Females have two
homologous X chromosomes as their sex chromosomes; males have an X and a Y
chromosome.
3. A karyotype is an ordered display of chromosomes arranged according to length
and the location of the centromere.
4. Various types of stains can be used to make chromosome bands more visible.
5. About 1 in 150 live births has a major diagnosable chromosome abnormality.
Chromosome abnormalities are the leading known cause of mental retardation and
miscarriage.
6. Polyploidy is a condition in which a euploid cell has some multiple of the normal
number of chromosomes. Humans have been observed to have triploidy (three
copies of each chromosome) and tetraploidy (four copies of each chromosome);
both conditions are lethal.
7. Somatic cells that do not have a multiple of 23 chromosomes are aneuploid.
Aneuploidy is usually the result of nondisjunction.
8. Trisomy is a type of aneuploidy in which one chromosome is present in three
copies in somatic cells. A partial trisomy is one in which only part of a
chromosome is present in three copies.
9. Monosomy is a type of aneuploidy in which one chromosome is present in only
one copy in somatic cells.
10. In general, monosomies cause more severe physical defects than do trisomies,
illustrating the principle that the loss of chromosome material has more severe
consequences than the duplication of chromosome material.
11. Down syndrome, a trisomy of chromosome 21, is the best-known disease caused
by a chromosome aberration. It affects 1 in 800 live births and is much more likely
to occur in the offspring of women older than 35 years.
12. Most aneuploidies of the sex chromosomes have less severe consequences than
those of the autosomes.
13. The most commonly observed sex chromosome aneuploidies are the 47,XXX
karyotype, 45,X karyotype (Turner syndrome), 47,XXY karyotype (Klinefelter
syndrome), and 47,XYY karyotype.
14. Abnormalities of chromosome structure include deletions, duplications,
inversions, and translocations.
Elements of Formal Genetics
1. Mendelian traits are caused by single genes, each of which occupies a position, or
locus, on a chromosome.
2. Alleles are different forms of genes located at the same locus on a chromosome.
3. At any given locus in a somatic cell, an individual has two genes, one from each
parent. An individual may be homozygous or heterozygous for a locus.
4. An individual’s genotype is his or her genetic makeup, and the phenotype reflects
the interaction of genotype and environment.
5. In a heterozygote, a dominant gene’s effects mask those of a recessive gene. The
recessive gene is expressed only when it is present in two copies.
Transmission of Genetic Diseases
1. Genetic diseases caused by single genes usually follow autosomal dominant,
autosomal recessive, or X-linked recessive modes of inheritance.
2. Pedigree charts are important tools in the analysis of modes of inheritance.
3. Recurrence risks specify the probability that future offspring will inherit a genetic
disease. For single-gene diseases, recurrence risks remain the same for each
offspring, regardless of the number of affected or unaffected offspring.
4. The recurrence risk for autosomal dominant diseases is usually 50%.
5. Germline mosaicism can alter recurrence risks for genetic diseases because
unaffected parents can produce multiple affected offspring. This situation occurs
because the germline of one parent is affected by a mutation but the parent’s somatic
cells are unaffected.
6. Skipped generations are not seen in classic autosomal dominant pedigrees.
7. Males and females are equally likely to exhibit autosomal dominant diseases and
to pass them on to their offspring.
8. Many genetic diseases have a delayed age of onset.
9. A gene that is not always expressed phenotypically is said to have incomplete
penetrance.
10. Variable expressivity is a characteristic of many genetic diseases.
11. Genomic imprinting, which is associated with methylation, results in differing
expression of a disease gene, depending on which parent transmitted the gene.
12. Epigenetics involves changes, such as the methylation of DNA bases, that do not
alter the DNA sequence but can alter the expression of genes.
13. Most commonly, biologic parents of children with autosomal recessive diseases
are both heterozygous carriers of the disease gene.
14. The recurrence risk for autosomal recessive diseases is 25%.
15. Males and females are equally likely to be affected by autosomal recessive
diseases.
16. Consanguinity is sometimes present in families with autosomal recessive
diseases, and it becomes more prevalent with rarer recessive diseases.
17. Carrier detection tests for an increasing number of autosomal recessive diseases
are available.
18. The frequency of genetic diseases approximately doubles in the offspring of
first-cousin matings.
19. In each normal female somatic cell, one of the two X chromosomes is
inactivated early in embryogenesis.
20. X inactivation is random, fixed, and incomplete (i.e., only part of the
chromosome is actually inactivated). It may involve methylation.
21. Gender is determined embryonically by the presence of the SRY gene on the Y
chromosome. Embryos that have a Y chromosome (and thus the SRY gene) become
males, whereas those lacking the Y chromosome become females. When the Y
chromosome lacks the SRY gene, an XY female can be produced. Similarly, an X
chromosome that contains the SRY gene can produce an XX male.
22. X-linked genes are those that are located on the X chromosome. Nearly all
known X-linked diseases are caused by X-linked recessive genes.
23. Males are hemizygous for genes on the X chromosome.
24. X-linked recessive diseases are seen much more often in males than in females
because males need only one copy of the gene to express the disease.
25. Biologic fathers cannot pass X-linked genes to their sons.
26. Skipped generations often are seen in X-linked recessive disease pedigrees
because the gene can be transmitted through carrier females.
27. Recurrence risks for X-linked recessive diseases depend on the carrier and
affected status of the mother and father.
28. A sex-limited trait is one that occurs only in one sex (gender).
29. A sex-influenced trait is one that occurs more often in one sex than in the other.
Linkage Analysis and Gene Mapping
1. During meiosis I, crossover occurs and can cause recombinations of alleles
located on the same chromosome.
2. The frequency of recombinations can be used to infer the map distance between
loci on the same chromosome.
3. A marker locus, when closely linked to a disease-gene locus, can be used to
predict whether an individual will develop a genetic disease.
4. The major goals of the Human Genome Project were to find the locations of all
human genes (the “gene map”) and to determine the entire human DNA sequence.
These goals have now been accomplished and the genes responsible for more than
4000 mendelian conditions have been identified.
Multifactorial Inheritance
1. Traits that result from the combined effects of several loci are polygenic. When
environmental factors also influence the trait, it is multifactorial.
2. Many multifactorial traits have a threshold of liability. Once the threshold of
liability has been crossed, the disease may be expressed.
3. Empirical risks, based on direct observation of large numbers of families, are
used to estimate recurrence risks for multifactorial diseases.
4. Recurrence risks for multifactorial diseases become higher if more than one
biologic family member is affected or if the expression of the disease in the
proband is more severe.
5. Recurrence risks for multifactorial diseases decrease rapidly for more remote
relatives.
Key Terms
Adenine, 38
Allele, 49
Amino acid, 39
Aneuploid cell, 42
Anticodon, 41
Autosome, 42
Barr body, 54
Base pair substitution, 39
Carrier, 49
Carrier detection test, 54
Chromosomal mosaic, 46
Chromosome, 38
Chromosome band, 42
Chromosome breakage, 47
Chromosome theory of inheritance, 50
Clastogen, 47
Codominance, 49
Codon, 39
Complementary base pairing, 39
Consanguinity, 54
CpG islands, 52
Cri du chat syndrome, 48
Crossover, 56
Cytokinesis, 42
Cytosine, 38
Delayed age of onset, 51
Deletion, 48
Deoxyribonucleic acid (DNA), 38
Diploid cell, 42
DNA methylation, 52
DNA polymerase, 39
Dominant, 49
Dosage compensation, 54
Double-helix model, 38
Down syndrome, 46
Duplication, 48
Dystrophin, 55
Empirical risk, 58
Epigenetic, 52
Euploid cell, 42
Exon, 41
Expressivity, 51
Fragile site, 49
Frameshift mutation, 39
Gamete, 42
Gene, 38
Genomic imprinting, 52
Genotype, 49
Germline mosaicism, 51
Guanine, 38
Haploid cell, 42
Hemizygous, 54
Heterozygote, 49
Heterozygous, 49
Homologous, 42
Homozygote, 49
Homozygous, 49
Inbreeding, 54
Intron, 41
Inversion, 48
Karyotype (karyogram), 42
Klinefelter syndrome, 47
Linkage, 56
Locus, 49
Meiosis, 42
Messenger RNA (mRNA), 39
Metaphase spread, 42
Methylation, 52
Missense, 39
Mitosis, 42
Mode of inheritance, 49
Multifactorial inheritance, 58
Mutagen, 39
Mutation, 39
Mutational hot spot, 39
Nondisjunction, 45
Nonsense, 39
Nucleotide, 39
Obligate carrier, 51
Partial trisomy, 46
Pedigree, 50
Penetrance, 51
Phenotype, 49
Polygenic trait, 57
Polymorphic (polymorphism), 49
Polypeptide, 39
Polyploid cell, 42
Position effect, 48
Principle of independent assortment, 50
Principle of segregation, 50
Proband, 50
Promoter site, 39
Pseudoautosomal, 54
Purine, 38
Pyrimidine, 38
Recessive, 49
Reciprocal translocation, 48
Recombination, 56
Recurrence risk, 50
Ribonucleic acid (RNA), 39
Ribosomal RNA (rRNA), 41
Ribosome, 41
RNA polymerase, 39
Robertsonian translocation, 49
Sex-influenced trait, 55
Sex-limited trait, 55
Sex linked (inheritance), 54
Silent mutation, 39
Somatic cell, 42
Spontaneous mutation, 39
Template, 39
Termination sequence, 41
Tetraploidy, 42
Threshold of liability, 58
Thymine, 38
Transcription, 39
Transfer RNA (tRNA), 41
Translation, 41
Translocation, 48
Triploidy, 42
Trisomy, 42
Tumor-suppressor gene, 51
Turner syndrome, 47
X inactivation, 54
References
1. Jorde LB, et al. Medical genetics. ed 4. Mosby-Elsevier: St Louis; 2010.
2. Gardner RJM, et al. Chromosome abnormalities and genetic counseling.
Oxford University Press: Oxford; 2012.
3. Nagaoka SI, et al. Human aneuploidy: mechanisms and new insights into an
age-old problem. Nat Rev Genet. 2012;13(7):493–504.
4. Antonarakis SE, Epstein CJ. The challenge of Down syndrome. Trends Mol
Med. 2006;12(10):473–479.
5. Gravholt CH. Sex chromosome abnormalities. Rimoin DL, Pyeritz RE,
Korf BR. Emery and Rimoin’s principles and practice of medical genetics.
ed 6. Elsevier: Philadelphia; 2013.
6. Rooms L, Kooy RF. Advances in understanding fragile X syndrome and
related disorders. Curr Opin Pediatr. 2011;23(6):601–606.
7. Nelson DL, et al. The unstable repeats—three evolving faces of
neurological disease. Neuron. 2013;77(5):825–843.
8. Biesecker LG, Spinner NB. A genomic view of mosaicism and human
disease. Nat Rev Genet. 2013;14(5):307–320.
9. Foulkes WD. Inherited susceptibility to common cancers. N Engl J Med.
2008;359(20):2143–2153.
10. Pasmant E, et al. Neurofibromatosis type 1: from genotype to phenotype. J
Med Genet. 2012;49(8):483–489.
11. Fraga MF, et al. Epigenetic differences arise during the lifetime of
monozygotic twins. Proc Natl Acad Sci U S A. 2005;102(30):10604–10609.
12. Lyon MF. X-chromosome inactivation. Curr Biol. 1999;9(7):R235–R237.
13. Lee JT, Bartolomei MS. X-inactivation, imprinting, and long noncoding
RNAs in health and disease. Cell. 2013;152(6):1308–1323.
14. Larney C, et al. Switching on sex: transcriptional regulation of the testis-
determining gene Sry. Development. 2014;141(11):2195–2205.
15. Flanigan KM. The muscular dystrophies. Semin Neurol. 2012;32(3):255–263.
16. Lander ES. Initial impact of the sequencing of the human genome. Nature.
2011;470(7333):187–197.
17. Yang Y, et al. Clinical whole-exome sequencing for the diagnosis of
mendelian disorders. N Engl J Med. 2013;369(16):1502–1511.
18. Koboldt DC, et al. The next-generation sequencing revolution and its impact
on genomics. Cell. 2013;155(1):27–38.
3
Epigenetics and Disease
Diane P. Genereux, Lynn B. Jorde
CHAPTER OUTLINE
Epigenetic Mechanisms, 62
DNA Methylation, 62
Histone Modifications, 63
RNA-Based Mechanisms, 64
Epigenetics and Human Development, 64
Genomic Imprinting, 64
Prader-Willi and Angelman Syndromes, 65
Beckwith-Wiedemann Syndrome, 65
Russell-Silver Syndrome, 66
Long-Term and Multigenerational Persistence of Epigenetic States
Induced by Stochastic and Environmental Factors, 66
Epigenetics and Nutrition, 66
Epigenetics and Maternal Care, 66
Epigenetics and Mental Illness, 67
Twin Studies Provide Insights on Epigenetic
Modification, 68
Molecular Approaches to Understand Epigenetic
Disease, 68
Epigenetics and Cancer, 68
DNA Methylation and Cancer, 68
miRNAs and Cancer, 69
Epigenetic Screening for Cancer, 69
Emerging Strategies for the Treatment of
Epigenetic Disease, 69
Future Directions, 70
Human beings exhibit an impressive diversity of physical and behavioral features.
Some of this diversity is attributable to genetic variation. Another contributor to
human diversity is epigenetic (“upon genetic”) modification (a change in phenotype
or gene expression that does not involve DNA mutation or changes in nucleotide
sequence). Basically, epigenetics is the study of mechanisms that will switch genes
“on,” such that they are expressed, and “off,” such that they are silenced. Epigenetic
mechanisms include chemical modifications to DNA and associated histones, and
the production of small RNA molecules. Gene regulation by epigenetic processes
can occur at the level of either transcription or translation. Epigenetic modification
is critical for fundamental processes of human development, including the
differentiation of embryonic stem cells into specific cell types, and the inactivation
of one of the two X chromosomes in each cell of a genetic female. Some genes are
noted to be imprinted, a form of epigenetic regulation where the expression of a
gene depends on whether it is inherited from the mother or the father.
Epigenetic Mechanisms
A variety of diseases can result from abnormal epigenetic states. Metabolic disease
can occur when there is aberrant expression of both copies of a locus that is
typically imprinted. Environmental stressors can markedly increase the risk of
aberrant epigenetic modification and are strongly associated with some cancers. It is
because of their increasing clear role in a wide range of pathologies that abnormal
epigenetic states are currently a focus of both preventative efforts and
pharmaceutical intervention. Currently known epigenetic mechanisms include DNA
methylation, histone modifications, and RNA-based mechanisms (Figure 3-1).
FIGURE 3-1 Three Types of Epigenetic Processes. Investigators are studying three epigenetic
mechanisms: (1) DNA methylation, (2) histone modifications, and (3) RNA based-mechanisms.
See text for discussion.
DNA Methylation
DNA methylation (see Figure 3-1) occurs through the attachment of a methyl group
(CH3) to a cytosine. Dense DNA methylation can be thought of as “insulation” that
renders genes silent by blocking access by transcription factors. Dense methylation
is typically coincident with hypoacetylation (decrease of the functional group
acetyl) of the histone proteins around which the DNA is wound (see Histone
Modifications). Together, DNA methylation and histone hypoacetylation can render
a gene transcriptionally silent, preventing production of the encoded protein.
Methylated cytosines have been found to occur principally at cytosines that are
followed by a guanine base (sometimes known as cytosines in “CpG
dinucleotides”). In human embryonic stem cells, methylation also can occur at
cytosines outside of the CpG context (see Figure 2-24).
DNA methylation plays a prominent role in both human health and disease. For
example, in each cell of a normal human female, one of the two X chromosomes is
silenced by dense methylation and associated molecular marks, whereas the other X
chromosome is transcriptionally active and largely devoid of methylation. During
early embryonic development, there is epigenetic inactivation of one of the two X
chromosomes in each cell of a human female—either the X chromosome inherited
from her mother or the X chromosome inherited from her father. The
determination of which chromosome is to be silenced occurs at random and
independently in each of the cells present at this stage of development; the silent
state of that chromosome is inherited by all subsequent copies. If a woman’s two X
chromosomes carry different alleles at a given locus, random X inactivation can
lead to somatic mosaicism, wherein the alleles active in two different cells can
confer two very different traits. Striking examples include the patchy coloration of
calico cats and anhidrotic ectodermal dysplasia, a condition characterized by patchy
presence and absence of sweat glands in the skin of human females who have one X
chromosome bearing a normal allele and one X chromosome bearing a mutant
allele at the X-encoded locus. Because of the somatic mosaicism that arises through
random inactivation of the X chromosome, females tend to have less severe
phenotypes than do males for a variety of X-linked disorders, including color
blindness and fragile X syndrome.
Aberrant DNA methylation, either the presence of dense methylation where it is
typically absent or the absence of methylation where it is typically present, can lead
to misregulation of tumor-suppressor genes and oncogenes. Abnormal DNA
methylation states are a common feature of several human cancers, including those
of the colon1-3 (see Figures 3-1 and 3-6 [p. 69]; also see Chapter 10).
Histone Modifications
Histone modifications (see Figure 3-1) include histone acetylation (adding an
acetyl group) and deacetylation (deletion of an acetyl group) to the end of a histone
protein. Like DNA methylation, these changes can alter the expression state of
chromatin. Histones are proteins that facilitate compaction of genomic DNA into
the nucleus of a cell, much as a spool helps to organize a long piece of thread for
storage in a small space. When the DNA of the human genome is wound around
histones, it is only ≈1/40,000 as long as it would be in its uncondensed state.
Chemical modification of histones in a region of DNA can either up-regulate or
down-regulate nearby gene expression by increasing or decreasing the tightness of
the interaction between DNA and histones, thus modulating the extent to which DNA
is accessible to transcription factors. DNA in association with histones is referred to
as “chromatin.” At any given time, various regions of chromatin are typically in
one of two forms: euchromatin, an open state in which most or all nearby genes are
transcriptionally active; or heterochromatin, a closed state in which most or all
nearby genes are transcriptionally inactive.
Chromatin structure plays a critical role in determining the developmental
potential of a given cell lineage, and can undergo dramatic changes during
organismal development. For example, chromatin states differ substantially between
embryonic stem cells, which are poised to give rise to all of the different cell types
that make up an individual, and terminally differentiated cells, which are committed
to a specific developmental path. The fraction of DNA that is in the heterochromatic
state increases as cells differentiate, consistent with the reduction in the number of
genes that are active as a cell lineage transitions from pluripotency to terminal
differentiation.4 Mutations in genes that encode histone-modifying proteins have
been implicated in congenital heart disease,5 for example, highlighting histone
modification states as critical for normal development.
In contrast to the vast majority of other cell types, including oocytes, sperm cells
express not histones but protamines, which are evolutionarily derived from
histones.6 Protamines enable sperm DNA to wind into an even more compact state
than does the histone-bound DNA in somatic cells. This tight compaction improves
the hydrodynamic features of the sperm head, facilitating its movement toward the
egg.
RNA-Based Mechanisms
Noncoding RNAs (ncRNAs) and other RNA-based mechanisms (see Figure 3-1)
play an important role in regulating a wide variety of cellular processes, including
RNA splicing and DNA replication. These ncRNAs have been likened to “sponges”
in so far as they can “sop up” complementary RNAs, thus inhibiting their function
(see, for example, www.ncbi.nlm.nih.gov/pmc/articles/PMC2957044/). Of particular
relevance to gene regulation are the hairpin-shaped microRNAs (miRNAs), which
are encoded by DNA sequences of approximately 22 nucleotides, typically within
the introns (a segment of a DNA molecule that does not code for proteins) of genes
or in noncoding DNA located between genes (see Chapter 2). In contrast to DNA
methylation and histone modification, both of which principally affect gene
expression at the level of transcription, miRNAs typically modulate the stability and
translational efficiency of existing messenger RNAs (mRNAs) encoded at other
loci. Interaction between miRNAs and mRNAs target for degradation is typically
mediated by regions of partial sequence complementarity. As a result, miRNAs can
at once be specific enough so that they do not bind to all of the mRNAs in a cell and
general enough to regulate a large number of different mRNA sequences. miRNAs
also directly modulate translation by impairing ribosomal function. miRNAs
regulate diverse signaling pathways; those that stimulate cancer development and
progression are called oncomirs. For example, miRNAs have been linked to
carcinogenesis because they alter the activity of oncogenes and tumor-suppressor
genes (see Chapter 10).
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2957044/
Epigenetics and Human Development
Each of the cells in the very early embryo has the potential to give rise to a somatic
cell of any type. These embryonic stem cells are therefore said to be totipotent
(“possessing all powers”). A key process in early development then is the
differential epigenetic modification of specific DNA nucleotide sequences in these
embryonic stem cells, ultimately leading to the differential gene-expression profiles
that characterize the various differentiated somatic cell types. These early
modifications ensure that specific genes are expressed only in the cells and tissue
types in which their gene products typically function (e.g., factor VIII expression
primarily in hepatocytes, or dopamine receptor expression in neurons).
Epigenetic modifications early in development also highlight a fundamental
feature of genetics as compared to epigenetic information: all of the cells in a given
individual contain almost exactly the same genetic information. It is the epigenetic
information eventually placed on top of these sequences that enables them to achieve
the diverse functions of differentiated somatic cells. A small percentage of genes,
termed housekeeping genes, are necessary for the function and maintenance of all
cells. These genes escape epigenetic silencing and remain transcriptionally active in
all or nearly all cells. Housekeeping genes include encoding histones, DNA and
RNA polymerases, and ribosomal RNA genes.
How do embryonic stem cells achieve epigenetic states typical of totipotency,
whereby they can give rise to all of the diverse cell types that make up a fully
developed organism? One explanation is that early embryogenesis (approximately
the 10 days just after fertilization) is characterized by rapid fluctuation in genome-
wide DNA methylation densities. Fertilization triggers a global loss of DNA
methylation at most loci in both the oocyte-contributed and the sperm-contributed
genomes. This loss of methylation is accomplished in part by suppression of the
DNA methyltransferases, the enzymes that add methyl groups to DNA. Methylation
is not directly copied by the DNA replication process. Instead, immediately
following replication, the methyltransferases read the pattern of methylation on the
parent DNA strand and use that information to determine which daughter-strand
cytosines should be methylated. As embryonic cell division proceeds in the absence
of DNA methyltransferases, cell division continues, eventually yielding cells that
have nearly all of their loci in unmethylated, transcriptionally active states. Around
the time of implantation in the uterus, the DNA methyltransferases become active
again, permitting establishment of the cell-lineage–specific marks required for the
establishment of organ systems.
Genomic Imprinting
A baby inherits two copies of each autosomal gene: one from its mother and one
from its father. For a large subset of these genes, expression is biallelic, meaning
that both the maternally and the paternally inherited copies contribute to offspring
phenotype. For another, smaller subset of these genes, expression is stochastically
monoallelic,7 meaning that the maternal copy is randomly chosen for inactivation in
some somatic cells and the paternal copy is randomly chosen for inactivation in
other somatic cells. For a third and smaller subset of autosomes (about 1%) either
the maternal copy or the paternal copy is imprinted, meaning that either the copy
inherited through the sperm or the copy inherited through the egg is inactivated and
remains in this inactive state in all of the somatic cells of the individual.
The subset of genes that are subject to imprinting is highly enriched for loci
relevant to organismal growth. The genetic conflict hypothesis7 was developed as a
potential explanation for this pattern. Although both the mother and the father
benefit genetically from the birth and survival of offspring, their interests are not
entirely aligned. Because a mother makes a large physiologic investment in each
child, it is in her evolutionary best interest to limit the flow of energetic resources
to any given offspring so as to maintain her physiologic capacity to bear subsequent
children. By contrast, except in cases of certain permanent, certain monogamy, it is
in the best interest of the father for his child to extract maximal resources from its
mother, as his own future fecundity, or fertility, is not contingent on the sustained
fecundity of the mother. In general, imprinting of maternally inherited genes tends
to reduce offspring size; imprinting of paternally inherited genes tends to increase
offspring size. One hallmark of imprinting-associated disease is that the phenotype
of affected individuals is critically dependent on whether the mutation is inherited
from the mother or from the father. Some examples are included in the following
syndromes.
Prader-Willi and Angelman Syndromes
A well-known disease example of imprinting is associated with a deletion of about 4
million base (Mb) pairs of the long arm of chromosome 15. When this deletion is
inherited from the father, the child manifests Prader-Willi syndrome, with features
including short stature, hypotonia, small hands and feet, obesity, mild to moderate
intellectual disability, and hypogonadism8 (Figure 3-2, A). The same 4-Mb deletion,
when inherited from the mother, causes Angelman syndrome, which is
characterized by severe intellectual disability, seizures, and an ataxic gait (Figure 3-
2, B).9 These diseases are each observed in about 1 of every 15,000 live births;
chromosome deletions are responsible for about 70% of cases of both diseases. The
deletions that cause Prader-Willi and Angelman syndromes are indistinguishable at
the DNA sequence level and affect the same group of genes.
FIGURE 3-2 Prader-Willi and Angelman Syndromes. A, A child with Prader-Willi syndrome
(truncal obesity, small hands and feet, inverted V-shaped upper lip). B, A child with Angelman
syndrome (characteristic posture, ataxic gait, bouts of uncontrolled laughter). (From Jorde LB, Carey JC,
Bamshad MJ: Medical genetics, ed 4, Philadelphia, 2010, Mosby.)
For several decades, it was unclear how the same deletion could produce such
disparate results in different individuals. Further analysis showed that the 4-Mb
deletion (the critical region) contains several genes that are normally transcribed
only on the copy of chromosome 15 that is inherited from the father.10 These genes
are transcriptionally inactive (imprinted) on the copy of chromosome 15 inherited
from the mother. Similarly, other genes in the critical region are transcriptionally
active only on the chromosome copy inherited from the mother and are inactive on
the chromosome inherited from the father. Thus, several genes in this region are
normally active on only one chromosome copy (Figure 3-3). If the single active
copy of one of these genes is lost because of a chromosome deletion, then no gene
product is produced, resulting in disease.
FIGURE 3-3 Prader-Willi Syndrome Pedigrees. These pedigrees illustrate the inheritance
patterns of Prader-Willi syndrome, which can be caused by a 4-Mb deletion of chromosome 15q
when inherited from the father. In contrast, Angelman syndrome can be caused by the same
deletion but only when it is inherited from the mother. The reason for this difference is that
different genes in this region are normally imprinted (inactivated) in the copies of 15q
transmitted by the mother and the father. (From Jorde LB, Carey JC, Bamshad MJ: Medical genetics, ed 4, Philadelphia, 2010,
Mosby.)
Molecular analysis has revealed much about genes in this critical region of
chromosome 15.10 The gene responsible for Angelman syndrome encodes a ligase
involved in protein degradation during brain development (consistent with the
mental retardation and ataxia observed in this disorder). In brain tissue, this gene is
active only on the chromosome copy inherited from the mother. Consequently, a
maternally transmitted deletion removes the single active copy of this gene. Several
genes in the critical region are associated with Prader-Willi syndrome and they are
transcribed only on the chromosome transmitted by the father. A paternally
transmitted deletion removes the only active copies of these genes producing the
features of Prader-Willi syndrome.
Beckwith-Wiedemann Syndrome
Another well-known example of imprinting is Beckwith-Wiedemann syndrome, an
overgrowth condition accompanied by an increased predisposition to cancer.
Beckwith-Wiedemann syndrome is usually identifiable at birth because of the
presence of large size for gestational age, neonatal hypoglycemia, a large tongue,
creases on the earlobe, and omphalocele (birth defect of infant intestines).11
Children with Beckwith-Wiedemann syndrome have an increased risk of developing
Wilms tumor or hepatoblastoma. Both of these tumors can be treated effectively if
they are detected early; thus screening at regular intervals is an important part of
management. Some children with Beckwith-Wiedemann syndrome also develop
asymmetric overgrowth of a limb or one side of the face or trunk
(hemihyperplasia).
As with Angelman syndrome, a minority of Beckwith-Wiedemann syndrome
cases (about 20% to 30%) are caused by the inheritance of two copies of a
chromosome from the father and no copy of the chromosome from the mother
(uniparental disomy, in this case affecting chromosome 11). Several genes on the
short arm of chromosome 11 are imprinted on either the paternally or the
maternally transmitted chromosome. These genes are found in two separate,
differentially methylated regions (DMRs). In DMR1, the gene that encodes insulin-
like growth factor 2 (IGF2) is inactive on the maternally transmitted chromosome
but active on the paternally transmitted chromosome. Thus, a normal individual has
only one active copy of IGF2. When two copies of the paternal chromosome are
inherited (i.e., paternal uniparental disomy) or there is loss of imprinting on the
maternal copy of IGF2, an active IGF2 gene is present in double dose. These
changes produce increased levels of insulin-like growth factor 2 during fetal
development, contributing to the overgrowth features of Beckwith-Wiedemann
syndrome. Note that, in contrast to Prader-Willi and Angelman syndromes, which
are produced by a missing gene product, Beckwith-Wiedemann syndrome is caused,
in part, by overexpression of a gene product.
Russell-Silver Syndrome
Russell-Silver syndrome is characterized by growth retardation, proportionate
short stature, leg length discrepancy, and a small, triangular face. About one third of
Russell-Silver syndrome cases are caused by imprinting abnormalities of
chromosome 11p15.5 that lead to down-regulation of IGF2 and therefore
diminished growth. Another 10% of cases of Russell-Silver syndrome are caused by
maternal uniparental disomy. Thus, whereas up-regulation, or extra copies, of active
IGF2 causes overgrowth in Beckwith-Wiedemann syndrome, down-regulation of
IGF2 causes the diminished growth seen in Russell-Silver syndrome.
Quick Check 3-1
1. Define epigenetics.
2. What are the three kinds of epigenetic mechanisms?
3. What is meant by the genetic conflict hypothesis?
4. Compare and contrast the molecular and phenotypic features of Prader-Willi and
Angelman syndromes.
Long-Term and Multigenerational
Persistence of Epigenetic States Induced by
Stochastic and Environmental Factors
It is increasingly clear that imprinted genes are not the only loci for which
epigenetic modifications persist over time. Conditions encountered in utero, during
childhood, and even during adolescence or later can have long-term impacts on
epigenetic states, sometimes with impacts that can be transmitted across generations.
A few such examples are listed below.
Epigenetics and Nutrition
During the winter of 1943, millions of people in urban areas of the Netherlands
suffered starvation conditions as a result of a Nazi blockage that prevented
shipments of food from agricultural areas. When researchers sought to investigate
how exposure to famine in utero had affected individuals born in a historically
prosperous country, they found individuals who suffered nutritional deprivation in
utero were more likely to suffer from obesity and diabetes as adults than individuals
in the Netherlands who had not experienced nutritional deprivation during gestation.
There also seemed to be a transgenerational impact, in that the children of
individuals who were in utero during the Dutch Hunger Winter were found to be
significantly smaller than the children of those not affected by the blockade. Other
data sets reveal elevated risk of cardiovascular and metabolic disease for offspring
of individuals exposed during early development to fluctuations in agricultural
yields.12
The specific molecular mechanisms that may mediate these apparent relationships
between nutritional deprivation and disease risk on one or more generations are
largely unknown. From some animal models, it seems that the insulin-like growth
factor 2 gene (IGF2) is a possible target of epigenetic modifications arising through
nutritional deprivation. Exposure in utero and through lactation to some chemicals
(including bisphenol A, a constituent of plastics sometimes used in food preparation
and storage) seems to lead to epigenetic modifications similar to those that arise
through nutritional deprivation in early life.13
Epigenetics and Maternal Care
It is increasingly clear that parenting style can affect epigenetic states, and that this
information can be transmitted from one generation to the next. Mice and other
rodents can exhibit two alternate styles of nursing behavior: frequent arched-back
nursing with a high level of licking and grooming behavior, and an alternate style
with infrequent arched-back nursing and much reduced licking and grooming
behavior. In one especially compelling study,14 pups of mothers that engaged in
frequent arched-backed nursing were found to have significantly lower methylation
levels and higher transcription activity of a glucocorticoid receptor–encoding
locus. Because the glucocorticoid receptor is involved in a pathway that intensifies
fearfulness and response to stress, these findings suggest that alteration to
methylation states could help explain the finding that exposure to stress early in life
can modulate behavior in adulthood. These findings also highlight the concept that
epigenetic processes can help store information about the environment, and that the
relevant epigenetic modifications can modulate behavior later in life.
Epigenetics and Mental Illness
Epigenetics and Ethanol Exposure During Gestation
The impact of ethanol exposure in utero on skeletal and neural development was
first reported in 197315 and led to broad awareness of fetal alcohol syndrome. It was
not until recently, however, that population-based and molecular-level studies began
to clarify the epigenetic signals that mediate these impacts. At first, researchers
found alcohol exposure in utero can affect the DNA methylation states of various
genomic elements but without specific emphasis on loci directly relevant to skeletal
and neural development.11 More recently, it has been found that treating cultured
neural stem cells with ethanol impairs their ability to differentiate to functional
neurons; this impairment seems to be correlated with aberrant, dense methylation at
loci that are active in normal neuronal tissue.16 One possible explanation for these
effects is that ethanol exposure in utero modulates fetal expression of the DNA
methyltransfereases.17
Epigenetic Disease in the Context of Genetic Abnormalities
In some diseases, both genetic and epigenetic factors contribute to the origin of
abnormal phenotypes. For example, several abnormal phenotypes can arise in
individuals with mutations at the fragile X locus FMR1 (Figure 3-4, A). Some of
these phenotypes arise in individuals for whom epigenetic changes are coincident
with genetic changes. The most common genetic abnormality at FMR1 involves
expansion in the number of cytosine-guanine (CG) dinucleotide repeats in the gene
promoter. Females who have CG repeats in excess of the approximately 35 that are
typical at this locus are at risk for fragile X–associated primary ovarian
insufficiency, characterized by an elevated risk of early menopause.18 Males with
moderate expansions are at risk of fragile X tremor ataxia syndrome (FXTAS),
characterized by a late-onset intention tremor.19 Both of these conditions seem to
arise through accumulation of excess levels of FMR1 mRNAs in nuclear inclusion
bodies.18,20 Individuals with 200 repeats are at risk of fragile X syndrome,
characterized by reduced IQ and a set of behavioral abnormalities. Remarkably,
although possession of a large CG repeat in the FMR1 promoter dramatically
increases the probability that an individual will have fragile X syndrome, the disease
can be present in males who have the large repeat but be absent in their brothers
who have inherited an allele of very similar size.21 This can be explained, at least in
part, by the observation that acquisition of methylation-based silencing at FMR1 is
stochastic, meaning that the presence of a large repeat increases the probability of
the dense promoter methylation that could lead to gene silencing, but does not
guarantee it. It remains to be seen whether dietary or environmental features can
modulate the probability that dense methylation at FMR1 will accrue in individuals
with the full-mutation allele.
FIGURE 3-4 Comparing the Molecular Mechanisms of Fragile X and FSHD. A, FMR1 in normal,
expanded permutation, and full-mutation states. B, DUX4 in normal and contracted states.
In another genetic-epigenetic disease, fascioscapulohumeral muscular
dystrophy (FSHMD) (see Figure 3-4, B), the disease phenotype arises through loss
of normal methylation rather than gain of abnormal methylation. Symptoms of the
disease include adverse impacts on skeletal musculature. Though lifespan is not
typically reduced by the disease, wheelchair use becomes necessary late in life for a
subset of individuals. The primary genetic event in FSHMD is deletion of a
nucleotide repeat in the DUX4 gene (see Figure 3-4, A). In normal individuals, the
D4Z4 gene promoter has between 11 and 150 copies. This number is typically found
to have been reduced by mutation in individuals with FSHMD, who usually have
only 1 to 10 such repeats. In healthy individuals with a normal-sized allele, the D4Z4
promoter typically has dense methylation. In individuals with reduced copy-counts,
the normally dense methylation is lost (see Figure 3-4, A).22 The disease allele
typically also has fewer repressive histone marks than does the normal allele.23
Together, fragile X syndrome and FSHMD highlight that both abnormal gain and
abnormal loss of epigenetic modifications can result in disease.
Twin Studies Provide Insights on Epigenetic
Modification
Identical (monozygotic) twin pairs, whose DNA sequences are essentially the same,
offer a unique opportunity to isolate and examine the impacts of epigenetic
modifications. A recent study found that as twins age, they exhibit increasingly
substantial differences in methylation patterns of the DNA sequences of their
somatic cells; these changes are often reflected in increasing numbers of phenotypic
differences. Twins with significant lifestyle differences (e.g., smoking versus
nonsmoking) tend to accumulate larger numbers of differences in their methylation
patterns. These results, along with findings generated in animal studies, suggest that
changes in epigenetic patterns may be an important part of the aging process24
(Figure 3-5).
FIGURE 3-5 Twins and Aging. A, Twins as babies look very much alike but, B, as adults, have
slight differences in appearance, possibly because of epigenetics. (A, vgm/Shutterstock. B, Stacey
Bates/Shutterstock.)
Molecular Approaches to Understand Epigenetic
Disease
Because epigenetic information is not encoded by DNA molecules but instead by
chemical modifications to those molecules, conventional sequencing approaches
are not sufficient to reveal epigenetic differences between normal individuals and
those who have epigenetic modifications associated with disease. To collect
information on DNA methylation states of individual nucleotides, DNA is typically
subjected to bisulfite conversion before sequencing. Bisulfite treatment does not
alter most nucleotides, including methylated cytosines, but deaminates unmethylated
cytosines to uracil.25 Because uracil complements adenine, not guanine, methylated
and unmethylated cytosines can be distinguished in resulting sequence data, so long
as the genetic sequence is known. Histone modification states can be assayed
through the use of antibodies specific for histones with various modifications.26
Quick Check 3-2
1. Evaluate the statement: “Epigenetic information is highly dynamic in early
development.”
2. How does the epigenetic regulation of imprinted genes compare with that of the
rest of the genome?
3. Compare and contrast the molecular mechanisms leading to FX syndrome and to
FSHMD.
Epigenetics and Cancer
DNA Methylation and Cancer
Some of the most extensive evidence for the role of epigenetic modification in
human disease comes from studies of cancer (Figure 3-6).27,28 Tumor cells typically
exhibit genome-wide hypomethylation (decreased methylation), which can increase
the activity of oncogenes (see Chapter 10). Hypomethylation increases as tumors
progress from benign neoplasms to malignancy. In addition, the promoter regions
of tumor-suppressor genes are often hypermethylated, which decreases their rate of
transcription and their ability to inhibit tumor formation. Hypermethylation of the
promoter region of the RB1 gene is often seen in retinoblastoma29;
hypermethylation of the BRCA1 gene is seen in some cases of inherited breast
cancer (Chapter 33).30
FIGURE 3-6 Global Epigenomic Alterations and Cancer. Oncogenesis often occurs through a
combination of genetic mutations and epigenetic change. In cancer cells, the promoters of
tumor-suppressor genes typically become hypermethylated, leading, in combination with
histone modifications, to abnormal gene silencing. Because tumor-suppressor genes typically
help to control cell division, their silencing can result in tumor progression. Global
hypomethylation leads to chromosomal instability and fragility, and increases the risk of
additional genetic mutations. Additionally, these modifications create abnormal mRNA and
miRNA expression, which leads to activation of oncogenes and silencing of tumor-suppressor
genes. (Adapted from Sandoval J, Esteller M: Cancer epigenomics: beyond genomics, Curr Opin Genet Dev 22:50-55, 2012.)
A major cause of one form of inherited colon cancer (hereditary nonpolyposis
colorectal cancer [HNPCC]) is the methylation of the promoter region of a gene,
MLH1, whose protein product repairs damaged DNA. When MLH1 becomes
inactive, DNA damage accumulates, eventually resulting in colon tumors31,32.
Abnormal methylation of tumor-suppressor genes also is common in the
progression of Barrett esophagus, a condition in which the lining of the esophagus
is replaced by cells that have features associated with the lower intestinal tract, and
to adenocarcinoma possibly through up-regulation of one of the enzymes that adds
methyl groups to DNA.33
miRNAs and Cancer
Hypermethylation also is seen in microRNA genes, which encode small (22 base
pair) RNA molecules that bind to the ends of mRNAs, degrading them and
preventing their translation. More than 1000 microRNA sequences have been
identified in humans, and hypermethylation of specific subgroups of microRNAs is
associated with tumorigenesis. When microRNA genes are methylated, their mRNA
targets are overexpressed, and this overexpression has been associated with
metastasis.27
Epigenetic Screening for Cancer
The common finding of epigenetic alteration in cancerous tissue raises the
possibility that epigenetic screening approaches could complement or even replace
existing early-detection methods. In some cases, epigenetic screening could be done
using bodily fluids, such as urine or sputum, eliminating the need for the more
invasive, costly, and risky strategies currently in place. Monitoring for
misregulation of miRNAs has shown promise as a tool for early diagnosis of
cancers of the colon,34 breast,35 and prostate.36 Other epigenetics-based screening
approaches have shown promise for detection of cancers of the bladder,37 lung,38
and prostate.39
Emerging Strategies for the Treatment of
Epigenetic Disease
Epigenetic modifications are potentially reversible: DNA can be demethylated,
histones can be modified to change the transcriptional state of nearby DNA, and
miRNA-encoding loci can be up-regulated or down-regulated. This raises the
prospect for treating epigenetic disease with pharmaceutical agents that directly
reverse the changes associated with the disease phenotype. In recent years,
interventions involving all three types of epigenetic modulators (DNA methylation,
histone modification, and miRNAs) have shown considerable promise for the
treatment of disease.
DNA Demethylating Agents
5-Azacytidine (Figure 3-7) has been used as a therapeutic drug in the treatment of
leukemia and myelodysplastic syndrome.40 A cytosine analog, 5-azacytidine, is
incorporated into DNA opposite its complementary nucleotide, guanine. 5-
Azacytidine differs from cytosine in that it has a nitrogen, rather than a carbon, in
the 5th position of its cytidine ring. As result, the DNMTs cannot add methyl groups
to 5-azacytidine, and DNAs that contain 5-azacytidine decline in their methylation
density over successive rounds of DNA replication.41 Administration of 5-
azacytidine is associated with various side effects, including digestive disturbance,
but has shown promise in the treatment of diseases, including pancreatic cancer42
and myelodysplastic syndromes.43,44
FIGURE 3-7 5-Azacytosine as Demethylating Agent. A, Unmethylated cytosines in DNA are
typically subject to the addition of methyl groups by DNMT1, a DNA methyltransferase, using
methyl groups supplied by the methyl donor S-adenosylmethionine. B, In 5-Azacytosine, the 5′
carbon of cytosine is replaced with a nitrogen. This chemical difference is sufficient both to
block the addition of a methyl group and to confer irreversible binding to DNMT1. Incorporation
of 5-Azacytosine into DNA is therefore sufficient to drive passive loss of methylation from
replicating DNA, and thus to reactivate hypermethylated loci. 5-Azacytosine, bound to a sugar,
can be integrated into DNA, and has been administered with some success in treating
epigenetic diseases that arise through hypermethylation of individual loci.
Histone Deacetylase Inhibitors
The activity of the histone deacetylases (HDACs) increases chromatin compaction,
decreasing transcriptional activity (see Figure 3-7). In many cases, excessive activity
of HDACs results in transcriptional inactivation of tumor-suppressor genes, leading
ultimately to the development of tumors. Treatment with HDAC inhibitors, either
alone or in combination with other drugs, has shown promise in the treatment of
cancers of the breast45 and prostate,46 but only very limited success in the treatment
of pancreatic cancer.47
miRNA Coding
A major challenge in developing drugs that modify epigenetic alterations is to
target only the genes responsible for a specific cancer. Therapeutic approaches that
use microRNA offer a potential solution to this problem as treatment can be targeted
to individual loci using sequence characteristics of relevant RNA molecules.
Quick Check 3-3
1. Assess the statement that cancer is, in many cases, an epigenetic disease.
2. Discuss the role of miRNAs in cancer.
3. Describe a potential strategy for the treatment of epigenetic disease.
Future Directions
Robust experimental observations are clarifying the roles of epigenetic states in
determining cell fates and disease phenotypes. The well-documented involvement of
epigenetic abnormalities in carcinogenesis and the mounting evidence for these
epigenetic changes in other common diseases (discussed in other chapters) will
likely elucidate possibilities for reversing the epigenetic abnormalities and possibly
preventing their establishment in utero.
Did You Understand?
Overview
1. Why are pairs of identical twins especially useful in the study of epigenetic
phenomena?
2. Describe some of the challenges of developing pharmaceutical approaches to
remedy abnormal epigenetic states.
Epigenetics and Human Development
1. Epigenetics modification alters gene expression without changes to DNA
sequence.
2. Investigators are studying three major types of epigenetic processes: (1) DNA
methylation, which results from attachment of a methyl group to a cytosine; in the
somatic cells, all or nearly all methylation occurs at cytosines that are followed by
guanines (“CpG dinucleotides”); (2) histone modification, through the addition of
various chemical groups, including methyl and acetyl; and (3) noncoding RNAs
(ncRNAs or miRNAs), short nucleotides derived from introns of protein coding
genes or transcribed as independent genes from regions of the genome whose
functions, if any, remain poorly understood. MicroRNAs regulate diverse signaling
pathways.
3. DNA methylation is, at present, the best-studied epigenetic process. When a gene
becomes heavily methylated the DNA is less likely to be transcribed into mRNA.
4. Methylation, along with histone hypoacetylation and condensation of chromatin,
inhibits the binding of proteins that promote transcription, such that the gene
becomes transcriptionally inactive.
5. Environmental factors, such as diet and exposure to certain chemicals, may cause
epigenetic modifications.
6. The heritable transmission to future generations of epigenetic modifications is
called transgenerational inheritance.
7. As twins age, they demonstrate increasing differences in methylation patterns of
their DNA sequences, causing increasing numbers of phenotypic differences.
8. In studies of twins with significant lifestyle differences (e.g., smoking versus
nonsmoking) large numbers of differences in their methylation patterns are
observed to accrue over time.
Genomic Imprinting
1. Gregor Mendel’s experiments with garden peas demonstrated that the phenotype
is the same whether a given allele is inherited from the mother or the father. This
principle, which has long been part of the central dogma of genetics, does not
always hold. For some human genes, a given gene is transcriptionally active on
only one copy of a chromosome (e.g., the copy inherited from the father). On the
other copy of the chromosome (the one inherited from the mother) the gene is
transcriptionally inactive. This process of gene silencing, in which genes are
silenced depending on which parent transmits them, is known as imprinting; the
transcriptionally silenced genes are said to be “imprinted.”
2. When an allele is imprinted, it typically has heavy methylation. By contrast, the
nonimprinted allele is typically not methylated.
3. A well-known disease example of imprinting is associated with a deletion of
about 4 million base pairs (Mb) of the long arm of chromosome 15. When this
deletion is inherited from the father, the child manifests Prader-Willi syndrome.
4. The same 4 Mb deletion, when inherited from the mother, causes Angelman
syndrome.
5. Another well-known example of imprinting is Beckwith-Wiedemann syndrome,
an overgrowth condition accompanied by an increased predisposition to cancer.
6. Whereas up-regulation, or extra copies, of active IGF2 causes overgrowth in
Beckwith-Wiedemann syndrome, down-regulation of IGF2 causes the diminished
growth seen in Russell-Silver syndrome.
Long-Term and Multigenerational Persistence of
Epigenetic States Induced by Stochastic and
Environmental Factors
1. Events encountered in utero, in childhood, and in adolescence can result in
specific epigenetic changes that yield a wide range of phenotypic abnormalities,
including metabolic syndromes.
2. Fetal alcohol syndrome, which results from ethanol exposure in utero, may be
mediated by the repressive impact of ethanol on the DNA methyltransferases.
3. Both abnormal gain of methylation, as in the case of fragile X syndrome, and
abnormal loss of methylation, as in the case of FSHMD, can produce disease
phenotypes.
Epigenetics and Cancer
1. The best evidence for epigenetic effects on disease risk comes from studies of
human cancer.
2. Methylation densities decline as tumors progress, which can increase the activity
of oncogenes, causing tumors to progress from benign neoplasms to malignancy.
Additionally, the promoter regions of tumor-suppressor genes are often
hypermethylated. These elevated methylation levels decreases their rate of
transcription at these critical genes, thus reducing the ability to inhibit tumor
formation.
3. Hypermethylation also is seen in microRNA genes and is associated with
tumorigenesis.
4. Unlike DNA sequence mutations, epigenetic modifications can be reversed
through pharmaceutical intervention. For example, 5-azacytidine, a demethylating
agent, has been used as a therapeutic drug in the treatment of leukemia and
myelodysplastic syndrome.
Future Directions
1. Robust experimental observations are defining the roles of epigenetic states in
shaping cell fates.
2. The well-documented involvement of epigenetic abnormalities in carcinogenesis
and the mounting evidence for these epigenetic changes in other common diseases
(discussed throughout the text) will likely elucidate new therapies with the
possibilities of reversing the epigenetic abnormalities.
Key Terms
5-Azacytidine, 70
Angelman syndrome, 65
Beckwith-Wiedemann syndrome, 65
Biallelic, 64
DNA methylation, 62
Embryonic stem cell, 64
Epigenetics, 62
Fascioscapulohumeral muscular dystrophy (FSHMD), 68
Fragile X, 67
Histone, 63
Histone modification, 63
Housekeeping genes, 64
Imprinted, 64
MicroRNA (miRNA), 64
Monoallelic, 64
Noncoding RNA (ncRNA), 64
Prader-Willi syndrome, 65
Russell-Silver syndrome, 66
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4
Altered Cellular and Tissue Biology
Kathryn L. McCance, Todd Cameron Grey
CHAPTER OUTLINE
Cellular Adaptation, 74
Atrophy, 74
Hypertrophy, 75
Hyperplasia, 76
Dysplasia: Not a True Adaptive Change, 77
Metaplasia, 77
Cellular Injury, 77
General Mechanisms of Cell Injury, 78
Hypoxic Injury, 78
Free Radicals and Reactive Oxygen Species—
Oxidative Stress, 81
Chemical or Toxic Injury, 84
Unintentional and Intentional Injuries, 93
Infectious Injury, 96
Immunologic and Inflammatory Injury, 96
Manifestations of Cellular Injury: Accumulations, 96
Water, 97
Lipids and Carbohydrates, 98
Glycogen, 98
Proteins, 98
Pigments, 99
Calcium, 100
Urate, 101
Systemic Manifestations, 101
Cellular Death, 101
Necrosis, 102
Apoptosis, 104
Autophagy, 105
Aging and Altered Cellular and Tissue Biology, 107
Normal Life Span, Life Expectancy, and Quality-
Adjusted Life Year, 108
Degenerative Extracellular Changes, 108
Cellular Aging, 108
Tissue and Systemic Aging, 109
Frailty, 109
Somatic Death, 109
The majority of diseases are caused by many factors acting together (i.e.,
multifactorial) or interacting with a genetically susceptible person. Injury to cells
and their surrounding environment, called the extracellular matrix, leads to tissue
and organ injury. Although the normal cell is restricted by a narrow range of
structure and functions, including metabolism and specialization, it can adapt to
physiologic demands or stress to maintain a steady state called homeostasis.
Adaptation is a reversible, structural, or functional response both to normal or
physiologic conditions and to adverse or pathologic conditions. For example, the
uterus adapts to pregnancy—a normal physiologic state—by enlarging.
Enlargement occurs because of an increase in the size and number of uterine cells.
In an adverse condition, such as high blood pressure, myocardial cells are
stimulated to enlarge by the increased work of pumping. Like most of the body’s
adaptive mechanisms, however, cellular adaptations to adverse conditions are
usually only temporarily successful. Severe or long-term stressors overwhelm
adaptive processes and cellular injury or death ensues. Altered cellular and tissue
biology can result from adaptation, injury, neoplasia, accumulations, aging, or
death. (Neoplasia is discussed in Chapters 10 and 11.)
Knowledge of the structural and functional reactions of cells and tissues to
injurious agents, including genetic defects, is vital to understanding disease
processes. Cellular injury can be caused by any factor that disrupts cellular
structures or deprives the cell of oxygen and nutrients required for survival. Injury
may be reversible (sublethal) or irreversible (lethal) and is classified broadly as
chemical, hypoxic (lack of sufficient oxygen), free radical, intentional,
unintentional, immunologic, infection, and inflammatory. Cellular injuries from
various causes have different clinical and pathophysiologic manifestations. Stresses
from metabolic derangements may be associated with intracellular accumulations
and include carbohydrates, proteins, and lipids. Sites of cellular death can cause
accumulations of calcium resulting in pathologic calcification. Cellular death is
confirmed by structural changes seen when cells are stained and examined under a
microscope. The two main types of cell death include necrosis and apoptosis and
nutrient deprivation can initiate autophagy that results in cell death. All of these
pathways of cellular death are discussed later in this chapter.
Cellular aging causes structural and functional changes that eventually may lead
to cellular death or a decreased capacity to recover from injury. Mechanisms
explaining how and why cells age are not known, and distinguishing between
pathologic changes and physiologic changes that occur with aging is often difficult.
Aging clearly causes alterations in cellular structure and function, yet senescence,
growing old, is both inevitable and normal.
Cellular Adaptation
Cells adapt to their environment to escape and protect themselves from injury. An
adapted cell is neither normal nor injured—its condition lies somewhere between
these two states. Adaptations are reversible changes in cell size, number, phenotype,
metabolic activity, or functions of cells.1 Adaptive responses have limits, however,
and additional cell stresses can affect essential cell function leading to cell injury.
Cellular adaptations also can be a common and central part of many disease states.
In the early stages of a successful adaptive response, cells may have enhanced
function; thus, it is hard to distinguish a pathologic response from an extreme
adaptation to an excessive functional demand. The most significant adaptive changes
in cells include atrophy (decrease in cell size), hypertrophy (increase in cell size),
hyperplasia (increase in cell number), and metaplasia (reversible replacement of
one mature cell type by another less mature cell type or a change in the phenotype).
Dysplasia (deranged cellular growth) is not considered a true cellular adaptation but
rather an atypical hyperplasia. These changes are shown in Figure 4-1.
FIGURE 4-1 Adaptive and Dysplastic Alterations in Simple Cuboidal Epithelial Cells.
Atrophy
Atrophy is a decrease or shrinkage in cellular size. If atrophy occurs in a sufficient
number of an organ’s cells, the entire organ shrinks or becomes atrophic. Atrophy
can affect any organ, but it is most common in skeletal muscle, the heart, secondary
sex organs, and the brain. Atrophy can be classified as physiologic or pathologic.
Physiologic atrophy occurs with early development. For example, the thymus
gland undergoes physiologic atrophy during childhood. Pathologic atrophy
occurs as a result of decreases in workload, pressure, use, blood supply, nutrition,
hormonal stimulation, and nervous system stimulation (Figure 4-2). Individuals
immobilized in bed for a prolonged time exhibit a type of skeletal muscle atrophy
called disuse atrophy. Aging causes brain cells to become atrophic and endocrine-
dependent organs, such as the gonads, to shrink as hormonal stimulation decreases.
Whether atrophy is caused by normal physiologic conditions or by pathologic
conditions, atrophic cells exhibit the same basic changes.
FIGURE 4-2 Atrophy. A, Normal brain of a young adult. B, Atrophy of the brain in an 82-year-old
male with atherosclerotic cerebrovascular disease, resulting in reduced blood supply. Note that
loss of brain substance narrows the gyri and widens the sulci. The meninges have been
stripped from the right half of each specimen to reveal the surface of the brain. (From Kumar V et al,
editors: Robbins and Cotran pathologic basis of disease, ed 9, Philadelphia, 2015, Elsevier.)
The atrophic muscle cell contains less endoplasmic reticulum (ER) and fewer
mitochondria and myofilaments (part of the muscle fiber that controls contraction)
than found in the normal cell. In muscular atrophy caused by nerve loss, oxygen
consumption and amino acid uptake are immediately reduced. The mechanisms of
atrophy include decreased protein synthesis, increased protein catabolism, or both.
A new hypothesis includes ribosome function and its role as translation machinery
or the conversion of mRNA into protein called ribosome biogenesis. Ribosome
biogenesis has an important role in the regulation of skeletal muscle mass.2 The
primary pathway of protein catabolism is the ubiquitin-proteasome pathway and
catabolism involves proteasomes (protein-degrading complexes. Proteins degraded
in this pathway are first conjugated to ubiquitin (another small protein) and then
degraded by proteasomes. An increase in proteasome activity is characteristic of
atrophic muscle changes. Deregulation of this pathway often leads to abnormal cell
growth and is associated with cancer and other diseases.
Atrophy as a result of chronic malnutrition is often accompanied by a “self-
eating” process called autophagy that creates autophagic vacuoles (see p. 105).
These vacuoles are membrane-bound vesicles within the cell that contain cellular
debris and hydrolytic enzymes, which function to break down substances to the
simplest units of fat, carbohydrate, or protein. The levels of hydrolytic enzymes rise
rapidly in atrophy. The enzymes are isolated in autophagic vacuoles to prevent
uncontrolled cellular destruction. Thus the vacuoles form as needed to protect
uninjured organelles from the injured organelles and are eventually engulfed and
destroyed by lysosomes. Certain contents of the autophagic vacuole may resist
destruction by lysosomal enzymes and persist in membrane-bound residual bodies.
An example of this is granules that contain lipofuscin, the yellow-brown age
pigment. Lipofuscin accumulates primarily in liver cells, myocardial cells, and
atrophic cells.
Hypertrophy
Hypertrophy is a compensatory increase in the size of cells in response to
mechanical stimuli (also called mechanical load or stress, such as from repetitive
stretching, chronic pressure, or volume overload) and consequently increases the
size of the affected organ (Figures 4-3 and 4-4). The cells of the heart and kidneys
are particularly prone to enlargement. Hypertrophy, as an adaptive response
(muscular enlargement), occurs in the striated muscle cells of both the heart and
skeletal muscles. Initial cardiac enlargement is caused by dilation of the cardiac
chambers, is short lived, and is followed by increased synthesis of cardiac muscle
proteins, allowing muscle fibers to do more work. The increase in cellular size is
associated with an increased accumulation of protein in the cellular components
(plasma membrane, ER, myofilaments, mitochondria) and not with an increase in
cellular fluid. Yet, individual protein pools may expand or shrink.3 Cardiac
hypertrophy involves changes in signaling and transcription factor pathways
resulting in increased protein synthesis leading to left ventricular hypertrophy
(LVH). Emerging evidence suggests that the ubiquitin-proteasome system (UPS) not
only attends to damaged, misfolded, or mutant proteins by protein breakdown but
also may attend to cell growth eventually leading to LVH.4 With time, cardiac
hypertrophy is characterized by extracellular matrix remodeling and increased
growth of adult myocytes. The myocytes progressively increase in size and reach a
limit beyond which no further hypertrophy can occur.5,6
FIGURE 4-3 Hypertrophy of Cardiac Muscle in Response to Valve Disease. A, Transverse slices
of a normal heart and a heart with hypertrophy of the left ventricle (L, normal thickness of left
ventricular wall; T, thickened wall from heart in which severe narrowing of aortic valve caused
resistance to systolic ventricular emptying). B, Histology of cardiac muscle from the normal
heart. C, Histology of cardiac muscle from a hypertrophied heart. (From Stevens A, Lowe J: Pathology:
illustrated review in color, ed 2, Edinburgh, 2000, Mosby.)
FIGURE 4-4 Mechanisms of Myocardial Hypertrophy. Mechanical sensors appear to be the
main stimulators for physiologic hypertrophy. Other stimuli possibly more important for
pathologic hypertrophy include agonists (initiators) and growth factors. These factors then
signal transcription pathways whereby transcription factors then bind to DNA sequences,
activating muscle proteins that are responsible for hypertrophy. These pathways include
induction of embryonic/fetal genes, increased synthesis of contractile proteins, and production
of growth factors. (Adapted from Kumar V et al, editors: Robbins and Cotran pathologic basis of disease, ed 9, Philadelphia, 2015,
Elsevier.)
Although hypertrophy can be classified as physiologic or pathologic, time may be
the critical factor or determinant of the transition from physiologic to pathologic
cardiac hypertrophy. With physiologic hypertrophy, preservation of myocardial
structure characterizes postnatal development, moderate endurance exercise
training, pregnancy, and the early phases of increased pressure and volume loading
on the adult human heart. This physiologic response is temporary; however, aging,
strenuous exercise, and sustained workload or stress lead to pathologic hypertrophy
with structural and functional manifestations. Pathologic hypertrophy in the heart is
secondary to hypertension, coronary heart disease, or problem valves and is
presumably a key risk factor for heart failure. Additionally, it is associated with
increased interstitial fibrosis, cell death, and abnormal cardiac function (see Figure
4-3). Historically, the progression of pathologic cardiac hypertrophy has been
considered irreversible. Emerging data, however, from experimental studies and
clinical observations show in certain cases reversal of pathologic cardiac
hypertrophy. Cardiac hypertrophy can be reversed when the increased wall stress is
normalized, a process termed regression.7 For example, unloading of hemodynamic
stress by a left ventricular assist device (used in individuals with heart failure for
bridging to heart transplantation) induces regression of cardiac hypertrophy and
improvement of left ventricular (LV) function in those with end-stage heart failure.8
Regression of cardiac hypertrophy is accompanied by activation of unique sets of
genes, including fetal-type genes and those involved in protein degradation.9,10
However, the signaling mechanisms mediating regression of cardiac hypertrophy
have been poorly understood. Improvement in new blood vessel development
(angiogenesis) in the hypertrophic heart can lead to regression of the hypertrophy
and prevention of heart failure.11,12 In mice, dietary supplementation of
physiologically relevant levels of copper can reverse pathologic cardiac
hypertrophy.12,13
When a diseased kidney is removed, the remaining kidney adapts to the increased
workload with an increase in both the size and the number of cells. The major
contributing factor to this renal enlargement is hypertrophy. Another example of
normal or physiologic hypertrophy is the increased growth of the uterus and
mammary glands in response to pregnancy.
Hyperplasia
Hyperplasia is an increase in the number of cells, resulting from an increased rate
of cellular division. Hyperplasia, as a response to injury, occurs when the injury has
been severe and prolonged enough to have caused cell death. Loss of epithelial cells
and cells of the liver and kidney triggers deoxyribonucleic acid (DNA) synthesis
and mitotic division. Increased cell growth is a multistep process involving the
production of growth factors, which stimulate the remaining cells to synthesize new
cell components and, ultimately, to divide. Hyperplasia and hypertrophy often occur
together, and both take place if the cells can synthesize DNA.
Two types of normal, or physiologic, hyperplasia are compensatory hyperplasia
and hormonal hyperplasia. Compensatory hyperplasia is an adaptive mechanism
that enables certain organs to regenerate. For example, removal of part of the liver
leads to hyperplasia of the remaining liver cells (hepatocytes) to compensate for the
loss. Even with removal of 70% of the liver, regeneration is complete in about 2
weeks. Several growth factors and cytokines (chemical messengers) are induced and
play critical roles in liver regeneration.
Not all types of mature cells have the same capacity for compensatory
hyperplastic growth. Nondividing tissues contain cells that can no longer (i.e.,
postnatally) go through the cell cycle and undergo mitotic division. These highly
specialized cells, for example, neurons and skeletal muscle cells, never divide again
once they have differentiated—that is, they are terminally differentiated.14 In human
cells, cell growth and cell division depend on signals from other cells; but cell
growth, unlike cell division, does not depend on the cell-cycle control system.14
Nerve cells and most muscle cells do most of their growing after they have
terminally differentiated and permanently ceased dividing.14 Significant
compensatory hyperplasia occurs in epidermal and intestinal epithelia, hepatocytes,
bone marrow cells, and fibroblasts; and some hyperplasia is noted in bone,
cartilage, and smooth muscle cells. Another example of compensatory hyperplasia
is the callus, or thickening, of the skin as a result of hyperplasia of epidermal cells
in response to a mechanical stimulus.
Hormonal hyperplasia occurs chiefly in estrogen-dependent organs, such as the
uterus and breast. After ovulation, for example, estrogen stimulates the
endometrium to grow and thicken in preparation for receiving the fertilized ovum.
If pregnancy occurs, hormonal hyperplasia, as well as hypertrophy, enables the
uterus to enlarge. (Hormone function is described in Chapters 19 and 33.)
Pathologic hyperplasia is the abnormal proliferation of normal cells, usually in
response to excessive hormonal stimulation or growth factors on target cells
(Figure 4-5). The most common example is pathologic hyperplasia of the
endometrium (caused by an imbalance between estrogen and progesterone
secretion, with oversecretion of estrogen) (see Chapter 33). Pathologic endometrial
hyperplasia, which causes excessive menstrual bleeding, is under the influence of
regular growth-inhibition controls. If these controls fail, hyperplastic endometrial
cells can undergo malignant transformation. Benign prostatic hyperplasia is another
example of pathologic hyperplasia and results from changes in hormone balance. In
both of these examples, if the hormonal imbalance is corrected, hyperplasia
regresses.1
FIGURE 4-5 Hyperplasia of the Prostate with Secondary Thickening of the Obstructed Urinary
Bladder (Bladder Cross Section). The enlarged prostate is seen protruding into the lumen of the
bladder, which appears trabeculated. These “trabeculae” result from hypertrophy and
hyperplasia of smooth muscle cells that occur in response to increased intravesical pressure
caused by urinary obstruction. (From Damjanov I: Pathology for the health professions, ed 4, St Louis, 2012, Saunders.)
Dysplasia: Not a True Adaptive Change
Dysplasia refers to abnormal changes in the size, shape, and organization of mature
cells (Figure 4-6). Dysplasia is not considered a true adaptive process but is related
to hyperplasia and is often called atypical hyperplasia. Dysplastic changes often are
encountered in epithelial tissue of the cervix and respiratory tract, where they are
strongly associated with common neoplastic growths and often are found adjacent
to cancerous cells. Importantly, however, the term dysplasia does not indicate cancer
and may not progress to cancer. Dysplasia is often classified as mild, moderate, or
severe; yet, because this classification scheme is somewhat subjective, it has
prompted some to recommend the use of either “low grade” or “high grade”
instead. If the inciting stimulus is removed, dysplastic changes often are reversible.
(Dysplasia is discussed further in Chapter 10.)
FIGURE 4-6 Dysplasia of the Uterine Cervix. A, Mild dysplasia. B, Severe dysplasia. (From Damjanov I,
Linder J: Anderson’s pathology, ed 10, St Louis, 1996, Mosby.)
Metaplasia
Metaplasia is the reversible replacement of one mature cell type (epithelial or
mesenchymal) by another, sometimes less differentiated, cell type. It is thought to
develop, as an adaptive response better suited to withstand the adverse environment,
from a reprogramming of stem cells that exist on most epithelia or of
undifferentiated mesenchymal (tissue from embryonic mesoderm) cells present in
connective tissue. These precursor cells mature along a new pathway because of
signals generated by growth factors in the cell’s environment. The best example of
metaplasia is replacement of normal columnar ciliated epithelial cells of the
bronchial (airway) lining by stratified squamous epithelial cells (Figure 4-7). The
newly formed cells do not secrete mucus or have cilia, causing loss of a vital
protective mechanism. Bronchial metaplasia can be reversed if the inducing
stimulus, usually cigarette smoking, is removed. With prolonged exposure to the
inducing stimulus, however, dysplasia and cancerous transformation can occur.
FIGURE 4-7 Reversible Changes in Cells Lining the Bronchi.
Cellular Injury
Injury to cells and to the extracellular matrix (ECM) leads to injury of tissues and
organs, ultimately determining the structural patterns of disease. Loss of function is
derived from cell and ECM injury and cell death. Cellular injury occurs if the cell is
unable to maintain homeostasis—a normal or adaptive steady state—in the face of
injurious stimuli or stress. Injured cells may recover (reversible injury) or die
(irreversible injury). Injurious stimuli include chemical agents, lack of sufficient
oxygen (hypoxia), free radicals, infectious agents, physical and mechanical factors,
immunologic reactions, genetic factors, and nutritional imbalances. Types of
injuries and their responses are summarized in Table 4-1 and Figure 4-8.
TABLE 4-1
Types of Progressive Cell Injury and Responses
Type Responses
Adaptation Atrophy, hypertrophy, hyperplasia, metaplasia
Active cell injury Immediate response of “entire” cell
Reversible Loss of ATP, cellular swelling, detachment of ribosomes, autophagy of lysosomes
Irreversible “Point of no return” structurally when severe vacuolization of mitochondria occurs and Ca++ moves into cell
Necrosis Common type of cell death with severe cell swelling and breakdown of organelles
Apoptosis, or programmed cell death Cellular self-destruction for elimination of unwanted cell populations
Autophagy Eating of self, cytoplasmic vesicles engulf cytoplasm and organelles, recycling factory
Chronic cell injury (subcellular alterations) Persistent stimuli response may involve only specific organelles or cytoskeleton (e.g., phagocytosis of bacteria)
Accumulations or infiltrations Water, pigments, lipids, glycogen, proteins
Pathologic calcification Dystrophic and metastatic calcification
ATP, Adenosine triphosphate; Ca++, calcium.
FIGURE 4-8 Stages of Cellular Adaptation, Injury, and Death. The normal cell responds to
physiologic and pathologic stresses by adapting (atrophy, hypertrophy, hyperplasia, metaplasia).
Cell injury occurs if the adaptive responses are exceeded or compromised by injurious agents,
stress, and mutations. The injury is reversible if it is mild or transient, but if the stimulus
persists the cell suffers irreversible injury and eventually death.
The extent of cellular injury depends on the type, state (including level of cell
differentiation and increased susceptibility to fully differentiated cells), and adaptive
processes of the cell, as well as the type, severity, and duration of the injurious
stimulus. Two individuals exposed to an identical stimulus may incur varying
degrees of cellular injury. Modifying factors, such as nutritional status, can
profoundly influence the extent of injury. The precise “point of no return” that leads
to cellular death is a biochemical puzzle, but once changes to the nucleus occur and
cell membranes are disrupted, the cell moves to irreversible injury and death.
General Mechanisms of Cell Injury
Common biochemical themes are important to understanding cell injury and cell
death regardless of the injuring agent. These include adenosine triphosphate (ATP)
depletion, mitochondrial damage, oxygen and oxygen-derived free radical
membrane damage (depletion of ATP), protein folding defects, DNA damage
defects, and calcium level alterations (Table 4-2). Examples of common forms of
cell injury are (1) hypoxic injury, (2) free radicals and reactive oxygen species
injury, and (3) chemical injury.
TABLE 4-2
Common Themes in Cell Injury and Cell Death
Theme Comments
ATP depletion Loss of mitochondrial ATP and decreased ATP synthesis; results include cellular swelling, decreased protein synthesis, decreased membrane
transport, and lipogenesis, all changes that contribute to loss of integrity of plasma membrane
Reactive
oxygen species
(↑ROS)
Lack of oxygen is key in progression of cell injury in ischemia (reduced blood supply); activated oxygen species (ROS, , H
2
O
2
, OH•)
cause destruction of cell membranes and cell structure
Ca++ entry Normally intracellular cytosolic calcium concentrations are very low; ischemia and certain chemicals cause an increase in cytosolic Ca++
concentrations; sustained levels of Ca++ continue to increase with damage to plasma membrane; Ca++ causes intracellular damage by
activating a number of enzymes
Mitochondrial
damage
Can be damaged by increases in cytosolic Ca++, ROS; two outcomes of mitochondrial damage are loss of membrane potential, which causes
depletion of ATP and eventual death or necrosis of cell, and activation of another type of cell death (apoptosis) (see p. 104)
Membrane
damage
Early loss of selective membrane permeability found in all forms of cell injury, lysosomal membrane damage with release of enzymes
causing cellular digestion
Protein
misfolding,
DNA damage
Proteins may misfold, triggering unfolded protein response that activates corrective responses; if overwhelmed, response activates cell
suicide program or apoptosis; DNA damage (genotoxic stress) also can activate apoptosis (see p. 104)
ATP, Adenosine triphosphate; Ca++, calcium.
Hypoxic Injury
Hypoxia, or lack of sufficient oxygen within cells, is the single most common cause
of cellular injury (Figure 4-9). Hypoxia can result from a reduced amount of
oxygen in the air, loss of hemoglobin or decreased efficacy of hemoglobin,
decreased production of red blood cells, diseases of the respiratory and
cardiovascular systems, and poisoning of the oxidative enzymes (cytochromes)
within the cells. Hypoxia plays a role in physiologic processes including cell
differentiation, angiogenesis, proliferation, erythropoiesis, and overall cell
viability.15 The main consumers of oxygen are mitochondria and the cellular
responses to hypoxia are reported to be mediated by the production of reactive
oxygen species (ROS) at the mitochondrial complex III.15 Investigators are studying
the role of ROS as hypoxia signaling molecules. More commonly, hypoxia is
associated with the pathophysiologic conditions such as inflammation, ischemia,
and cancer. Hypoxia can induce inflammation and inflamed lesions can become
hypoxic (Figure 4-10).16 The cellular mechanisms involved in hypoxia and
inflammation are emerging and include activation of immune responses and
oxygen-sensing compounds called prolyl hydroxylases (PHDs) and hypoxia-
inducible transcription factor (HIF). The hypoxia-inducible factor (HIF) is a
family of transcription regulators that coordinate the expression of many genes in
response to oxygen deprivation. Mammalian development occurs in a hypoxic
environment.17 Hypoxia-induced signaling involves complicated crosstalk between
hypoxia and inflammation, linking hypoxia and inflammation to inflammatory
bowel disease, certain cancers, and infections.16 Research is ongoing to understand
the mechanisms of how tumors adapt to low oxygen levels by inducing
angiogenesis, increasing glucose consumption, and promoting the metabolic state
of glycolysis.18
FIGURE 4-9 Hypoxic Injury Induced by Ischemia. A, Consequences of decreased oxygen
delivery or ischemia with decreased ATP. The structural and physiologic changes are reversible
if oxygen is delivered quickly. Significant decreases in ATP result in cell death, mostly by
necrosis. B, Mitochondrial damage can result in changes in membrane permeability, loss of
membrane potential, and decrease in ATP concentration. Between the outer and inner
membranes of the mitochondria are proteins that can activate the cell’s suicide pathways,
called apoptosis. C, Calcium ions are critical mediators of cell injury. Calcium ions are usually
maintained at low concentrations in the cell’s cytoplasm; thus ischemia and certain toxins can
initially cause an increase in the release of Ca++ from intracellular stores and later an increased
movement (influx) across the plasma membrane. (Adapted from Kumar V et al, editors: Pathology, St Louis, 2014,
Elsevier.)
FIGURE 4-10 Hypoxia and Inflammation. Shown is a simplified drawing of clinical conditions
characterized by tissue hypoxia that causes inflammatory changes (left) and inflammatory
diseases that ultimately lead to hypoxia (right). These diseases and conditions are discussed in
more detail in their respective chapters. (Adapted from Eltzschig HK, Carmeliet P: Hypoxia and inflammation, N Engl J Med
364:656-665, 2011.)
The most common cause of hypoxia is ischemia (reduced blood supply).
Ischemic injury often is caused by the gradual narrowing of arteries
(arteriosclerosis) or complete blockage by blood clots (thrombosis), or both.
Progressive hypoxia caused by gradual arterial obstruction is better tolerated than
the acute anoxia (total lack of oxygen) caused by a sudden obstruction, as with an
embolus (a blood clot or other blockage in the circulation). An acute obstruction in
a coronary artery can cause myocardial cell death (infarction) within minutes if the
blood supply is not restored, whereas the gradual onset of ischemia usually results
in myocardial adaptation. Myocardial infarction and stroke, which are common
causes of death in the United States, generally result from atherosclerosis (a type of
arteriosclerosis) and consequent ischemic injury. (Vascular obstruction is discussed
in Chapter 24.)
Cellular responses to hypoxic injury caused by ischemia have been demonstrated
in studies of the heart muscle. Within 1 minute after blood supply to the
myocardium is interrupted, the heart becomes pale and has difficulty contracting
normally. Within 3 to 5 minutes, the ischemic portion of the myocardium ceases to
contract because of a rapid decrease in mitochondrial phosphorylation, causing
insufficient ATP production. Lack of ATP leads to increased anaerobic metabolism,
which generates ATP from glycogen when there is insufficient oxygen. When
glycogen stores are depleted, even anaerobic metabolism ceases.
A reduction in ATP levels causes the plasma membrane’s sodium-potassium (Na+-
K+) pump and sodium-calcium exchange mechanism to fail, which leads to an
intracellular accumulation of sodium and calcium and diffusion of potassium out of
the cell. Sodium and water then can enter the cell freely, and cellular swelling, as
well as early dilation of the endoplasmic reticulum (ER), results. Dilation causes the
ribosomes to detach from the rough ER, reducing protein synthesis. With continued
hypoxia, the entire cell becomes markedly swollen, with increased concentrations of
sodium, water, and chloride and decreased concentrations of potassium. These
disruptions are reversible if oxygen is restored. If oxygen is not restored, however,
vacuolation (formation of vacuoles) occurs within the cytoplasm and swelling of
lysosomes and marked mitochondrial swelling result from damage to the outer
membrane. Continued hypoxic injury with accumulation of calcium subsequently
activates multiple enzyme systems resulting in membrane damage, cytoskeleton
disruption, DNA and chromatin degradation, ATP depletion, and eventual cell death
(see Figures 4-9, C, and 4-27). Structurally, with plasma membrane damage,
extracellular calcium readily moves into the cell and intracellular calcium stores are
released. Increased intracellular calcium levels activate cell enzymes (caspases) that
promote cell death by apoptosis (see Figures 4-29 and 4-33). Persistent ischemia is
associated with irreversible injury and necrosis. Irreversible injury is associated
structurally with severe swelling of the mitochondria, severe damage to plasma
membranes, and swelling of lysosomes. Overall, death is mainly by necrosis but
apoptosis also contributes.1
Restoration of blood flow and oxygen, however, can cause additional injury
called ischemia-reperfusion injury (Figure 4-11). Ischemia-reperfusion injury is
very important clinically because it is associated with tissue damage during
myocardial and cerebral infarction. Several mechanisms are now proposed for
ischemia-reperfusion injury and include:
• Oxidative stress—Reoxygenation causes the increased generation of reactive
oxygen species (ROS) and nitrogen species.1 Highly reactive oxygen intermediates
(oxidative stress) generated include hydroxyl radical (OH−), superoxide radical (
), and hydrogen peroxide (H2O2) (see pp. 82-83). The nitrogen species include
nitric oxide (NO) generated by endothelial cells, macrophages, neurons, and other
cells. These radicals can all cause further membrane damage and mitochondrial
calcium overload. The white blood cells (neutrophils) are especially affected with
reperfusion injury, including neutrophil adhesion to the endothelium. Antioxidant
treatment not only reverses neutrophil adhesion but also can reverse neutrophil-
mediated heart injury. In one study of individuals undergoing elective
percutaneous coronary intervention (PCI), pretreatment with vitamin C was
associated with less myocardial injury.19 The PREVEC Trial (Prevention of
reperfusion damage associated with percutaneous coronary angioplasty following
acute myocardial infarction) seeks to evaluate whether vitamins C and E reduce
infarct size in patients subjected to percutaneous coronary angioplasty after acute
myocardial infarction.20
• Increased intracellular calcium concentration—Intracellular and mitochondrial
calcium overload the cell; this process begins during acute ischemia. Reperfusion
causes even more calcium influx because of cell membrane damage and ROS-
induced injury to the sarcoplasmic reticulum. The increased calcium increases
mitochondrial permeability, eventually leading to depletion of ATP and further cell
injury.
• Inflammation—Ischemic injury increases inflammation because danger signals
(from cytokines) are released by resident immune cells when cells die and this
signaling initiates inflammation.
• Complement activation—The activation of complement may increase the tissue
damage from reperfusion-ischemia injury.1
Quick Check 4-1
1. When does a cell become irreversibly injured?
2. Discuss the pathogenesis of hypoxic injury?
3. What are the mechanisms of ischemia-reperfusion injury?
FIGURE 4-11 Reperfusion Injury. Without oxygen, or anoxia, the cells display hypoxic injury and
become swollen. With reoxygenation, reperfusion injury increases because of the formation of
reactive oxygen radicals that can cause cell necrosis. (Redrawn from Damjanov I: Pathology for the health
professions, ed 3, St Louis, 2006, Saunders.)
Free Radicals and Reactive Oxygen Species—
Oxidative Stress
An important mechanism of cellular injury is injury induced by free radicals,
especially by reactive oxygen species (ROS); this form of injury is called oxidative
stress. Oxidative stress occurs when excess ROS overwhelm endogenous
antioxidant systems. A free radical is an electrically uncharged atom or group of
atoms that has an unpaired electron. Having one unpaired electron makes the
molecule unstable; the molecule becomes stabilized either by donating or by
accepting an electron from another molecule. When the attacked molecule loses its
electron, it becomes a free radical. Therefore it is capable of injurious chemical
bond formation with proteins, lipids, and carbohydrates—key molecules in
membranes and nucleic acids. Free radicals are difficult to control and initiate chain
reactions. They are highly reactive because they have low chemical specificity,
meaning they can react with most molecules in their proximity. Oxidative stress can
activate several intracellular signaling pathways because ROS can modulate
enzymes and transcription factors. This is an important mechanism of cell damage
in many conditions including chemical and radiation injury, ischemia-reperfusion
injury, cellular aging, and microbial killing by phagocytes, particularly neutrophils
and macrophages.1
Free radicals may be generated within cells, first by the reduction-oxidation
reactions (redox reactions) in normal metabolic processes such as respiration.
Under normal physiologic conditions ROS serve as “redox messengers” in the
regulation of intracellular signaling; however, excess ROS may produce
irreversible damage to cellular components. All biologic membranes contain redox
systems, which also are important for cell defense (e.g., inflammation, iron uptake,
growth and proliferation, and signal transduction) (Figure 4-12). Second,
absorption of extreme energy sources (e.g., ultraviolet light, radiation) produces
free radicals. Third, enzymatic metabolism of exogenous chemicals or drugs (e.g.,
, a product of carbon tetrachloride [CCl4]) results in the formation of free
radicals. Fourth, transition metals (i.e., iron and copper) donate or accept free
electrons during intracellular reactions and activate the formation of free radicals
such as in the Fenton reaction (see Figure 4-12). Finally, nitric oxide (NO) is an
important colorless gas that is an intermediate in many reactions generated by
endothelial cells, neurons, macrophages, and other cell types. NO can act as a free
radical and can be converted to highly reactive peroxynitrite anion (ONOO−), NO2,
and . Table 4-3 describes the most significant free radicals.
FIGURE 4-12 Generation of Reactive Oxygen Species and Antioxidant Mechanisms in Biologic
Systems. Free radicals are generated within cells in several ways, including from normal
respiration; absorption of radiant energy; activation of leukocytes during inflammation;
metabolism of chemicals or drugs; transition metals, such as iron (Fe+++) or copper (Cu+), where
the metals donate or accept electrons as in the Fenton reaction; nitric oxide (NO) generated by
endothelial cells (not shown); and reperfusion injury. Ubiquinone (coenzyme Q), a lipophilic
molecule, transfers electrons in the inner membrane of mitochondria, ultimately enabling their
interaction with oxygen (O2) and hydrogen (H2) to yield water (H2O). In so doing, the transport
allows free energy change and the synthesis of 1 mole of adenosine triphosphate (ATP). With
the transport of electrons, free radicals are generated within the mitochondria. Reactive oxygen
species ( , H2O2, OH•) act as physiologic modulators of some mitochondrial functions but
may also cause cell damage. O2 is converted to superoxide ( ) by oxidative enzymes in the
mitochondria, endoplasmic reticulum (ER), plasma membrane, peroxisomes, and cytosol. O2 is
converted to H2O2 by superoxide dismutase (SOD) and further to OH• by the Cu/Fe Fenton
reaction. Superoxide catalyzes the reduction of Fe++ to Fe+++, thus increasing OH• formation by
the Fenton reaction. H2O2 is also derived from oxidases in peroxisomes. The three reactive
oxygen species (H2O2, OH•, and ) cause free radical damage to lipids (peroxidation of the
membrane), proteins (ion pump damage), and DNA (impaired protein synthesis). The major
antioxidant enzymes include SOD, catalase, and glutathione peroxidase.
TABLE 4-3
Biologically Relevant Free Radicals
Reactive oxygen species (ROS)
Superoxide
Generated either (1) directly during autoxidation in mitochondria or (2) enzymatically by enzymes in cytoplasm,
such as xanthine oxidase or cytochrome P-450; once produced, it can be inactivated spontaneously or more rapidly
by enzyme superoxide dismutase (SOD):
Hydrogen peroxide (H2O2)
Or
Oxidases present in peroxisomes
O
2
peroxisome
Generated by SOD or directly by oxidases in intracellular peroxisomes; NOTE: SOD is considered an antioxidant
because it converts superoxide to H2O2; catalase (another antioxidant) can then decompose H2O2 to O2 + H2O.)
Hydroxyl radicals (OH−)
Or
Or
Generated by hydrolysis of water caused by ionizing radiation or by interaction with metals—especially iron (Fe)
and copper (Cu); iron is important in toxic oxygen injury because it is required for maximal oxidative cell damage
Nitric oxide (NO) NO by itself is an important mediator that can act as a free radical; it can be converted to another radical—
peroxynitrite anion (ONOO
−
), as well as and
Data from Cotran RS et al: Robbins pathologic basis of disease, ed 6, Philadelphia, 1999, Saunders.
Free radicals cause several damaging effects by (1) lipid peroxidation, which is
the destruction of polyunsaturated lipids (the same process by which fats become
rancid), leading to membrane damage and increased permeability; (2) protein
alterations, causing fragmentation of polypeptide chains that can lead to loss and
protein misfolding; and (3) DNA damage, causing mutations (Figure 4-13; also see
p. 39). Because of the increased understanding of free radicals, a growing number
of diseases and disorders have been linked either directly or indirectly to these
reactive species (Box 4-1).
FIGURE 4-13 The Role of Reactive Oxygen Species (ROS) in Cell Injury. The production of ROS
can be initiated by many cell stressors, such as radiation, toxins, and reperfusion of oxygen.
Free radicals are removed by normal decay and enzymatic systems. ROS accumulates in cells
because of insufficient removal or excess production leading to cell injury, including lipid
peroxidation, protein modifications, and DNA damage or mutations. (Adapted from Kumar V et al, editors:
Robbins and Cotran pathologic basis of disease, ed 9, Philadelphia, 2015, Elsevier.)
Box 4-1
Diseases and Disorders Linked to Oxygen-
Derived Free Radicals
Deterioration noted in aging
Atherosclerosis
Ischemic brain injury
Alzheimer disease
Neurotoxins
Cancer
Cardiac myopathy
Chronic granulomatous disease
Diabetes mellitus
Eye disorders
Macular degeneration
Cataracts
Inflammatory disorders
Iron overload
Lung disorders
Asbestosis
Oxygen toxicity
Emphysema
Nutritional deficiencies
Radiation injury
Reperfusion injury
Rheumatoid arthritis
Skin disorders
Toxic states
Xenobiotics (CCl4, paraquat, cigarette smoke, etc.)
Metal irons (Ni, Cu, Fe, etc.)
The body can eliminate free radicals. The oxygen free radical superoxide may
spontaneously decay into oxygen and hydrogen peroxide. Table 4-4 summarizes
other methods that contribute to inactivation or termination of free radicals. The
toxicity of certain drugs and chemicals can be attributed either to conversion of
these chemicals to free radicals or to the formation of oxygen-derived metabolites
(see the following discussion).
TABLE 4-4
Methods Contributing to Inactivation or Termination of Free Radicals
Method Process
Antioxidants Endogenous or exogenous; either blocks synthesis or inactivates (e.g., scavenges) free radicals; includes vitamin E, vitamin C, cysteine,
glutathione, albumin, ceruloplasmin, transferrin, γ-lipoacid, others
Enzymes Superoxide dismutase,* which converts superoxide to H2O2; catalase* (in peroxisomes) decomposes H2O2; glutathione peroxidase* decomposes
OH• and H2O2
*These enzymes are important in modulating the cellular destructive effects of free radicals, also released
in inflammation.
Mitochondrial Effects
Mitochondria are key players in cell injury and cell death because they produce ATP
or life-sustaining energy. Mitochondria can be damaged by ROS and by increases of
cytosolic Ca++ concentration (see Figure 4-9). Box 4-2 summarizes the three major
types and consequences of mitochondrial damage. Currently, investigators are
trying to identify the polypeptides (i.e., proteomes) directly involved in diseases
associated with mitochondrial dysfunction. ROS not only damage proteins and
mitochondria but also can promote damage in neighboring cells. An important area
of research emphasis is that protein aggregates can increase mitochondrial damage
and damaged mitochondria can further induce protein damage, thus resulting in
neurodegeneration. An emerging area of research concerns mitochondrial DNA that
escapes from autophagy, which may be a mechanism of tissue inflammation.21
Box 4-2
Three Major Types and Consequences of
Mitochondrial Damage
1. Damage to the mitochondria results in the formation of the mitochondrial
permeability transition pore, a high-conductance channel or pore. The opening of
this channel results in the loss of mitochondrial membrane potential, causing
failure of oxidative phosphorylation, depletion of ATP, and damage to
mitochondrial DNA (mtDNA), leading to necrosis of the cell.
2. Altered oxidative phosphorylation leads to the formation of ROS that can damage
cellular components.
3. Because mitochondria store several proteins between their membranes, increased
permeability of the outer membrane may result in leakage of pro-apoptotic
proteins and cause cell death by apoptosis.
Data from Kumar V et al, editors: Robbins and Cotran pathologic basis of disease, ed 9, Philadelphia, 2015,
Elsevier.
Chemical or Toxic Injury
Mechanisms
Humans are exposed to thousands of chemicals that have inadequate toxicologic
data.22 The given societal considerations of time, cost, and reduced animal use have
increased the need to develop new methods for toxicity testing. To meet this public
health need, many agencies have partnered to investigate how chemicals interact
with biologic systems. Advances in molecular and systems biology, computational
toxicology, and bioinformatics have increased the development of powerful new
tools.
The systems biology approach includes delineation of toxicity pathways that may
be defined as cellular response pathways, which when disturbed are expected to
result in adverse health effects. Using this model of testing, investigators proposed
screening and classifying compounds using a “cellular stress response pathway.”
Components or mechanisms of these pathways include oxidative stress, heat shock
response, DNA damage response, hypoxia, ER stress (see Chapter 1), mental stress,
inflammation, and osmotic stress. Many chemicals have already been classified
under these mechanisms.
Humans are constantly exposed to a variety of compounds termed xenobiotics
(Greek xenos, “foreign”; bios, “life”) that include toxic, mutagenic, and
carcinogenic chemicals (Figure 4-14). Some of these chemicals are found in the
human diet, for example, fungal mycotoxins such as aflatoxin B1. Many xenobiotics
are toxic to the liver (hepatotoxic). The liver is the initial site of contact for many
ingested xenobiotics, drugs, and alcohol, making this organ most susceptible to
chemically induced injury. The toxicity of many chemicals results from absorption
through the gastrointestinal tract after oral ingestion. A main cause for withdrawing
medications from the market is hepatotoxicity. Dietary supplements, for example,
chaparral and ma huang, are potent hepatotoxins.23 Other common routes of
exposure for xenobiotics are absorption through the skin and inhalation. The
severity of chemically induced liver injury varies from minor liver injury to acute
liver failure, cirrhosis, and liver cancer.24
FIGURE 4-14 Human Exposure to Pollutants. Pollutants contained in air, water, and soil are
absorbed through the lungs, gastrointestinal tract, and skin. In the body, the pollutants may act
at the site of absorption but are generally transported through the bloodstream to various
organs where they can be stored or metabolized. Metabolism of xenobiotics may result in the
formation of water-soluble compounds that are excreted, or a toxic metabolite may be created
by activation of the agent. (From Kumar V et al, editors: Robbins and Cotran pathologic basis of disease, ed 9, Philadelphia, 2015,
Elsevier.)
The liver as the principal site for xenobiotic metabolism, called
biotransformation, converts the lipophilic xenobiotics to more hydrophilic forms
for efficient excretion. Biotransformation, however, also can produce short-lived
unstable highly reactive chemical intermediates that can lead to adverse effects.25
These harmful intermediates, classified and cataloged, are called toxicophores. The
intermediates include electrophiles, nucleophiles, free radicals, and redox-active
reactants. Electrophiles (electron lovers) are an atom or molecule attracted to
electrons and accepts a pair of electrons to make a covalent bond. This process
creates a partially or fully charged center in electrophilic molecules.25 A
nucleophile is an atom or molecule that donates an electron pair to an electrophile
to make a chemical bond. All chemical species with a free pair of electrons can act
as nucleophiles. Nucleophiles are strongly attracted to positively charged regions in
other chemicals and can be oxidized to free radicals and electrophiles.25 In general,
the majority of all reactive chemical species are electrophilic because the formation
of nucleophiles is rare25 (for a discussion on free radicals, see p. 81). The
generation of these excess reactive chemical species leads to molecular damage in
liver cells (Figure 4-15). These reactive intermediates can interact with cellular
macromolecules (such as proteins and DNA), can covalently bind to proteins and
form protein adducts (chemical bound to protein) and DNA adducts, or can react
directly with cell structures to cause cell damage.26 Adduct formation can lead to
adverse conditions including disruption in protein function, excess formation of
fibrous connective tissue (fibrogenesis), and activation of immune responses.25 The
identity of proteins modified by xenobiotics can be found in the resource known as
the reactive metabolite target protein database.27 The body has two major defense
systems for counteracting these effects: (1) detoxification enzymes and their
cofactors and (2) antioxidant systems (see p. 82). Phases of detoxification include
phase I enzymes, such as cytochrome P-450 (CYP) oxidases, which are the most
important oxidative reactions. Other phase I detoxification enzymes include those
for reduction and hydrolysis. In phase II detoxification, conjugation enzymes, such
as glutathione (GSH), detoxify reactive electrophiles and produce polar metabolites
that cannot diffuse across membranes. Most conjugation enzymes are located in the
cytosol. Phase III detoxification is often called the efflux transporter system because
enzymes remove the parent drugs, metabolites, and xenobiotics from cells. The
liver has the highest supply of biotransformation enzymes of all organs and,
therefore, has the key role in protection from chemical toxicity.25 Figure 4-16 is a
summary of chemically induced liver injury.
FIGURE 4-15 Liver Toxicants: Chemical Injury.
FIGURE 4-16 Chemical Liver Injury. Liver injury is a result of genetic, environmental, biologic,
and dietary factors. Certain chemicals can form toxic or chemically reactive metabolites. The
risk of liver injury also can increase with increasing doses of a toxicant. Xenobiotic enzyme
induction can lead to altered metabolism of chemicals, and drugs can either inhibit or induce
drug-metabolizing enzymes. These changes can lead to greater toxicity. The dose at the site of
action is controlled by the Phase I to III xenobiotic metabolites and metabolizing enzymes are
encoded by numerous different genes. Therefore, the metabolism and toxicity outcomes can
vary greatly among individuals. Additionally, all aspects of xenobiotic metabolism are regulated
by certain transcription factors (cellular mediators of gene regulation). Overall, the extent of cell
damage depends on the balance between reactive chemical species and protective responses
aimed at decreasing oxidative stress, repairing macromolecular damage, or preserving cell
health by inducing apoptosis or cell death. Significant clinical outcomes of chemical-induced
liver injury occur with necrosis and the immune response. Covalent binding of reactive
metabolites to cellular proteins can produce new antigens (haptens) that initiate autoantibody
production and cytotoxic T-cell responses. Necrosis, a form of cell death (see p. 102), can result
from extensive damage to the plasma membrane with altered ion transport, changes of
membrane potential, cell swelling, and eventual dissolution. Altogether the pathogenesis of
chemically induced liver injury is determined by genetics, environmental factors, and other
underlying pathologic conditions. Green arrows are pathways leading to cell recovery; red
arrows indicate pathways to cell damage or death; black arrows are pathways leading to
chemically induced liver injury. (Adapted from Gu X, Manautou JE: Molecular mechanisms underlying chemical liver injury, Exp Rev
Mol Med 14:e4, 2013.)
The consequence of self-propagating chain reactions of free radicals is lipid
peroxidation (also see p. 82). Free radicals react mainly with polyunsaturated fatty
acids in membranes and can initiate lipid peroxidation. The breakdown of
membrane lipids results in altered function of the mitochondria, ER, plasma
membranes, and Golgi apparatus, and therefore has a role in acute liver cell death
(necrosis) and progression of liver injury (Figure 4-17).25
FIGURE 4-17 Chemical Injury of Liver Cells Induced by Carbon Tetrachloride (CCl4)
Poisoning. Light blue boxes are mechanisms unique to chemical injury, purple boxes involve
hypoxic injury, and green boxes are clinical manifestations.
Chemical Agents Including Drugs
Numerous chemical agents cause cellular injury. Because chemical injury remains a
constant problem in clinical settings, it is a major limitation to drug therapy. Over-
the-counter and prescribed drugs can cause cellular injury, sometimes leading to
death. The leading cause of child poisoning is medications (see Health Alert: The
Percentage of Child Medication–Related Poisoning Deaths Is Increasing). The site
of injury is frequently the liver, where many chemicals and drugs are metabolized
(see Figure 4-17). Long-term exposure to air pollutants, insecticides, and herbicides
can cause cellular injury (see Health Alert: Air Pollution Reported as Largest
Single Environmental Health Risk).
Health alert
The Percentage of Child Medication–Related Poisoning Deaths Is
Increasing
Today, the leading cause of child poisoning is medications. Each year, more than
500,000 children, ages 5 and younger, experience a potential poisoning related to
medications. More than 60,000 children are treated in emergency departments
because of accidental medication exposure or overdose. Of every 150 2-year-old
children, one is being sent to the emergency department for an unintentional
medication overdose. Among children younger than age 5, 95% of emergency
department visits are caused by unsupervised accidental ingestions and about 5%
from dosing errors made by clinicians.
Importantly, investigators analyzed records from the American Association of
Poison Control Centers’ National Poison Data System (NPDS), an electronic
database of all calls to the 61 poison control centers across the United States. Their
analysis included all calls for children age 5 years or younger who were seen in a
hospital emergency department between 2001 and 2008 for either unintentional
self-exposure to a single drug (prescription or over-the-counter [OTC]) or
unintentional therapeutic error for a single drug (prescription or OTC). The
number of such calls during this 8-year period totaled 453,559. Medication-related
poisoning deaths among children 5 years and younger now most frequently involve
exposures to opioid analgesics and cardiovascular medications. About half of all
poisoning-related deaths involve analgesics, antihistamines, and sedatives.
Development of new medications also has led to more of them being available in
American homes. With aging, more adults are taking OTC and prescription
medications as well as multiple medications. Oxycodone, morphine, and methadone
prescriptions have increased between 159% and 559% between 2000 and 2009,
depending on the drug; the number of prescribed cardiovascular drugs (e.g., meto-
prolol) has increased about fivefold. Additionally, more medications, such as those
utilized for attention-deficit disorder and diabetes, are being prescribed to younger
adults and children. Prescription pain killer overdose is a growing epidemic,
especially among women.
How can we increase the safety of children exposed to so many medications?
Safe storage is the most important solution and safe dosing from clinicians will
reduce dosing errors. Additionally, improvements are continuing through
improved packaging and labeling of medications as well as education of parents
and consumers on dosing information.
Data from Bond GR et al: J Pediatr 160(2):265-270, 2011; Bronstein AC et al: Clin Toxicol 49:910-941, 2011;
Budnitz DS, Lovegrove MC: J Pediatr 160(2):190-192, 2012; Bunitz DS, Salis S: Pediatrics 127(6):e1597-
e1599, 2011; Centers for Disease Control and Prevention: Available at
www.cdc/gov/features/medicationstorage/. Accessed February 9, 2010.
Health Alert
Air Pollution Reported as Largest Single Environmental Health
Risk
The World Health Organization (WHO) reports that about 7 million people died in
2012 as a result of air pollution exposure. Improved measurements and better
technology have enabled scientists to make more detailed analyses of health risks.
These findings confirm that air pollution is now the world’s largest single
environmental health risk and reducing air pollution could save millions of lives.
New data show a stronger link between indoor and outdoor air pollution exposure
and cardiovascular diseases, for example, strokes and ischemic heart disease, as
well as the link between air pollution and cancer. These data are in addition to the
role of air pollution and the development of respiratory diseases including
infections and chronic obstructive pulmonary diseases. Using these 2012 data for
low- and middle-income countries, Southeast Asia and Western Pacific regions had
the largest air pollution burden. Included in the analysis is a breakdown of deaths
for adults and children attributed to specific diseases:
Outdoor Air Pollution–Caused Deaths—Breakdown by Disease:
• 40% ischemic heart disease
• 40% stroke
http://www.cdc/gov/features/medicationstorage/
• 11% chronic obstructive pulmonary disease (COPD)
• 6% lung cancer
• 3% acute lower respiratory tract infections in children
Indoor Air Pollution–Caused Deaths—Breakdown by Disease:
• 34% stroke
• 26% ischemic heart disease
• 22% COPD
• 12% acute lower respiratory tract infections in children
• 6% lung cancer
The WHO estimates that indoor air pollution was linked to 4.3 million deaths in
2012 from cooking over coal, wood, dung, and biomass stoves. Outdoor air
pollution estimates were 3.7 million deaths in 2012 from urban and rural sources.
Data from World Health Organization (WHO): 7 million premature deaths annually linked to air pollution.
Available from www.who.int/mediacentre/news/releases/2014/air-pollution/en/#.
Another way to classify mechanisms by which drug actions, chemicals, and toxins
produce injury includes (1) direct damage, also called on-target toxicity; (2)
exaggerated response at the target, including overdose; (3) biologic activation to
toxic metabolites, including free radicals; (4) hypersensitivity and related
immunologic reactions; and (5) rare toxicities.28 These mechanisms are not
mutually exclusive; thus several may be operating concurrently.
Direct damage is when chemicals and drugs injure cells by combining directly
with critical molecular substances. For example, cyanide is highly toxic (e.g.,
poisonous) because it inhibits mitochondrial cytochrome oxidase and hence blocks
electron transport. Many chemotherapeutic drugs, known as antineoplastic agents,
induce cell damage by direct cytotoxic effects. Exaggerated pharmacologic
responses at the target include tumors caused by industrial chemicals and the birth
defects attributed to thalidomide.28 Importantly, another example includes common
drugs of abuse (Table 4-5). Drug abuse can involve mind-altering substances
beyond therapeutic or social norms (Table 4-6). Drug addiction and overdose are
serious public health issues.
http://www.who.int/mediacentre/news/releases/2014/air-pollution/en/
TABLE 4-5
Common Drugs of Abuse
Class Molecular Target Example
Opioid narcotics Mu opioid receptor (agonist) Heroin, hydromorphone (Dilaudid)
Oxycodone (Percodan, Percocet, OxyContin)
Methadone (Dolophine)
Meperidine (Demerol)
Sedative-hypnotics GABAA receptor (agonist) Barbiturates
Ethanol
Methaqualone (Quaalude)
Glutethimide (Doriden)
Ethchlorvynol (Placidyl)
Psychomotor stimulants Dopamine transporter (antagonist)
Serotonin receptors (toxicity)
Cocaine
Amphetamines
3,4-Methylenedioxymethamphetamine (MDMA, ecstasy)
Phencyclidine-like drugs NMDA glutamate receptor channel (antagonist) Phencyclidine (PCP, angel dust)
Ketamine
Cannabinoids CB1 cannabinoid receptors (agonist) Marijuana
Hashish
Hallucinogens Serotonin 5-HT2 receptors (agonist) Lysergic acid diethylamide (LSD)
Mescaline
Psilocybin
CB1, Cannabinoid receptor type 1; GABA, γ-aminobutyric acid; 5-HT2, 5-hydroxytryptamine; NMDA, N-
methyl-D-aspartate.
From Kumar V et al: Cellular responses to stress and toxic insults: adaptation, injury, and death. In Kumar
V et al, editors: Robbins and Cotran pathologic basis of disease, ed 9, St Louis, 2014, Saunders; Hyman
SE: JAMA 286:2586, 2001.
TABLE 4-6
Social or Street Drugs and Their Effects
Type of Drug Description and Effects
Marijuana (pot) Active substance: Δ9-Tetrahydrocannabinol (THC), found in resin of Cannabis sativa plant
With smoking (e.g., “joints”), about 5% to 10% is absorbed through lungs; with heavy use the following adverse effects have been
reported: alterations of sensory perception; cognitive and psychomotor impairment (e.g., inability to judge time, speed, distance); it
increases heart rate and blood pressure; increases susceptibility to laryngitis, pharyngitis, bronchitis; causes cough and hoarseness; may
contribute to lung cancer (different dosages need study; contains large number of carcinogens); data from animal studies only indicate
reproductive changes include reduced fertility, decreased sperm motility, and decreased levels of circulatory testosterone; fetal abnormalities
include low birth weight; increased frequency of infectious illness is thought to be result of depressed cell-mediated and humoral
immunity; beneficial effects include decreased nausea secondary to cancer chemotherapy and decreased pain in certain chronic conditions
Methamphetamine
(meth)
An amine derivation of amphetamine (C10H15N) used as crystalline hydrochloride
CNS stimulant; in large doses causes irritability, aggressive (violent) behavior, anxiety, excitement, auditory hallucinations, and
paranoia (delusions and psychosis); mood changes are common and abuser can swiftly change from friendly to hostile; paranoiac swings
can result in suspiciousness, hyperactive behavior, and dramatic mood swings
Appeals to abusers because body’s metabolism is increased and produces euphoria, alertness, and perception of increased energy
Stages:
Low intensity: User is not psychologically addicted and uses methamphetamine by swallowing or snorting
Binge and high intensity: User has psychologic addiction and smokes or injects to achieve a faster, stronger high
Tweaking: Most dangerous stage; user is continually under the influence, not sleeping for 3-15 days, extremely irritated, and paranoid
Cocaine and crack Extracted from leaves of cocoa plant and sold as a water-soluble powder (cocaine hydrochloride) liberally diluted with talcum powder or
other white powders; extraction of pure alkaloid from cocaine hydrochloride is “free-base” called crack because it “cracks” when heated
Crack is more potent than cocaine; cocaine is widely used as an anesthetic, usually in procedures involving oral cavity; it is a potent CNS
stimulant, blocking reuptake of neurotransmitters norepinephrine, dopamine, and serotonin; also increases synthesis of norepinephrine
and dopamine; dopamine induces sense of euphoria, and norepinephrine causes adrenergic potentiation, including hypertension,
tachycardia, and vasoconstriction; cocaine can therefore cause severe coronary artery narrowing and ischemia; reason cocaine increases
thrombus formation is unclear; other cardiovascular effects include dysrhythmias, sudden death, dilated cardiomyopathy, rupture of
descending aorta (i.e., secondary to hypertension); effects on fetus include premature labor, retarded fetal development, stillbirth,
hyperirritability
Heroin Opiate closely related to morphine, methadone, and codeine
Highly addictive, and withdrawal causes intense fear (“I’ll die without it”); sold “cut” with similar-looking white powder; dissolved in
water it is often highly contaminated; feeling of tranquility and sedation lasts only a few hours and thus encourages repeated intravenous
or subcutaneous injections; acts on the receptors enkephalins, endorphins, and dynorphins, which are widely distributed throughout body
with high affinity to CNS; effects can include infectious complications, especially Staphylococcus aureus, granulomas of lung, septic
embolism, and pulmonary edema—in addition, viral infections from casual exchange of needles and HIV; sudden death is related to
overdosage secondary to respiratory depression, decreased cardiac output, and severe pulmonary edema
CNS, Central nervous system; HIV, human immunodeficiency virus.
Data from Kumar V et al, editors: Robbins and Cotran pathologic basis of disease, ed 9, Philadelphia,
2015, Elsevier; Nahas G et al: N Engl J Med 343(7):514, 2000.
Most toxic chemicals are not biologically active in their parent (native) form but
must be converted to reactive metabolites, which then act on target molecules. This
conversion is usually performed by the cytochrome P-450 oxidase enzymes in the
smooth ER of the liver and other organs. These toxic metabolites cause membrane
damage and cell injury mostly from formation of free radicals and subsequent
membrane damage from lipid peroxidation (see Figure 4-17). For example,
acetaminophen (paracetamol) is converted to a toxic metabolite in the liver, causing
cell injury (Figure 4-18). Acetaminophen is one of the most common causes of
poisoning worldwide.29 Many investigators are studying hepatoprotective
strategies.30
FIGURE 4-18 Acetaminophen Metabolism and Toxicity. CYP2E1, A cytochrome; GSH,
glutathione; NAPQI, toxic byproduct.
Hypersensitivity reactions are a common drug toxicity and range from mild skin
rashes to immune-mediated organ failure.28 One type of hypersensitivity reaction is
the delayed-onset reaction, which occurs after multiple doses of a drug are
administered. Some protein drugs and large polypeptide drugs (e.g., insulin) can
directly stimulate antibody production (see Chapter 8). Most drugs, however, act as
haptens and bind covalently to serum or cell-bound proteins. The binding makes the
protein immunogenic, stimulating antidrug antibody production, T-cell responses
against the drug, or both. For example, penicillin itself is not antigenic but its
metabolic degradation products can become antigenic and cause an allergic
reaction. Rare toxicities simply mean infrequent occurrences as described
previously by the other four mechanisms. These toxicities reflect individual genetic
predispositions that affect drug or chemical metabolism, disposition, and immune
responses.
Carbon monoxide, carbon tetrachloride, and social drugs, such as alcohol, can
significantly alter cellular function and injure cellular structures. Accidental or
suicidal poisonings by chemical agents cause numerous deaths. The injurious
effects of some agents—lead, carbon monoxide, ethyl alcohol, mercury—are
common cellular injuries.
Lead.
Lead (Pb) is a heavy toxic metal that persists in older homes, the environment, and
the workplace. Lead may be found in hazardous concentrations in food, water, and
air and it is one of the most common overexposures found in industry.31 Despite
efforts to reduce exposure through government regulation, exposure still persists
for many people and toxicity is still a primary hazard for children32 (see Health
Alert: Low-Level Lead Exposure Harms Children: A Renewed Call for Primary
Prevention). Although Pb was removed from paint in Europe in 1922 and removed
in the United States in 1978, many homes in the United States still contain leaded
paint and chipped and peeling leaded paint constitutes a major source of current
childhood exposure.33-36 The chipped paint can disintegrate at friction surfaces to
form Pb dust.36 Another source of contamination is Pb dust dispersed along
roadways from previous leaded gasoline emissions.36 When Pb was removed from
gasoline, blood lead levels (BLLs) dropped significantly.37-39 Previous emissions of
leaded fuel created large dispersions of lead dust in the environment. Particulate
lead (2 to 10 µm) does not degrade and persists in the environment, making it a
notable source of human exposure.40 Other airborne sources include smelters and
piston-engine airplanes.41 Drinking water exposed to Pb occurs from outdated
fixtures, plumbing without corrosion control, and solders.36 Because well water is
not subject to EPA regulation it may not be tested for Pb.36 Although the average
blood levels of Pb in children in the United States have dropped since the 1970s,
there are at-risk populations with higher than average BLLs.36 Children of lower
social economic status or racial minority status are still at higher risk of Pb
poisoning and some regions in the United States have an increased prevalence of
higher BLLs in children.36 Importantly, the CDC reports “no safe blood lead level in
children has been identified.”42 Common sources of Pb are included in Table 4-7.
Health Alert
Low-Level Lead Exposure Harms Children: A Renewed Call for
Primary Prevention
An advisory committee of the CDC recently suggested that the current threshold for
harmful lead exposure in children should be cut in half because even lower levels
cause irreversible harm. The report noted that studies have found reduced
intelligence quotients (IQs) and behavioral problems in children with exposure
levels less than 10 mcg/dl and that such low levels have effects on cardiovascular,
endocrine, and immunologic systems. Based on these data, the panel recommended
reducing the threshold for harmful levels of lead in the blood to 5 mcg/dl. Despite
progress in reducing blood lead levels (BLLs), racial and income disparities
persist. An internal review process from both the Centers for Disease Control and
Prevention and the U.S. Department of Health and Human Services will determine
how to implement any accepted recommendations. This is a very important process
because BLLs appear to be irreversible, underscoring the need for primary
prevention.
Data from Advisory Committee for Childhood Lead Poisoning Prevention of the Centers for Disease Control and
Prevention: Low level lead exposure harms children: a renewed call for primary prevention, 2012. Available at
www.cdc.gov/nceh/lead/ACCLPP/FinalDocument030712.pdf. Accessed September 24, 2012.
TABLE 4-7
Common Sources of Lead Exposure
Exposure Source
Environmental Lead paint, soil, or dust near roadways or lead-painted homes; plastic window blinds; plumbing materials (from pipes or solder); pottery
glazes and ceramic ware; lead-core candle wicks; leaded gasoline; water (pipes)
Occupational Lead mining and refining, plumbing and pipe fitting, auto repair, glass manufacturing, battery manufacturing and recycling, printing shop,
construction work, plastic manufacturing, gas station attendant, firing-range attendant
Hobbies Glazed pottery making, target shooting at firing ranges, lead soldering, preparing fishing sinkers, stained-glass making, painting, car or boat
repair
Other Gasoline sniffing, costume jewelry, cosmetics, contaminated herbal products
Data from Sanborn MD et al: CMAJ 166(10):1287-1292, 2002.
Children are more susceptible to the effects of Pb than adults for several reasons,
including (1) children have increased hand-to-mouth behavior and exposure from
the ingestion of Pb dust; (2) the blood-brain barrier in children is immature during
fetal development, contributing to greater accumulation in the developing brain; and
(3) infant absorption of Pb is greater than that in adults and bone turnover (in adults
the body burden of lead is found in bone) in children from skeletal growth results in
continuous leaching of Pb into blood, causing constant body exposure.36,42 If
nutrition is compromised, especially if dietary intake of iron and calcium is
insufficient, children are more likely to have elevated BLLs.36 Particularly
worrisome is lead exposure during pregnancy because the developing fetal nervous
system is especially vulnerable; lead exposure can result in lower IQs, learning
disorders, hyperactivity, and attention problems.32
The organ systems primarily affected by lead ingestion include the nervous
system, the hematopoietic system (tissues that produce blood cells), and the kidneys
http://www.cdc.gov/nceh/lead/ACCLPP/FinalDocument030712.pdf
of the urologic system. The neurologic effect of Pb in exposed children is the
driving factor for reducing Pb levels in the environment.36 Elevated BLLs not only
are linked to cognitive deficits but also are associated with behavioral changes
including antisocial behavior, acting out in school, and difficulty paying attention.36
The cognitive and behavioral changes of Pb-exposed children persist after complete
cessation of Pb exposure.36 In 1991 the CDC lowered the definition of Pb
intoxication to 10 µm/dl BLL because several studies reported that children with
BLLs of at least 10 µm/dl had impaired intellectual functioning36 (Figure 4-19).
Studies in animals have led to the hypothesis that Pb targets the learning and
memory processes by inhibiting the N-methyl-D-aspartate receptor (NMDAR),
which is necessary for hippocampus-mediated learning and memory.36,43 Similar
changes also have been found in cultured neuron systems.36 Inhibition of either
voltage-gated calcium channels or NMDARs by Pb results in reduction of Ca++ entry
into the cell, thereby disrupting the necessary Ca++ signaling for
neurotransmission.44,45 Lead induces cellular damage by increasing oxidative
stress.46 Lead toxicity involves the direct formation of ROS (singlet oxygen,
hydrogen peroxides, hydroperoxides) and depletion of antioxidants.46 Pb exposure
leads to lowered levels of glutathione; and because glutathione is important for the
metabolism of specific drugs and other toxins, low Pb levels can increase their
toxicity, as well as the levels of other metals.46 From animal studies and human
population studies, low-level lead exposure may cause hypertension.47 Lead
interferes with the normal remodeling of cartilage and bone in children. From
radiologic studies of bone, “lead lines” are detectable and lead also can be found in
the gums as a result of hyperpigmentation. Lead inhibits several enzymes involved
in hemoglobin synthesis and causes anemia (most obvious is a microcytic
hypochromic anemia). Renal lesions can cause tubular dysfunction resulting in
glycosuria (glucose in the urine), aminoaciduria (amino acids in the urine), and
hyperphosphaturia (excess phosphate in the urine). Gastrointestinal symptoms are
less severe and include nausea, loss of appetite, weight loss, and abdominal
cramping.
FIGURE 4-19 Lead Poisoning in Children Related to Blood Levels. (From Kumar V et al, editors: Robbins and
Cotran pathologic basis of disease, ed 9, Philadelphia, 2015, Elsevier.)
Carbon monoxide.
Gaseous substances can be classified according to their ability to asphyxiate
(interrupt respiration) or irritate. Toxic asphyxiants, such as carbon monoxide,
hydrogen cyanide, and hydrogen sulfide, directly interfere with cellular respiration.
Carbon monoxide (CO) is an odorless, colorless, nonirritating, and undetectable
gas unless it is mixed with a visible or odorous pollutant. CO is produced by the
incomplete combustion of fuels such as gasoline. Although CO is a chemical agent,
the ultimate injury it produces is a hypoxic injury—namely, oxygen deprivation. As
a systemic asphyxiant, CO causes death by inducing central nervous system (CNS)
depression. Normally, oxygen molecules are carried to tissues bound to
hemoglobin in red blood cells (see Chapter 27). Because CO’s affinity for
hemoglobin is 300 times greater than that of oxygen, CO quickly binds with the
hemoglobin, preventing the oxygen molecules’ ability to bind to the hemoglobin.
Minute amounts of CO can produce a significant percentage of carboxyhemoglobin
(carbon monoxide bound with hemoglobin). With increasing levels of
carboxyhemoglobin, hypoxia occurs insidiously, evoking widespread ischemic
changes in the CNS, and individuals are often unaware of their plight. The diagnosis
is made from measurement of carboxyhemoglobin levels in the blood.
Symptoms related to CO poisoning include headache, giddiness, tinnitus (ringing
in the ears), chest pain, confusion, nausea, weakness, and vomiting. CO is an air
pollutant found in combustion fumes produced by cars and trucks, small gasoline
engines, stoves, gas ranges, gas refrigerators, heating systems, lanterns, burning
charcoal or wood, and cigarette smoke. Chronic exposure can occur in people
working in confined spaces, such as underground garages and tunnels. Fumes can
accumulate in enclosed or semi-enclosed spaces, and poisoning from breathing CO
can occur in humans and animals. High levels of CO can cause loss of
consciousness and death. Death can occur in individuals sleeping or intoxicated
before experiencing any symptoms. Although all people and animals are at risk,
those most susceptible to poisoning include unborn babies, infants, and people with
chronic heart disease, respiratory problems, and anemia. For information on
preventing CO poisoning from home appliances and proper venting, see the Centers
for Disease Control and Prevention (CDC) website at www.cdc.gov/co/faqs.htm.
Ethanol.
Alcohol (ethanol) is the primary choice among mood-altering drugs available in
the United States. It is estimated there are more than 10 million chronic alcoholics in
the United States. Alcohol contributes to more than 100,000 deaths annually with
50% of these deaths from drunk driving accidents, alcohol-related homicides, and
suicides.48 A blood concentration of 80 mg/dl is the legal definition for drunk
driving in the United States. This level of alcohol in an average person may be
reached after consumption of three drinks (three 12-ounce bottles of beer, 15 ounces
of wine, and 4 to 5 ounces of distilled liquor). The effects of alcohol vary by age,
gender, and percent body fat; the rate of metabolism affects the blood alcohol level.
Because alcohol is not only a psychoactive drug but also a food, it is considered
part of the basic food supply in many societies.
A large intake of alcohol has enormous effects on nutritional status. Liver and
nutritional disorders are the most serious consequences of alcohol abuse. Major
http://www.cdc.gov/co/faqs.htm
nutritional deficiencies include magnesium, vitamin B6, thiamine, and phosphorus.
Folic acid deficiency is a common problem in chronic alcoholic populations.
Ethanol alters folic acid (folate) homeostasis by decreasing intestinal absorption of
folate, increasing liver retention of folate, and increasing the loss of folate through
urinary and fecal excretion.49 Folic acid deficiency becomes especially serious in
pregnant women who consume alcohol and may contribute to fetal alcohol
syndrome (see p. 92).
Most of the alcohol in blood is metabolized to acetaldehyde in the liver by three
enzyme systems: alcohol dehydrogenase (ADH), the microsomal ethanol-oxidizing
system (MEOS; CYP2E1), and catalase (Figure 4-20). The major pathway involves
ADH, an enzyme located in the cytosol of hepatocytes. The microsomal ethanol
oxidizing system (MEOS) depends on cytochrome P-450 (CYP2E1), an enzyme
needed for cellular oxidation. Activation of CYP2E1 requires a high ethanol
concentration and thus is thought to be important in the accelerated ethanol
metabolism (i.e., tolerance) noted in persons with chronic alcoholism. Acetaldehyde
has many toxic tissue effects and is responsible for some of the acute effects of
alcohol and for development of head and neck cancer (HNC).48 A recent and first
study showed that head and neck cancer risk may be influenced by alcohol-
metabolizing genes (ADH1B and ALDH2) and oral hygiene.50
FIGURE 4-20 Ethanol Metabolism Pathway. Ethanol is metabolized into acetaldehyde through
the cytosolic enzyme alcohol dehydrogenase (ADH), the microsomal enzyme cytochrome P-450
2E1 (CYP2E1), and the peroxisomal enzyme catalase. The ADH enzyme reaction is the main
ethanol metabolic pathway involving an intermediate carrier of electrons, namely, nicotinamide
adenine dinucleotide (NAD+), which is reduced by two electrons to form NADH. Acetaldehyde is
metabolized mainly by aldehyde dehydrogenase 2 (ALDH2) in the mitochondria to acetate and
NADH before being cleared into the systemic circulation. (Adapted from Zhang Y, Ren J: Pharmacol Ther 132[1]:86-92,
2011.)
The major effects of acute alcoholism involve the central nervous system (CNS).
After alcohol is ingested, it is absorbed, unaltered, in the stomach and small
intestine. Fatty foods and milk slow absorption. Alcohol then is distributed to all
tissues and fluids of the body in direct proportion to the blood concentration.
Individuals differ in their capability to metabolize alcohol. Genetic differences in
the metabolism of liver alcohol, including levels of aldehyde dehydrogenases, have
been identified.51 These genetic polymorphisms may account for ethnic and gender
differences in ethanol metabolism. Persons with chronic alcoholism develop
tolerance because of production of enzymes, leading to an increased rate of
metabolism (e.g., P-450).
Numerous studies have validated the so-called J- or U-shaped inverse association
between alcohol and overall or cardiovascular mortality, such as from myocardial
infarction and ischemic stroke. These studies have found that light to moderate
(nonbinge) drinkers tend to have lower mortality than nondrinkers and heavy
drinkers have higher mortality.52 For both men and women, former drinkers and
regular heavy drinkers had higher mortality.52 Light to moderate drinkers in the
United States may have reduced mortality but this may be confounded by medical
care and social relationships, especially among women.52,53 These relationships
need further study. The suggested mechanisms for cardioprotection for light to
moderate drinkers include increase in levels of high-density lipoprotein–cholesterol
(HDL-C), decrease in levels of low-density lipoprotein (LDL), prevention of clot
formation, reduction in platelet aggregation, decrease in blood pressure, increase in
coronary vessel vasodilation, increase in coronary blood flow, decrease in
coronary inflammation, decrease in atherosclerosis, limited ischemia-reperfusion
injury (I/R injury), and a decrease in diabetic vessel pathology.54 The American
Heart Association recommends no more than two drinks per day for men and one
drink per day for women (one 12-oz beer, 4 oz of wine, 1.5 oz of 80-proof spirits,
or 1 oz of 100-proof spirits). Drinking more alcohol can increase the risks of
alcoholism, high blood pressure, obesity, stroke, breast cancer, suicide, and
accidents.55 Individuals who do not consume alcohol should not be encouraged to
start drinking.56
Acute alcoholism (drunkenness) affects the CNS (see Health Alert: Alcohol:
Global Burden, Adolescent Onset, Chronic or Binge Drinking). Alcohol
intoxication causes CNS depression. Depending on the amount consumed, CNS
depression is associated with sedation, drowsiness, loss of motor coordination,
delirium, altered behavior, and loss of consciousness. Toxic amounts (300 to
400 mg/dl) result in a lethal coma or respiratory arrest because of medullary center
depression. Investigators studied the effects of snoring and multiple variables
including alcohol. They found that a low level of self-reported physical activity is a
risk factor for future habitual snoring complaints in women independent of alcohol
dependence, smoking, current weight, and weight gain. Furthermore, increased
physical activity can modify the risk.57 Acute alcoholism may induce reversible
hepatic and gastric changes.48 Acute alcoholism contributes significantly to motor
vehicle fatalities.
Health Alert
Alcohol: Global Burden, Adolescent Onset, Chronic or Binge
Drinking
Alcohol is widely consumed worldwide, and in the United States 50% of the adult
population (18 years and older) consumes alcohol regularly. Alcohol continues to
be the drug of choice among teens and young adults with one third of twelfth
graders and 40% of college students reporting “binge drinking” (four standard
alcohol drinks on one occasion in females and five in males). Alcohol abuse is the
leading cause of liver-related morbidity and mortality. Chronic and binge drinking
causes alcoholic liver disease (ALD) with a spectrum from hepatic steatosis (fatty
change) to steatohepatitis (fatty change and inflammation) and cirrhosis (see
Chapter 36). These alterations can eventually lead to hepatocellular carcinoma. The
pathogenesis of ALD is not fully characterized and recent studies reveal a major
role of mitochondria. Animal studies have shown that alcohol causes mitochondrial
DNA damage, lipid accumulation, and oxidative stress. Understanding the role of
the mitochondria may help identify therapeutic targets.
Investigations of adolescent drinking behaviors, especially binge drinking, is
providing evidence of neurocognitive changes, including changes in both gray and
white matter. These studies are examining risk-taking behaviors that begin in
adolescence and coincide with vulnerable and significant neurodevelopmental
changes.
Data from Adams PF et al: Vital Health Stat 10(255), 2012; available from
www.cdc.gov/nchs/data/series/sr_10/sr10_255.pdf; Hicks BM et al: Addiction 107:540-548, 2012; Johnston LD
et al: Monitoring the future national results on adolescent drug use: overview of key findings, Bethesda, Md,
2009, National Institute on Drug Abuse; Lisdahl KM et al: Front Psychiatry 4:53, 2013; Mathews S et al: Am J
Physiol Gastrointest Liver Physiol 2014 Apr 3 [Epub ahead of print]; Nassir F, Ibdah JA: World J Gastroenterol
20(9):2136-2142, 2014; White HR et al: Alcohol Clin Exp Res 35:295-303, 2010.
Chronic alcoholism causes structural alterations in practically all organs and
tissues in the body because most tissues contain enzymes capable of ethanol
oxidation or nonoxidative metabolism. The most significant activity, however,
occurs in the liver. Alcohol is the leading cause of liver-related morbidity and
mortality.58 In general, hepatic changes, initiated by acetaldehyde, include
inflammation, deposition of fat, enlargement of the liver, interruption of
microtubular transport of proteins and their secretion, increase in intracellular
water, depression of fatty acid oxidation in the mitochondria, increase in membrane
rigidity, and acute liver cell necrosis (see Chapter 36). Specifically, chronic or
binge alcohol consumption causes alcoholic liver disease (ALD) with a spectrum
ranging from simple fatty liver (steatosis), to steatohepatitis (fatty with
inflammation), to cirrhosis (Figure 4-21) (see Chapter 36). Cirrhosis is associated
with portal hypertension and an increased risk for hepatocellular carcinoma.
Cellular damage is increased by reactive oxygen species (ROS) and oxidative stress
(see p. 81). Activation of proinflammatory cytokines from neutrophils and
lymphocytes mediates liver damage.59 Oxidative stress is associated with cell
membrane phospholipid depletion, which alters the fluidity and function of cell
membranes as well as intercellular transport. Chronic alcoholism is related to
several disorders, including injury to the myocardium (alcoholic cardiomyopathy);
http://www.cdc.gov/nchs/data/series/sr_10/sr10_255.pdf
increased tendency to hypertension, acute gastritis, and acute and chronic
pancreatitis; and regressive changes in skeletal muscle. Chronic alcohol
consumption is associated with an increased incidence of cancer of the oral cavity,
liver, esophagus, and breast (see Health Alert: Alcohol: Global Burden, Adolescent
Onset, Chronic or Binge Drinking).
FIGURE 4-21 Alcoholic Hepatitis. Chicken-wire fibrosis extending between hepatocytes
(Mallory trichrome stain). (From Damjanov I, Linder J, editors: Anderson’s pathology, ed 10, St Louis, 1996, Mosby.)
Ethanol is implicated in the onset of a variety of immune defects, including
effects on the production of cytokines involved in inflammatory responses. Alcohol
can induce epigenetic variations in the developmental pathways of many types of
immune cells (e.g., granulocytes, macrophages, and T-lymphocytes) that promote
increased inflammation.60 Alcohol increases the development of serious medical
conditions related to immune system dysfunction, including acute respiratory
distress syndrome (ARDS) as well as liver cancer and alcoholic liver disease
(ALD).60 Binge and chronic drinking increases susceptibility to many infectious
microorganisms and can enhance the progression of human immunodeficiency
virus (HIV) by affecting innate and adaptive immunity.60
The deleterious effects of prenatal alcohol exposure can cause mental deficiency
and neurobehavioral disorders, as well as fetal alcohol syndrome. Fetal alcohol
syndrome includes growth retardation, facial anomalies, cognitive impairment, and
ocular malformations (Figure 4-22). It is among the common causes of mental
deficiency.61 Evidence of epigenetic alterations has led to the hypothesis that alcohol
effects on fetal development may be caused not only by maternal alcohol
consumption but also by the father’s exposure as well.61 Epigenetic alterations may
be carried through the male germline for generations.62 Alcohol crosses the
placenta, reaching the fetus, and blood levels of the fetus may reach equivalent
levels to maternal levels in 1 to 2 hours.63 Research has demonstrated an unimpeded
bidirectional movement of alcohol between the fetus and the mother. The fetus may
completely depend on maternal hepatic detoxification because the activity of alcohol
dehydrogenase (ADH) in fetal liver is less than 10% of that in the adult liver.63
Additionally, the amniotic fluid acts as a reservoir for alcohol, prolonging fetal
exposure.63 The specific mechanisms of injury are unknown; however, acetaldehyde
can alter fetal development by disrupting differentiation and growth; DNA and
protein synthesis; modification of carbohydrates, proteins, and fats; flow of
nutrients across the placenta; and neuro-circuitry dysfunction that may be long-
lasting.61,63
FIGURE 4-22 Fetal Alcohol Syndrome. When alcohol enters the fetal blood, the potential result
can cause tragic congenital abnormalities, such as microcephaly (“small head”), low birth
weight, and cardiovascular defects, as well as developmental disabilities, such as physical and
intellectual disability, and even death. Note the small head, thinned upper lip, small eye openings
(palpebral fissures), epicanthal folds, and receded upper jaw (retrognathia) typical of fetal
alcohol syndrome. (From Fortinash KM, Holoday W orret PA: Psychiatric mental health nursing, ed 5, St Louis, 2012, Mosby.)
Mercury.
Mercury is a global threat to human and environmental health. A recent report
presents an overview of the Global Mercury Assessment 2013.64 This report
provides the most recent information on worldwide atmospheric mercury
emissions, releases to the aquatic environment, and the fate of mercury in the global
environment. Causes from human activity, called anthropogenic, are responsible
for about 30% of annual emissions of mercury to air, another 10% arise from
natural geologic sources, and the remainder (60%) occurs from re-emissions or
earlier released mercury that has increased over decades and centuries in surface
soil and water.64 The major sources of anthropogenic mercury emissions to air are
artisanal and small-scale gold mining (ASGM) and coal burning. The next major
sources are the production of ferrous and nonferrous metals, and cement
production. Importantly, investigators report that emissions from industrial sectors
have increased since 2005.64 Types of aquatic releases of mercury include industrial
sites (power plants, factories), old mines, landfills, and waste disposal locations.
Artisanal and small-scale gold mining are significant producers of aquatic mercury
release. It is estimated that more than 90% of mercury in marine animals is from
anthropogenic emissions.64 Large amounts of inorganic mercury have accumulated
in surface soils and in the oceans. Climate change, with thawing of enormous areas
of frozen lands, may release even more long-stored mercury and organic matter
into lakes, rivers, and oceans.64
Dental amalgams, or “silver fillings,” are made of two almost equal parts of
liquid mercury and a powder containing silver, tin, copper, zinc, and other metals.41
When amalgams are placed or removed they can release a small amount of mercury
vapor. Chewing can release a small amount of vapor and people absorb the vapor
by inhalation or ingestion.41 Researchers are studying the effects of exposure to
magnetic fields, such as from mobile phone use, and the release of mercury from
amalgams.65 Susceptibility to mercury toxicity varies in a dose-dependent fashion,
and among individuals based on multiple genes, not all have been identified.66,67
Worldwide efforts are under way to phase down or eliminate the use of mercury
dental amalgam.67 Thimerosal, a mercury-containing preservative, was removed
from all vaccines in 2001, with the exception of inactivated influenza vaccines.68
Quick Check 4-2
1. Why are children more susceptible to the toxic effects of lead exposure?
2. Discuss the sources of lead exposure?
3. Discuss the mechanisms of cell injury related to chronic alcoholism?
4. What are the sources of mercury exposure?
Unintentional and Intentional Injuries
Unintentional and intentional injuries are an important health problem in the United
States. In 2012 there were 192,945 deaths, an injury death rate of 60.2/100,000.69 The
number of deaths because of poisoning was 48,545 with 15.4 deaths per 100,000.
Motor vehicle traffic deaths were 33,804 with a rate of 10.7 deaths per 100,000.
Deaths from all firearms were 33,636 with a rate of 10.6 deaths per 100,000. From
data reporting in 2010, drug poisoning deaths were 12.4 per 100,000.69 Death from
injury is significantly more common for men than for women; the overall rate for
men is 83.46/100,000 versus 39.28/100,000 for women. Significant racial
differences are noted in the death rate, with whites at 64.85/100,000, blacks at
56.20/100,000, and other racial groups at a combined rate of 28.96/100,000. There
also is a bimodal age distribution for injury-related deaths, with peaks in the young
adult and elderly groups. Unintentional injury is the leading cause of death for
people between the ages of 1 and 34 years; intentional injury (suicide, homicide)
ranks between the second and fourth leading cause of death in this age group. The
1999 report published by the Institute of Medicine (IOM) indicated that between
44,000 and 98,000 unnecessary deaths per year occurred in hospitals alone as a
result of errors by healthcare professionals (see Health Alert: Unintentional Injury
Errors in Health Care and Patient Safety). Statistics on nonfatal injuries are harder
to document accurately, but they are known to be a significant cause of morbidity
and disability and to cost society billions of dollars annually. The more common
terms used to describe and classify unintentional and intentional injuries and brief
descriptions of important features of these injuries are discussed in Table 4-8.
Health Alert
Unintentional Injury Errors in Health Care and Patient Safety
According to a US Senate subcommittee hearing (July 17, 2014), despite more than
a decade of national efforts to improve patient safety, hospitals and ambulatory care
centers remain problematic for patients. This assessment follows the 15-year
anniversary of the release of the IOM report on patient safety. Testimony from the
senate hearings challenged the IOM report that patient harms were likely
underestimated. A more recent estimate suggests the number of U.S. deaths as a
result of medical error may be greater than 400,000 per year with more than 1000
each day.
Progress has been made in certain areas including the reduction of bloodstream
infections from central lines. Success with this program has been expanded
nationwide. Checklists are a very useful tool for improving patient safety. They
have become more widely implemented and their success depends on appropriately
targeting the intervention and utilizing a careful implementation strategy. Besides
checklists, other examples of patient safety primers include adverse events after
hospital discharge, computerized provider order entry, detection of safety hazards,
diagnostic errors, disruptive and unprofessional behavior, error disclosure,
handoffs and signouts, health care–associated infections, nursing and patient safety,
and medication errors. In a testimony at the hearings it was stated “that one of the
biggest barriers to improved patient safety is the lack of a robust national system
for tracking patient safety data.” Additionally, speakers testified that better systems
of care are needed in understanding that a complex set of factors—complexity of
hospital systems, time pressures, growing use of technology, financial incentives
that reward hospitals by paying them to care for patients’ complications, CEO
compensation not tied to quality of care—all contribute to poor patient outcomes.
The entrenched challenges of the U.S. health care system demand a transformed
approach. Left unchanged, health care will continue to underperform; cause
unnecessary harm; and strain national, state, and family budgets. The actions
required to reverse this trend will be notable, substantial, sometimes disruptive—
and absolutely necessary.” (IOM Best Care at Lower Cost; The Path to Continously
Learning Health Care in America Institute of Medicine Report Brief Washington
DC, 2012)
Data from Agency for Healthcare Research and Quality: Patient safety primers, Rockville, MD, 2014, U.S.
Department of Health and Human Services; James JT: J Patient Saf 9(3):122-128, 2013; Kohn LT et al, editors:
To err is human: building a safer health system, Washington DC, 1999, National Academy Press; Kuehn BM: J
Am Med Assoc 312(9):879-880, 2014.
TABLE 4-8
Unintentional and Intentional Injuries
Type of Injury Description
BLUNT-FORCE INJURIES Mechanical injury to body
resulting in tearing,
shearing, or crushing; most
common type of injury seen
in healthcare settings; caused
by blows or impacts; motor
vehicle accidents and falls
most common cause (see
photo, A)
Contusion (bruise):
Bleeding into skin or
underlying tissues; initial
color will be red-purple, then
blue-black, then yellow-
brown or green (see Figure
4-26); duration of bruise
depends on extent, location,
and degree of
vascularization; bruising of
soft tissue may be confined
to deeper structures;
hematoma is collection of
blood in soft tissue;
subdural hematoma is
blood between inner surface
of dura mater and surface of
brain; can result from
blows, falls, or sudden
acceleration/deceleration of
head as occurs in shaken
baby syndrome; epidural
hematoma is collection of
blood between inner surface
of skull and dura; is most
often associated with a skull
fracture
Laceration: Tear or rip
resulting when tensile
strength of skin or tissue is
exceeded; is ragged and
irregular with abraded edges;
an extreme example is
avulsion, where a wide area
of tissue is pulled away;
lacerations of internal
organs are common in
blunt-force injuries;
lacerations of liver, spleen,
kidneys, and bowel occur
from blows to abdomen;
thoracic aorta may be
lacerated in sudden
deceleration accidents; severe
blows or impacts to chest
may rupture heart with
lacerations of atria or
ventricles
Fracture: Blunt-force blows
or impacts can cause bone to
break or shatter (see Chapter
39)
SHARP-FORCE INJURIES Cutting and piercing injuries
accounted for 2734 deaths
in 2007; men have a higher
rate (1.37/100,000) than
women (0.44/100,000);
differences by race are
whites 0.71/100,000,
blacks 2.12/100,000, and
other groups 0.80/100,000
Incised wound: A wound
that is longer than it is
deep; wound can be straight
or jagged with sharp, distinct
edges without abrasion;
usually produces significant
external bleeding with little
internal hemorrhage; these
wounds are noted in sharp-
force injury suicides; in
addition to a deep, lethal cut,
there will be superficial
incisions in same area called
hesitation marks (see photo,
B)
Stab wound: A penetrating
sharp-force injury that is
deeper than it is long; if a
sharp instrument is used,
depths of wound are clean
and distinct but can be
abraded if object is inserted
deeply and wider portion
(e.g., hilt of a knife) impacts
skin; depending on size and
location of wound, external
bleeding may be surprisingly
small; after an initial spurt
of blood, even if a major
vessel or heart is struck,
wound may be almost
completely closed by tissue
pressure, thus allowing only
a trickle of visible blood
despite copious internal
bleeding
Puncture wound:
Instruments or objects with
sharp points but without
sharp edges produce puncture
wounds; classic example is
wound of foot after stepping
on a nail; wounds are prone
to infection, have abrasion
of edges, and can be very
deep
Chopping wound: Heavy,
edged instruments (axes,
hatchets, propeller blades)
produce wounds with a
combination of sharp- and
blunt-force characteristics
GUNSHOT WOUNDS Accounted for more than
33,636 deaths in the United
States in 2015; men more
likely to die than women
(18.16 vs. 2.73/100,000);
black men between ages of
15 and 24 have greatest
death rate (86.95/100,000);
gunshot wounds are either
penetrating (bullet remains
in body) or perforating
(bullet exits body); bullet
also can fragment; most
important factors or
appearances are whether it is
an entrance or exit wound
and range of fire
Entrance wound: All
wounds share some common
features; overall appearance
is most affected by range of
fire
Contact range entrance
wound: Distinctive type of
wound when gun is held so
muzzle rests on or presses
into skin surface; there is
searing of edges of wound
from flame and soot or
smoke on edges of wound in
addition to hole; hard contact
wounds of head cause severe
tearing and disruption of
tissue (because of thin layer
of skin and muscle
overlying bone); wound is
gaping and jagged, known
as blow back; can produce a
patterned abrasion that
mirrors weapon used (see
photo, C)
Intermediate (distance)
range entrance wound:
Surrounded by gunpowder
tattooing or stippling;
tattooing results from
fragments of burning or
unburned pieces of
gunpowder exiting barrel
and forcefully striking skin;
stippling results when
gunpowder abrades but does
not penetrate skin (see photo,
D)
Indeterminate range
entrance wound: Occurs
when flame, soot, or
gunpowder does not reach
skin surface but bullet does;
indeterminate is used rather
than distant because
appearance may be same
regardless of distance; for
example, if an individual is
shot at close range through
multiple layers of clothing
the wound may look the
same as if the shooting
occurred at a distance
Exit wound: Has the same
appearance regardless of
range of fire; most
important factors are speed
of projectile and degree of
deformation; size cannot be
used to determine if hole is
an exit or entrance wound;
usually has clean edges that
can often be reapproximated
to cover defect; skin is one of
toughest structures for a
bullet to penetrate; thus it is
not uncommon for a bullet
to pass entirely through
body but stopped just
beneath skin on “exit” side
Wounding potential of
bullets: Most damage done
by a bullet is a result of
amount of energy
transferred to tissue
impacted; speed of bullet has
much greater effect than
increased size; some bullets
are designed to expand or
fragment when striking an
object, for example, hollow-
point ammunition; lethality
of a wound depends on what
structures are damaged;
wounds of brain may not be
lethal; however, they are
usually immediately
incapacitating and lead to
significant long-term
disability; a person with a
“lethal” injury (wound of
heart or aorta) also may not
be immediately incapacitated
Asphyxial Injuries
Asphyxial injuries are caused by a failure of cells to receive or use oxygen.
Deprivation of oxygen may be partial (hypoxia) or total (anoxia). Asphyxial injuries
can be grouped into four general categories: suffocation, strangulation, chemical
asphyxiants, and drowning.
Suffocation.
Suffocation, or oxygen failing to reach the blood, can result from a lack of oxygen
in the environment (entrapment in an enclosed space or filling of the environment
with a suffocating gas) or blockage of the external airways. Classic examples of
these types of asphyxial injuries are a child who is trapped in an abandoned
refrigerator or a person who commits suicide by putting a plastic bag over his or
her head. A reduction in the ambient oxygen level to 16% (normal is 21%) is
immediately dangerous. If the level is below 5%, death can ensue within a matter of
minutes. The diagnosis of these types of asphyxial injuries depends on obtaining an
accurate and thorough history because there will be no specific physical findings.
Diagnosis and treatment in choking asphyxiation (obstruction of the internal
airways) depend on locating and removing the obstructing material. Injury or
disease also may cause swelling of the soft tissues of the airway, leading to partial
or complete obstruction and subsequent asphyxiation. Suffocation also may result
from compression of the chest or abdomen (mechanical or compressional
asphyxia), preventing normal respiratory movements. Usual signs and symptoms
include florid facial congestion and petechiae (pinpoint hemorrhages) of the eyes
and face.
Strangulation.
Strangulation is caused by compression and closure of the blood vessels and air
passages resulting from external pressure on the neck. This causes cerebral hypoxia
or anoxia secondary to the alteration or cessation of blood flow to and from the
brain. It is important to remember that the amount of force needed to close the
jugular veins (2 kg [4.5 lb]) or carotid arteries (5 kg [11 lb]) is significantly less
than that required to crush the trachea (15 kg [33 lb]). It is the alteration of cerebral
blood flow in most types of strangulation that causes injury or death—not the lack
of airflow. With complete blockage of the carotid arteries, unconsciousness can
occur within 10 to 15 seconds.
A noose is placed around the neck, and the weight of the body is used to cause
constriction of the noose and compression of the neck in hanging strangulations.
The body does not need to be completely suspended to produce severe injury or
death. Depending on the type of ligature used, there usually is a distinct mark on the
neck—an inverted V with the base of the V pointing toward the point of suspension.
Internal injuries of the neck are actually quite rare in hangings, and only in judicial
hangings, in which the body is weighted and dropped, is significant soft tissue or
cervical spinal trauma seen. Petechiae of the eyes or face may be seen, but they are
rare.
In ligature strangulation, the mark on the neck is horizontal without the inverted
V pattern seen in hangings. Petechiae may be more common because intermittent
opening and closure of the blood vessels may occur as a result of the victim’s
struggles. Internal injuries of the neck are rare.
Variable amounts of external trauma on the neck are found with contusions and
abrasions in manual strangulation caused either by the assailant or by the victim
clawing at his or her own neck in an attempt to remove the assailant’s hands. Internal
damage can be quite severe, with bruising of deep structures and even fractures of
the hyoid bone and tracheal and cricoid cartilages. Petechiae are common.
Chemical asphyxiants.
Chemical asphyxiants either prevent the delivery of oxygen to the tissues or block
its utilization. Carbon monoxide is the most common chemical asphyxiant (see p.
90). Cyanide acts as an asphyxiant by combining with the ferric iron atom in
cytochrome oxidase, thereby blocking the intracellular use of oxygen. A victim of
cyanide poisoning will have the same cherry-red appearance as a carbon monoxide
intoxication victim because cyanide blocks the use of circulating oxyhemoglobin.
An odor of bitter almonds also may be detected. (The ability to smell cyanide is a
genetic trait that is absent in a significant portion of the general population.)
Hydrogen sulfide (sewer gas) is a chemical asphyxiant in which victims of
hydrogen cyanide poisoning may have brown-tinged blood in addition to the
nonspecific signs of asphyxiation.
Drowning.
Drowning is an alteration of oxygen delivery to tissues resulting from the
inhalation of fluid, usually water. In 2012 there were 3391 drowning deaths in the
United States. Although research in the 1940s and 1950s indicated that changes in
blood electrolyte levels and volume as a result of absorption of fluid from the lungs
may be an important factor in some drownings, the major mechanism of injury is
hypoxemia (low blood oxygen levels). Even in freshwater drownings, where large
amounts of water can pass through the alveolar-capillary interface, there is no
evidence that increases in blood volume cause significant electrolyte disturbances
or hemolysis, or that the amount of fluid loading is beyond the compensatory
capabilities of the kidneys and heart. Airway obstruction is the more important
pathologic abnormality, underscored by the fact that in as many as 15% of
drownings little or no water enters the lungs because of vagal nerve–mediated
laryngospasms. This phenomenon is called dry-lung drowning.
No matter what mechanism is involved, cerebral hypoxia leads to
unconsciousness in a matter of minutes. Whether this progresses to death depends
on a number of factors, including the age and the health of the individual. One of the
most important factors is the temperature of the water. Irreversible injury develops
much more rapidly in warm water than it does in cold water. Submersion times of
up to 1 hour with subsequent survival have been reported in children who were
submerged in very cold water. Complete submersion is not necessary for a person
to drown. An incapacitated or helpless individual (epileptic, alcoholic, infant) may
drown in water that is only a few inches deep.
It is important to remember that no specific or diagnostic findings prove that a
person recovered from the water is actually a drowning victim. In cases where water
has entered the lung, there may be large amounts of foam exiting the nose and
mouth, although this also can be seen in certain types of drug overdoses. A body
recovered from water with signs of prolonged immersion could just as easily be a
victim of some other type of injury with the immersion acting to obscure the actual
cause of death. When working with a living victim recovered from water, it is
essential to keep in mind that an underlying condition may have led to the person’s
becoming incapacitated and submerged—a condition that also may need to be
treated or corrected while correcting hypoxemia and dealing with its sequelae.
Quick Check 4-3
1. Give examples of intentional and unintentional injury in the United States..
2. Discuss unintentional injury as a form of injury with health care delivery in the
United States.
3. What is the major mechanism of injury with drowning?
Infectious Injury
The pathogenicity (virulence) of microorganisms lies in their ability to survive and
proliferate in the human body, where they injure cells and tissues. The disease-
producing potential of a microorganism depends on its ability to (1) invade and
destroy cells, (2) produce toxins, and (3) produce damaging hypersensitivity
reactions. (See Chapter 8 for a description of infection and infectious organisms.)
Immunologic and Inflammatory Injury
Cellular membranes are injured by direct contact with cellular and chemical
components of the immune and inflammatory responses, such as phagocytic cells
(lymphocytes, macrophages) and substances such as histamine, antibodies,
lymphokines, complement, and proteases (see Chapter 6). Complement is
responsible for many of the membrane alterations that occur during immunologic
injury.
Membrane alterations are associated with a rapid leakage of potassium (K+) out
of the cell and a rapid influx of water. Antibodies can interfere with membrane
function by binding with and occupying receptor molecules on the plasma
membrane. Antibodies also can block or destroy cellular junctions, interfering with
intercellular communication. Other mechanisms of cellular injury are genetic and
epigenetic factors, nutritional imbalances, and physical agents. These are
summarized in Table 4-9.
TABLE 4-9
Mechanisms of Cellular Injury
Mechanism Characteristics Examples
Genetic
Factors
Alter cell’s nucleus and plasma membrane’s structure, shape, receptors, or transport
mechanisms
Sickle cell anemia, Huntington disease, muscular
dystrophy, abetalipoproteinemia, familial
hypercholesterolemia
Epigenetic
Factors
Induction of mitotically heritable alterations in gene expression without changing DNA Gene silencing in cancer
Nutritional
Imbalances
Pathophysiologic cellular effects develop when nutrients are not consumed in diet and
transported to body’s cells or when excessive amounts of nutrients are consumed and
transported
Protein deficiency, protein-calorie malnutrition,
glucose deficiency, lipid deficiency (hypolipidemia),
hyperlipidemia (increased lipoproteins in blood
causing deposits of fat in heart, liver, and muscle),
vitamin deficiencies
Physical Agents
Temperature
extremes
Hypothermic injury results from chilling or freezing of cells, creating high intracellular
sodium concentrations; abrupt drops in temperature lead to vasoconstriction and
increased viscosity of blood, causing ischemic injury, infarction, and necrosis; reactive
oxygen species (ROS) are important in this process
Frostbite
Hyperthermic injury is caused by excessive heat and varies in severity according to
nature, intensity, and extent of heat
Burns, burn blisters, heat cramps usually from
vigorous exercise with water and salt loss; heat
exhaustion with salt and water loss causes heme
contraction; heat stroke is life-threatening with a
clinical rectal temperature of 106° F
Tissue injury caused by compressive waves of air or fluid impinging on body, followed
by sudden wave of decreased pressure; changes may collapse thorax, rupture internal
solid organs, and cause widespread hemorrhage: carbon dioxide and nitrogen that are
normally dissolved in blood precipitate from solution and form small bubbles (gas
emboli), causing hypoxic injury and pain
Blast injury (air or immersion), decompression
sickness (caisson disease or “the bends”); recently
reported in a few individuals with subdural
hematomas after riding high-speed roller coasters
Ionizing
radiation
Refers to any form of radiation that can remove orbital electrons from atoms; source is
usually environment and medical use; damage is to DNA molecule, causing
chromosomal aberrations, chromosomal instability, and damage to membranes and
enzymes; also induces growth factors and extracellular matrix remodeling; uncertainty
exists regarding effects of low levels of radiation
X-rays, γ-rays, and α- and β-particles cause skin
redness, skin damage, chromosomal damage, cancer
Illumination Fluorescent lighting and halogen lamps create harmful oxidative stresses; ultraviolet
light has been linked to skin cancer
Eyestrain, obscured vision, cataracts, headaches,
melanoma
Mechanical
stresses
Injury is caused by physical impact or irritation; they may be overt or cumulative Faulty occupational biomechanics, leading to
overexertion disorders
Noise Can be caused by acute loud noise or cumulative effects of various intensities,
frequencies, and duration of noise; considered a public health threat
Hearing impairment or loss; tinnitus, temporary
threshold shift (TTS), or loss can occur as a
complication of critical illness, from mechanical
trauma, ototoxic medications, infections, vascular
disorders, and noise
Manifestations of Cellular Injury:
Accumulations
An important manifestation of cell injury is the intracellular accumulation of
abnormal amounts of various substances and the resultant metabolic disturbances.
Cellular accumulations, also known as infiltrations, not only result from sublethal,
sustained injury by cells, but also result from normal (but inefficient) cell function.
Two categories of substances can produce accumulations: (1) a normal cellular
substance (such as excess water, proteins, lipids, and carbohydrates) or (2) an
abnormal substance, either endogenous (such as a product of abnormal metabolism
or synthesis) or exogenous (such as infectious agents or a mineral). These products
can accumulate transiently or permanently and can be toxic or harmless. Most
accumulations are attributed to four types of mechanisms, all abnormal (Figure 4-
23). Abnormal accumulations of these substances can occur in the cytoplasm (often
in the lysosomes) or in the nucleus if (1) there is insufficient removal of the normal
substance because of altered packaging and transport, for example, fatty change in
the liver called steatosis; (2) an abnormal substance, often the result of a mutated
gene, accumulates because of defects in protein folding, transport, or abnormal
degradation; (3) an endogenous substance (normal or abnormal) is not effectively
catabolized, usually because of lack of a vital lysosomal enzyme, called storage
diseases; or (4) harmful exogenous materials, such as heavy metals, mineral dusts,
or microorganisms, accumulate because of inhalation, ingestion, or infection.
FIGURE 4-23 Mechanisms of Intracellular Accumulations. (From Kumar V et al, editors: Robbins and Cotran
pathologic basis of disease, ed 9, Philadelphia, 2015, Elsevier.)
In all storage diseases, the cells attempt to digest, or catabolize, the “stored”
substances. As a result, excessive amounts of metabolites (products of catabolism)
accumulate in the cells and are expelled into the extracellular matrix, where they are
consumed by phagocytic cells called macrophages (see Chapter 6). Some of these
scavenger cells circulate throughout the body, whereas others remain fixed in
certain tissues, such as the liver or spleen. As more and more macrophages and
other phagocytes migrate to tissues that are producing excessive metabolites, the
affected tissues begin to swell. This is the mechanism that causes enlargement of the
liver (hepatomegaly) or the spleen (splenomegaly) as a clinical manifestation of
many storage diseases.
Water
Cellular swelling, the most common degenerative change, is caused by the shift of
extracellular water into the cells. In hypoxic injury, movement of fluid and ions into
the cell is associated with acute failure of metabolism and loss of ATP production.
Normally, the pump that transports sodium ions (Na+) out of the cell is maintained
by the presence of ATP and adenosinetriphosphatase (ATPase), the active transport
enzyme. In metabolic failure caused by hypoxia, reduced levels of ATP and ATPase
permit sodium to accumulate in the cell while potassium (K+) diffuses outward. The
increased intracellular sodium concentration increases osmotic pressure, drawing
more water into the cell. The cisternae of the ER become distended, rupture, and
then unite to form large vacuoles that isolate the water from the cytoplasm, a
process called vacuolation. Progressive vacuolation results in cytoplasmic swelling
called oncosis (which has replaced the old term hydropic [water] degeneration) or
vacuolar degeneration (Figure 4-24). If cellular swelling affects all the cells in an
organ, the organ increases in weight and becomes distended and pale.
FIGURE 4-24 The Process of Oncosis (Formerly Referred to as “Hydropic Degeneration”). ATP,
Adenosine triphosphate.
Cellular swelling is reversible and is considered sublethal. It is, in fact, an early
manifestation of almost all types of cellular injury, including severe or lethal cell
injury. It is also associated with high fever, hypokalemia (abnormally low
concentrations of potassium in the blood; see Chapter 5), and certain infections.
Lipids and Carbohydrates
Certain metabolic disorders result in the abnormal intracellular accumulation of
carbohydrates and lipids. These substances may accumulate throughout the body but
are found primarily in the spleen, liver, and CNS. Accumulations in cells of the CNS
can cause neurologic dysfunction and severe intellectual disability. Lipids
accumulate in Tay-Sachs disease, Niemann-Pick disease, and Gaucher disease;
whereas in the diseases known as mucopolysaccharidoses, carbohydrates are in
excess. The mucopolysaccharidoses are progressive disorders that usually involve
multiple organs, including liver, spleen, heart, and blood vessels. The accumulated
mucopolysaccharides are found in reticuloendothelial cells, endothelial cells,
intimal smooth muscle cells, and fibroblasts throughout the body. These
carbohydrate accumulations can cause clouding of the cornea, joint stiffness, and
intellectual disability.
Although lipids sometimes accumulate in heart, muscle, and kidney cells, the
most common site of intracellular lipid accumulation, or fatty change (steatosis),
is liver cells (Figure 4-25). Because hepatic metabolism and secretion of lipids are
crucial to proper body function, imbalances and deficiencies in these processes lead
to major pathologic changes. In developed countries the most common cause of
fatty change in the liver is alcohol abuse. Other causes of fatty change include
diabetes mellitus, protein malnutrition, toxins, anoxia, and obesity. As lipids fill the
cells, vacuolation pushes the nucleus and other organelles aside. The liver’s outward
appearance is yellow and greasy. Alcohol abuse is one of the most common causes
of fatty liver (see Chapter 36).
FIGURE 4-25 Fatty Liver. The liver appears yellow. (From Damjanov I, Linder J: Pathology: a color atlas, St Louis, 2000,
Mosby.)
Lipid accumulation in liver cells occurs after cellular injury instigates one or
more of the following mechanisms:
1. Increased movement of free fatty acids into the liver (starvation, for example,
increases the metabolism of triglycerides in adipose tissue, releasing fatty acids that
subsequently enter liver cells)
2. Failure of the metabolic process that converts fatty acids to phospholipids,
resulting in the preferential conversion of fatty acids to triglycerides
3. Increased synthesis of triglycerides from fatty acids (increased levels of the
enzyme α-glycerophosphatase can accelerate triglyceride synthesis)
4. Decreased synthesis of apoproteins (lipid-acceptor proteins)
5. Failure of lipids to bind with apoproteins and form lipoproteins
6. Failure of mechanisms that transport lipoproteins out of the cell
7. Direct damage to the ER by free radicals released by alcohol’s toxic effects
Many pathologic states show accumulation of cholesterol and cholesterol esters.
These states include atherosclerosis, in which atherosclerotic plaques, smooth
muscle cells, and macrophages within the intimal layer of the aorta and large
arteries are filled with lipid-rich vacuoles of cholesterol and cholesterol esters.
Other states include cholesterol-rich deposits in the gallbladder and Niemann-Pick
disease (type C), which involve genetic mutations of an enzyme affecting
cholesterol transport.
Glycogen
Glycogen storage is important as a readily available energy source in the cytoplasm
of normal cells. Intracellular accumulations of glycogen are seen in genetic
disorders called glycogen storage diseases and in disorders of glucose and
glycogen metabolism. As with water and lipid accumulation, glycogen
accumulation results in excessive vacuolation of the cytoplasm. The most common
cause of glycogen accumulation is the disorder of glucose metabolism (i.e., diabetes
mellitus) (see Chapter 19).
Proteins
Proteins provide cellular structure and constitute most of the cell’s dry weight. The
proteins are synthesized on ribosomes in the cytoplasm from the essential amino
acids lysine, threonine, leucine, isoleucine, methionine, tryptophan, valine,
phenylalanine, and histidine. The accumulation of protein probably damages cells in
two ways. First, metabolites, produced when the cell attempts to digest some
proteins, are enzymes that when released from lysosomes can damage cellular
organelles. Second, excessive amounts of protein in the cytoplasm push against
cellular organelles, disrupting organelle function and intracellular communication.
Protein excess accumulates primarily in the epithelial cells of the renal
convoluted tubules of the nephron unit and in the antibody-forming plasma cells (B
lymphocytes) of the immune system. Several types of renal disorders cause
excessive excretion of protein molecules in the urine (proteinuria). Normally, little
or no protein is present in the urine, and its presence in significant amounts
indicates cellular injury and altered cellular function.
Accumulations of protein in B lymphocytes can occur during active synthesis of
antibodies during the immune response. The excess aggregates of protein are called
Russell bodies (see Chapter 6). Russell bodies have been identified in multiple
myeloma (plasma cell tumor) (see Chapter 21).
Mutations in protein can slow protein folding, resulting in the accumulation of
partially folded intermediates. An example is α1-antitrypsin deficiency, which can
cause emphysema. Certain types of cell injury are associated with the accumulation
of cytoskeleton proteins. For example, the neurofibrillary tangle found in the brain
in Alzheimer disease contains these types of proteins.
Pigments
Pigment accumulations may be normal or abnormal, endogenous (produced within
the body) or exogenous (produced outside the body). Endogenous pigments are
derived, for example, from amino acids (e.g., tyrosine, tryptophan). They include
melanin and the blood proteins porphyrins, hemoglobin, and hemosiderin. Lipid-
rich pigments, such as lipofuscin (the aging pigment), give a yellow-brown color to
cells undergoing slow, regressive, and often atrophic changes. The most common
exogenous pigment is carbon (coal dust), a pervasive air pollutant in urban areas.
Inhaled carbon interacts with lung macrophages and is transported by lymphatic
vessels to regional lymph nodes. This accumulation blackens lung tissues and
involved lymph nodes. Other exogenous pigments include mineral dusts containing
silica and iron particles, lead, silver salts, and dyes for tattoos.
Melanin
Melanin accumulates in epithelial cells (keratinocytes) of the skin and retina. It is an
extremely important pigment because it protects the skin against long exposure to
sunlight and is considered an essential factor in the prevention of skin cancer (see
Chapters 11 and 41). Ultraviolet light (e.g., sunlight) stimulates the synthesis of
melanin, which probably absorbs ultraviolet rays during subsequent exposure.
Melanin also may protect the skin by trapping the injurious free radicals produced
by the action of ultraviolet light on skin.
Melanin is a brown-black pigment derived from the amino acid tyrosine. It is
synthesized by epidermal cells called melanocytes and is stored in membrane-bound
cytoplasmic vesicles called melanosomes.
Melanin also can accumulate in melanophores (melanin-containing pigment
cells), macrophages, or other phagocytic cells in the dermis. Presumably these cells
acquire the melanin from nearby melanocytes or from pigment that has been
extruded from dying epidermal cells. This is the mechanism that causes freckles.
Melanin also occurs in the benign form of pigmented moles called nevi (see
Chapter 41). Malignant melanoma is a cancerous skin tumor that contains melanin.
A decrease in melanin production occurs in the inherited disorder of melanin
metabolism called albinism. Albinism is often diffuse, involving all the skin, the
eyes, and the hair. Albinism is also related to phenylalanine metabolism. In classic
types, the person with albinism is unable to convert tyrosine to DOPA (3,4-
dihydroxyphenylalanine), an intermediate in melanin biosynthesis. Melanocytes are
present in normal numbers, but they are unable to make melanin. Individuals with
albinism are very sensitive to sunlight and quickly become sunburned. They are also
at high risk for skin cancer.
Hemoproteins
Hemoproteins are among the most essential of the normal endogenous pigments.
They include hemoglobin and the oxidative enzymes, the cytochromes. Central to
an understanding of disorders involving these pigments is knowledge of iron
uptake, metabolism, excretion, and storage (see Chapter 20). Hemoprotein
accumulations in cells are caused by excessive storage of iron, which is transferred
to the cells from the bloodstream. Iron enters the blood from three primary sources:
(1) tissue stores, (2) the intestinal mucosa, and (3) macrophages that remove and
destroy dead or defective red blood cells. The amount of iron in blood plasma
depends also on the metabolism of the major iron transport protein, transferrin.
Iron is stored in tissue cells in two forms: as ferritin and, when increased levels
of iron are present, as hemosiderin. Hemosiderin is a yellow-brown pigment
derived from hemoglobin. With pathologic states, excesses of iron cause
hemosiderin to accumulate within cells, often in areas of bruising and hemorrhage
and in the lungs and spleen after congestion caused by heart failure. With local
hemorrhage, the skin first appears red-blue and then lysis of the escaped red blood
cells occurs, causing the hemoglobin to be transformed to hemosiderin. The color
changes noted in bruising reflect this transformation (Figure 4-26).
FIGURE 4-26 Hemosiderin Accumulation Is Noted as the Color Changes in a “Black Eye.”
Hemosiderosis is a condition in which excess iron is stored as hemosiderin in the
cells of many organs and tissues. This condition is common in individuals who have
received repeated blood transfusions or prolonged parenteral administration of
iron. Hemosiderosis is associated also with increased absorption of dietary iron,
conditions in which iron storage and transport are impaired, and hemolytic anemia.
Excessive alcohol (wine) ingestion also can lead to hemosiderosis. Normally,
absorption of excessive dietary iron is prevented by an iron absorption process in
the intestines. Failure of this process can lead to total body iron accumulations in the
range of 60 to 80 g, compared with normal iron stores of 4.5 to 5 g. Excessive
accumulations of iron, such as occur in hemochromatosis (a genetic disorder of
iron metabolism and the most severe example of iron overload), are associated with
liver and pancreatic cell damage.
Bilirubin is a normal, yellow-to-green pigment of bile derived from the
porphyrin structure of hemoglobin. Excess bilirubin within cells and tissues causes
jaundice (icterus), or yellowing of the skin. Jaundice occurs when the bilirubin level
exceeds 1.5 to 2 mg/dl of plasma, compared with the normal values of 0.4 to
1 mg/dl. Hyperbilirubinemia occurs with (1) destruction of red blood cells
(erythrocytes), such as in hemolytic jaundice; (2) diseases affecting the metabolism
and excretion of bilirubin in the liver; and (3) diseases that cause obstruction of the
common bile duct, such as gallstones or pancreatic tumors. Certain drugs
(specifically chlorpromazine and other phenothiazine derivatives), estrogenic
hormones, and halothane (an anesthetic) can cause the obstruction of normal bile
flow through the liver.
Because unconjugated bilirubin is lipid soluble, it can injure the lipid components
of the plasma membrane. Albumin, a plasma protein, provides significant protection
by binding unconjugated bilirubin in plasma. Unconjugated bilirubin causes two
cellular outcomes: uncoupling of oxidative phosphorylation and a loss of cellular
proteins. These two changes could cause structural injury to the various membranes
of the cell.
Calcium
Calcium salts accumulate in both injured and dead tissues (Figure 4-27). An
important mechanism of cellular calcification is the influx of extracellular calcium
in injured mitochondria. Another mechanism that causes calcium accumulation in
alveoli (gas-exchange airways of the lungs), gastric epithelium, and renal tubules is
the excretion of acid at these sites, leading to the local production of hydroxyl ions.
Hydroxyl ions result in precipitation of calcium hydroxide, Ca(OH)2, and
hydroxyapatite, (Ca3[PO4]2)3•Ca(OH)2, a mixed salt. Damage occurs when calcium
salts cluster and harden, interfering with normal cellular structure and function.
FIGURE 4-27 Free Cytosolic Calcium: A Destructive Agent. Normally, calcium is removed from
the cytosol by adenosine triphosphate (ATP)–dependent calcium pumps. In normal cells,
calcium is bound to buffering proteins, such as calbindin or parvalbumin, and is contained in the
endoplasmic reticulum and the mitochondria. If there is abnormal permeability of calcium-ion
channels, direct damage to membranes, or depletion of ATP (i.e., hypoxic injury), calcium
increases in the cytosol. If the free calcium cannot be buffered or pumped out of cells,
uncontrolled enzyme activation takes place, causing further damage. Uncontrolled entry of
calcium into the cytosol is an important final common pathway in many causes of cell death.
Pathologic calcification can be dystrophic or metastatic. Dystrophic calcification
occurs in dying and dead tissues in areas of necrosis (see also the types of necrosis:
coagulative, liquefactive, caseous, and fatty). It is present in chronic tuberculosis of
the lungs and lymph nodes, advanced atherosclerosis (narrowing of the arteries as a
result of plaque accumulation), and heart valve injury (Figure 4-28). Calcification
of the heart valves interferes with their opening and closing, causing heart murmurs
(see Chapter 24). Calcification of the coronary arteries predisposes them to severe
narrowing and thrombosis, which can lead to myocardial infarction. Another site of
dystrophic calcification is the center of tumors. Over time, the center is deprived of
its oxygen supply, dies, and becomes calcified. The calcium salts appear as gritty,
clumped granules that can become hard as stone. When several layers clump
together, they resemble grains of sand and are called psammoma bodies.
FIGURE 4-28 Aortic Valve Calcification. A, This calcified aortic valve is an example of
dystrophic calcification. B, This algorithm shows the dystrophic mechanism of calcification. (A
from Damjanov I: Pathology for the health professions, ed 4, St Louis, 2012, Saunders.)
Metastatic calcification consists of mineral deposits that occur in undamaged
normal tissues as the result of hypercalcemia (excess calcium in the blood; see
Chapter 5). Conditions that cause hypercalcemia include hyperparathyroidism, toxic
levels of vitamin D, hyperthyroidism, idiopathic hypercalcemia of infancy, Addison
disease (adrenocortical insufficiency), systemic sarcoidosis, milk-alkali syndrome,
and the increased bone demineralization that results from bone tumors, leukemia,
and disseminated cancers. Hypercalcemia also may occur in advanced renal failure
with phosphate retention. As phosphate levels increase, the activity of the
parathyroid gland increases, causing higher levels of circulating calcium.
Urate
In humans, uric acid (urate) is the major end product of purine catabolism because
of the absence of the enzyme urate oxidase. Serum urate concentration is, in
general, stable: approximately 5 mg/dl in postpubertal males and 4.1 mg/dl in
postpubertal females. Disturbances in maintaining serum urate levels result in
hyperuricemia and the deposition of sodium urate crystals in the tissues, leading to
painful disorders collectively called gout. These disorders include acute arthritis,
chronic gouty arthritis, tophi (firm, nodular, subcutaneous deposits of urate crystals
surrounded by fibrosis), and nephritis (inflammation of the nephron). Chronic
hyperuricemia results in the deposition of urate in tissues, cell injury, and
inflammation. Because urate crystals are not degraded by lysosomal enzymes, they
persist in dead cells.
Systemic Manifestations
Systemic manifestations of cellular injury include a general sense of fatigue and
malaise, a loss of well-being, and altered appetite. Fever is often present because of
biochemicals produced during the inflammatory response. Table 4-10 summarizes
the most significant systemic manifestations of cellular injury.
TABLE 4-10
Systemic Manifestations of Cellular Injury
Manifestation Cause
Fever Release of endogenous pyrogens (interleukin-1, tumor necrosis factor-alpha, prostaglandins) from bacteria or
macrophages; acute inflammatory response
Increased heart rate Increase in oxidative metabolic processes resulting from fever
Increase in leukocytes (leukocytosis) Increase in total number of white blood cells because of infection; normal is 5000-9000/mm3 (increase is directly
related to severity of infection)
Pain Various mechanisms, such as release of bradykinins, obstruction, pressure
Presence of cellular enzymes Release of enzymes from cells of tissue* in extracellular fluid
Lactate dehydrogenase (LDH) (LDH
isoenzymes)
Release from red blood cells, liver, kidney, skeletal muscle
Creatine kinase (CK) (CK
isoenzymes)
Release from skeletal muscle, brain, heart
Aspartate aminotransferase
(AST/SGOT)
Release from heart, liver, skeletal muscle, kidney, pancreas
Alanine aminotransferase
(ALT/SGPT)
Release from liver, kidney, heart
Alkaline phosphatase (ALP) Release from liver, bone
Amylase Release from pancreas
Aldolase Release from skeletal muscle, heart
*The rapidity of enzyme transfer is a function of the weight of the enzyme and the concentration gradient
across the cellular membrane. The specific metabolic and excretory rates of the enzymes determine how
long levels of enzymes remain elevated.
Cellular Death
In response to significant external stimuli, cell injury becomes irreversible and cells
are forced to die. Cell death has historically been classified as necrosis and
apoptosis. Necrosis is characterized by rapid loss of the plasma membrane
structure, swelling of organelles, dysfunction of the mitochondria, and lack of
typical features of apoptosis.70 Apoptosis is known as a regulated or programmed
cell process characterized by the “dropping off” of cellular fragments called
apoptotic bodies. Too little or too much apoptosis is linked to many disorders,
including neurodegenerative diseases, ischemic damage, autoimmune disorders,
and cancers. Yet, apoptosis can have normal functions, and unlike necrosis it is not
always linked with a pathologic process. Until recently, necrosis was only
considered passive or accidental cell death occurring after severe and sudden injury.
It is the main outcome in several common injuries including ischemia, toxin
exposure, certain infections, and trauma. It has now been proposed that under certain
conditions, such as activation of death proteases, necrosis may be regulated or
programmed in a well-orchestrated way as a back-up for apoptosis (apoptosis may
progress to necrosis)71—hence the new term programmed necrosis, or
necroptosis. Necroptosis shares traits with both necrosis and apoptosis.72 Although
the identification of the signaling mechanisms for necroptosis is incomplete,
necroptosis is recognized in both normal physiologic conditions and pathologic
conditions, including bone growth plate disorders, cell death in fatty liver disease,
acute pancreatitis, reperfusion injury, and certain neurodegenerative disorders, such
as Parkinson disease.1
Historically, programmed cell death only referred to apoptosis. Figure 4-29
illustrates the structural changes in cell injury resulting in necrosis or apoptosis.
Table 4-11 compares the unique features of necrosis and apoptosis. Other forms of
cell loss include autophagy (self-eating) (see p. 105).
FIGURE 4-29 Schematic Illustration of the Morphologic Changes in Cell Injury Culminating in
Necrosis or Apoptosis. Myelin figures come from degenerating cellular membranes and are
noted within the cytoplasm or extracellularly. (From Kumar V et al, editors: Robbins and Cotran pathologic basis of
disease, ed 9, Philadelphia, 2015, Elsevier.)
TABLE 4-11
Features of Necrosis and Apoptosis
Feature Necrosis Apoptosis
Cell size Enlarged (swelling) Reduced (shrinkage)
Nucleus Pyknosis → karyorrhexis → karyolysis Fragmentation into nucleosome-size fragments
Plasma membrane Disrupted Intact; altered structure, especially orientation of lipids
Cellular contents Enzymatic digestion; may leak out of cell Intact; may be released in apoptotic bodies
Adjacent
inflammation
Frequent No
Physiologic or
pathologic role
Invariably pathologic (culmination of
irreversible cell injury)
Often physiologic, means of eliminating unwanted cells; may be pathologic after some
forms of cell injury, especially DNA damage
From Kumar V et al: Cellular responses to stress and toxic insults: adaptation, injury, and death. In Kumar
V et al, editors: Robbins and Cotran pathologic basis of disease, ed 9, Philadelphia, 2015, Elsevier.
Necrosis
Cellular death eventually leads to cellular dissolution, or necrosis. Necrosis is the
sum of cellular changes after local cell death and the process of cellular self-
digestion, known as autodigestion or autolysis (see Figure 4-29). Cells die long
before any necrotic changes are noted by light microscopy.71 The structural signs
that indicate irreversible injury and progression to necrosis are dense clumping and
progressive disruption both of genetic material and of plasma and organelle
membranes. Because membrane integrity is lost, necrotic cell contents leak out and
may cause the signaling of inflammation in surrounding tissue. In later stages of
necrosis, most organelles are disrupted, and karyolysis (nuclear dissolution and
lysis of chromatin from the action of hydrolytic enzymes) is under way. In some
cells the nucleus shrinks and becomes a small, dense mass of genetic material
(pyknosis). The pyknotic nucleus eventually dissolves (by karyolysis) as a result of
the action of hydrolytic lysosomal enzymes on DNA. Karyorrhexis means
fragmentation of the nucleus into smaller particles or “nuclear dust.”
Although necrosis still refers to death induced by nonspecific trauma or injury
(e.g., cell stress or the heat shock response), with the very recent identification of
molecular mechanisms regulating the process of necrosis, the study of necrosis has
experienced a new twist. Unlike apoptosis, necrosis has been viewed as passive with
cell death occurring in a disorganized and unregulated manner. Some molecular
regulators governing programmed necrosis have been identified and demonstrated
to be interconnected by a large network of signaling pathways.71,73 Emerging
evidence shows that programmed necrosis is associated with pathologic diseases
and provides innate immune response to viral infection.71,73
Different types of necrosis tend to occur in different organs or tissues and
sometimes can indicate the mechanism or cause of cellular injury. The four major
types of necrosis are coagulative, liquefactive, caseous, and fatty. Another type,
gangrenous necrosis, is not a distinctive type of cell death but refers instead to
larger areas of tissue death. These necroses are summarized as follows:
1. Coagulative necrosis. Occurs primarily in the kidneys, heart, and adrenal glands;
commonly results from hypoxia caused by severe ischemia or hypoxia caused by
chemical injury, especially ingestion of mercuric chloride. Coagulation is a result
of protein denaturation, which causes the protein albumin to change from a
gelatinous, transparent state to a firm, opaque state (Figure 4-30, A). The area of
coagulative necrosis is called an infarct.
FIGURE 4-30 Types of Necrosis. A, Coagulative necrosis. A wedge-shaped kidney infarct
(yellow). B, Liquefactive necrosis of the brain. The area of infarction is softened as a result of
liquefaction necrosis. C, Caseous necrosis. Tuberculosis of the lung, with a large area of
caseous necrosis containing yellow-white and cheesy debris. D, Fat necrosis of pancreas.
Interlobular adipocytes are necrotic; acute inflammatory cells surround these. (A and C from Kumar V
et al, editors: Robbins and Cotran pathologic basis of disease, ed 9, Philadelphia, 2015, Elsevier. B from Damjanov I: Pathology for the health
professions, ed 4, St Louis, 2012, Saunders. D from Damjanov I, Linder J, editors: Anderson’s pathology, ed 10, St Louis, 1996, Mosby.)
2. Liquefactive necrosis. Commonly results from ischemic injury to neurons and
glial cells in the brain (Figure 4-30, B). Dead brain tissue is readily affected by
liquefactive necrosis because brain cells are rich in digestive hydrolytic enzymes
and lipids and the brain contains little connective tissue. Cells are digested by their
own hydrolases, so the tissue becomes soft, liquefies, and segregates from healthy
tissue, forming cysts. This can be caused by bacterial infection, especially
Staphylococci, Streptococci, and Escherichia coli.
3. Caseous necrosis. Usually results from tuberculous pulmonary infection,
especially by Mycobacterium tuberculosis (Figure 4-30, C). It is a combination of
coagulative and liquefactive necroses. The dead cells disintegrate, but the debris is
not completely digested by the hydrolases. Tissues resemble clumped cheese in that
they are soft and granular. A granulomatous inflammatory wall encloses areas of
caseous necrosis.
4. Fatty necrosis. Fat necrosis is cellular dissolution caused by powerful enzymes,
called lipases, that occur in the breast, pancreas, and other abdominal structures
(Figure 4-30, D). Lipases break down triglycerides, releasing free fatty acids that
then combine with calcium, magnesium, and sodium ions, creating soaps
(saponification). The necrotic tissue appears opaque and chalk-white.
5. Gangrenous necrosis. Refers to death of tissue but is not a specific pattern of cell
death and results from severe hypoxic injury, commonly occurring because of
arteriosclerosis, or blockage, of major arteries, particularly those in the lower leg
(Figure 4-31). With hypoxia and subsequent bacterial invasion, the tissues can
undergo necrosis. Dry gangrene is usually the result of coagulative necrosis. The
skin becomes very dry and shrinks, resulting in wrinkles, and its color changes to
dark brown or black. Wet gangrene develops when neutrophils invade the site,
causing liquefactive necrosis. This usually occurs in internal organs, causing the
site to become cold, swollen, and black. A foul odor is present, and if systemic
symptoms become severe, death can ensue.
FIGURE 4-31 Gangrene, a Complication of Necrosis. In certain circumstances, necrotic tissue
will be invaded by putrefactive organisms that are both saccharolytic and proteolytic. Foul-
smelling gases are produced, and the tissue becomes green or black as a result of breakdown
of hemoglobin. Obstruction of the blood supply to the bowel almost inevitably is followed by
gangrene.
6. Gas gangrene. Refers to a special type of gangrene caused by infection of
injured tissue by one of many species of Clostridium. These anaerobic bacteria
produce hydrolytic enzymes and toxins that destroy connective tissue and cellular
membranes and cause bubbles of gas to form in muscle cells. This can be fatal if
enzymes lyse the membranes of red blood cells, destroying their oxygen-carrying
capacity. Death is caused by shock.
Apoptosis
Apoptosis (“dropping off”) is an important distinct type of cell death that differs
from necrosis in several ways (see Figure 4-29 and Table 4-11). Apoptosis is an
active process of cellular self-destruction called programmed cell death and is
implicated in both normal and pathologic tissue changes. Cells need to die;
otherwise, endless proliferation would lead to gigantic bodies. The average adult
may create 10 billion new cells every day—and destroy the same number.74 Death by
apoptosis causes loss of cells in many pathologic states including the following:
• Severe cell injury. When cell injury exceeds repair mechanisms, the cell triggers
apoptosis. DNA damage can result either directly or indirectly from production of
free radicals.
• Accumulation of misfolded proteins. This may result from genetic mutations or
free radicals. Excessive accumulation of misfolded proteins in the ER leads to a
condition known as endoplasmic reticulum stress (ER stress) (see Chapter 1). ER
stress results in apoptotic cell death. This mechanism has been linked to several
degenerative diseases of the CNS and other organs (Figure 4-32).
FIGURE 4-32 The Unfolded Protein Response, Endoplasmic Stress, and Apoptosis. A, In normal
or healthy cells the newly made proteins are folded with help from chaperones and then
incorporated into the cell or secreted. B, Various stressors can cause ER stress whereby the
cell is challenged to cope with the increased load of misfolded proteins. The accumulation of
the protein load initiates the unfolded protein response in the ER; if restoration of the protein
fails, the cell dies by apoptosis. An example of a disease caused by misfolding of proteins is
Alzheimer disease. (From Kumar V et al, editors: Robbins and Cotran pathologic basis of disease, ed 9, Philadelphia, 2015, Elsevier.)
• Infections (particularly viral). Apoptosis may be the result of the virus directly or
indirectly by the host immune response. Cytotoxic T lymphocytes respond to viral
infections by inducing apoptosis and, therefore, eliminating the infectious cells.
This process can cause tissue damage and it is the same for cell death in tumors
and rejection of tissue transplants.
• Obstruction in tissue ducts. In organs with duct obstruction, including the pancreas,
kidney, and parotid gland, apoptosis causes pathologic atrophy.
Excessive or insufficient apoptosis is known as dysregulated apoptosis. A low
rate of apoptosis can permit the survival of abnormal cells, for example, mutated
cells that can increase cancer risk. Defective apoptosis may not eliminate
lymphocytes that react against host tissue (self-antigens), leading to autoimmune
disorders. Excessive apoptosis is known to occur in several neurodegenerative
diseases, from ischemic injury (such as myocardial infarction and stroke), and from
death of virus-infected cells (such as seen in many viral infections).
Apoptosis depends on a tightly regulated cellular program for its initiation and
execution.74 This death program involves enzymes that divide other proteins—
proteases, which are activated by proteolytic activity in response to signals that
induce apoptosis. These proteases are called caspases, a family of aspartic acid–
specific proteases. The activated suicide caspases cleave and, thereby, activate other
members of the family, resulting in an amplifying “suicide” cascade. The activated
caspases then cleave other key proteins in the cell, killing the cell quickly and neatly.
The two different pathways that converge on caspase activation are called the
mitochondrial (intrinsic) pathway and the death receptor (extrinsic) pathway (Figure
4-33). Cells that die by apoptosis release chemical factors that recruit phagocytes
that quickly engulf the remains of the dead cell, thus reducing chances of
inflammation. With necrosis, cell death is not tidy because cells that die as a result
of acute injury swell, burst, and spill their contents all over their neighbors, causing
a likely damaging inflammatory response.
FIGURE 4-33 Mechanisms of Apoptosis. The two pathways of apoptosis differ in their induction
and regulation, and both culminate in the activation of “executioner” caspases. The induction of
apoptosis by the mitochondrial pathway involves the Bcl-2 family, which causes leakage of
mitochondrial proteins. The regulators of the death receptor pathway involve the proteases,
called caspases. (Adapted from Kumar V et al, editors: Robbins and Cotran pathologic basis of disease, ed 9, Philadelphia, 2015,
Elsevier.)
Autophagy
The Greek term autophagy means “eating of self.” Autophagy, as a “recycling
factory,” is a self-destructive process and a survival mechanism. Basically,
autophagy involves the delivery of cytoplasmic contents to the lysosome for
degradation. Box 4-3 contains the terms used to describe autophagy.
Box 4-3
The Major Forms of Autophagy
Macroautophagy, the most common term to refer to autophagy, involves the
sequestration and transportation of parts (cargo) of the cytosol in an autophagic
vacuole (autophagosome).
Microautophagy is the inward invagination of the lysosomal membrane for cargo
delivery.
Chaperone-mediated autophagy is the chaperone-dependent proteins that direct
cargo across the lysosomal membrane.
When cells are starved or nutrient deprived, the autophagic process institutes
cannibalization and recycles the digested contents.48,75 Autophagy can maintain
cellular metabolism under starvation conditions and remove damaged organelles
under stress conditions, improving the survival of cells. With the central role of
autophagy in cell homeostasis, autophagy has been implicated in cancer, heart
disease, neurodegeneration diseases, inflammation, and infection.76 Autophagy
begins with a membrane, also known as a phagophore (although controversial)
(Figure 4-34).75 This cup-shaped, curved phagophore expands and engulfs
intracellular cargo—organelles, ribosomes, proteins—forming a double membrane
autophagosome. The cargo-laden autophagosome fuses with the lysosome, now
called an autophagolysosome, which promotes the degradation of the
autophagosome by lysosomal acid proteases. The phagophore membrane is highly
curved along the rim of the open cup, suggesting that mechanisms responsible for
its formation and growth may depend on membrane curvature-dependent events.77
Lysosomal transporters export amino acids and other byproducts of degradation out
of the cytoplasm where they can be reused for the synthesis of macromolecules and
for metabolism.78,79 ATP is generated and cellular damage is reduced during
autophagy that removes nonfunctional proteins and organelles.75
FIGURE 4-34 Autophagy. Cellular stresses, such as nutrient deprivation, activate autophagy
genes that create vacuoles in which cellular organelles are sequestered and then degraded
following fusion of the vesicles with lysosomes. The digested materials are recycled to provide
nutrients for the cell.
Investigators are excited about the utilization of autophagy for therapeutic
strategies. Autophagy is a critical garbage collecting and recycling process in
healthy cells, and this process becomes less efficient and less discriminating as the
cell ages. Consequently, harmful agents accumulate in cells, damaging cells and
leading to aging: for example, failure to clear protein products in neurons of the
CNS can cause dementia; failure to clear ROS-producing mitochondria can lead to
nuclear DNA mutations and cancer. Thus these processes may even partially define
aging. Therefore normal autophagy may potentially rejuvenate an organism and
prevent cancer development as well as other degenerative diseases.80 In addition,
autophagy may be the last immune defense against infectious microorganisms that
penetrate intracellularly.81
Quick Check 4-4
1. Why is an increase in the concentration of intracellular calcium injurious?
2. Compare and contrast necrosis and apoptosis.
3. Why is apoptosis significant?
4. Define autophagy.
Aging and Altered Cellular and Tissue
Biology
The terms aging and life span tend to be used synonymously; however, they are not
equivalent. Aging is usually defined as a normal physiologic process that is
universal and inevitable, whereas life span is the time from birth to death and has
been used to study the aging process.82 Aging is associated with a gradual loss of
homeostatic mechanisms whose underlying cause is perplexing,83 and is a complex
process because of a multiplicity of factors. Investigators are focused on genetic,
epigenetic, inflammatory, oxidative stress, and metabolic origins of aging,
including the study of genetic signatures in humans with exceptional longevity; the
identification and recent discovery of epigenetic mechanisms that modulate gene
expression; the role of intrauterine environment and lifelong patterns of health; the
effects of personality, behavior, and social support; the influence of insulin/insulin-
like growth factor 1 (IGF-1) signaling; and the contributions of cellular dysfunction
and senescence to an inflammatory microenvironment that leads to chronic disease,
frailty, and decreased life span. To focus more simply, the factors that may be most
important for aging include increased damage to the cell, reduced capacity to divide
(replicative senescence), reduced ability to repair damaged DNA, and increased
likelihood of defective protein balance or homeostasis.1 A major challenge of aging
research has been to separate the causes of cell and tissue aging from the vast
changes that accompany it.83 Public health issues related to healthy aging require
understanding of the nature of aging and the factors that predict healthy aging and
delayed transition to increasing vulnerability and frailty.
Aging traditionally has not been considered a disease because it is “normal”;
disease is usually considered “abnormal.” Conceptually, this distinction seems clear
until the concept of “injury” or “damage” is introduced; disease has been defined by
some pathologists as the result of injury. Chronologic aging has been defined as the
time-dependent loss of structure and function that proceeds very slowly and in such
small increments that it appears to be the result of the accumulation of small,
imperceptible injuries—a gradual result of wear and tear. One of the hallmarks of
aging is the accumulation of damaged macromolecules. DNA damage can lead to
cellular dysfunction both directly and indirectly as a consequence of cellular
responses to damage that can lead to altered gene expression.84,85 Age-related
changes to macromolecules for long-lived cells, such as neurons and myofibers,
lead to gradual loss of structure and function.
Replicative aging or senescence is the accumulation of cellular damage in
continuously dividing cells, for example, epithelia of the skin or gastrointestinal
tract. One mechanism of replicative senescence is the progressive shortening of
telomeres—the repeated sequences of DNA at the ends of chromosomes. Replicative
aging and chronologic aging are particularly important for adult stem cells because
they divide throughout life.86 As mutations increase with age, cell fates include
apoptosis, malignant transformation, cell cycle arrest, or senescence.87
Despite the fact that aging and death are inevitable, life span, on the other hand,
can be experimentally changed.83 Genetic and environmental interventions have
extended the life span of model organisms, such as the nematode worm
Caenorhabditis elegans (C. elegans), the fruit fly Drosophilia melanogaster, and
mice.88,89 Extending life span, however, is not equivalent to delaying aging!83 For
example, treatment of an acute infection can prevent death but the fundamental rate
of aging continues. Yet, investigators will study and try to isolate, manipulate, and
reset so-called longevity genes to slow the rate of aging.
Recent advances in stem cell biology have begun to reveal the molecular
mechanisms behind reprogramming events that occur during fertilization and when
the nucleus of a mature somatic cell is transferred to an enucleated oocyte. Called
somatic cell nuclear transfer (SCNT), this process gave rise to the first cloned
mammal, Dolly the sheep, and lead to the explosion of research in cloning.83 SCNT
is important in terms of demonstrating the ability of the oocyte cytoplasm to
reprogram the donor nucleus. These reprogramming events have led to the process
to create induced pluripotent stem cells (iPSCs).90 The major emphasis of
reprogramming research is the reversal of the differentiated program and
attainment of a pluripotent state (differentiated cells in all three germ layers of the
embryo) and not the reversal of aging.83,91 Nevertheless, each of these processes is
discussed in the context of resetting the aging clock.
Restoration of youthfulness to aged cells and tissues has created so-called
rejuvenating interventions. Experiments to test whether cells and tissues from an old
animal can be restored to a younger self include the approach called heterochronic
(i.e., young-to-old or old-to-young) transplantations and heterochronic parabiosis,
when the systemic circulations of two animals are joined. The systemic environment
may become more youthful with restoration of protein components in the blood and
tissues, especially chemokines and cytokines.92 For example, investigators found a
protein, GDF-11, may reverse age-associated cardiac hypertrophy when injected
into old animals.93
Administration of the drug rapamycin, an mTOR inhibitor, can extend the life
span of mice.94 These and future studies may not just change differentiation
programs of cells and tissue, but also possibly alter the aging clock. Observations in
C. elegans suggest strongly that the causes of aging may be largely epigenetic.83,95,96
Normal Life Span, Life Expectancy, and Quality-
Adjusted Life Year
The maximal life span of humans is between 80 and 100 years and does not vary
significantly among populations. Life expectancy is the average number of years of
life remaining at a given age, however, it does not include quality of life. The
quality-adjusted life year (QALY) is a measure of disease burden including quality
and not just quantity of live lived. The Centers for Disease Control and Prevention
reported in 2009 that the overall life expectancy at birth was 78.5 years. Between
2008 and 2009, life expectancy at birth increased for all groups reviewed. It
increased for males, from 75.6 to 76.0 years, and females, 80.6 to 80.9 years; for the
white population, 78.5 to 78.8 years; the black population, 74.0 to 74.5 years; the
Hispanic population, 81.0 to 81.2 years; the non-Hispanic white population, 78.4 to
78.7 years; and the non-Hispanic black population, 73.7 to 74.0 years.97
Degenerative Extracellular Changes
Extracellular factors that affect the aging process include the binding of collagen;
the increase in the effects of free radicals on cells; the structural alterations of
fascia, tendons, ligaments, bones, and joints; and the development of peripheral
vascular disease, particularly arteriosclerosis (see Chapter 24).
Aging affects the extracellular matrix with increased cross-linking (e.g., aging
collagen becomes more insoluble, chemically stable but rigid, resulting in
decreased cell permeability), decreased synthesis, and increased degradation of
collagen. The extracellular matrix determines the tissue’s physical properties.98
These changes, together with the disappearance of elastin and changes in
proteoglycans and plasma proteins, cause disorders of the ground substance that
result in dehydration and wrinkling of the skin (see Chapter 41). Other age-related
defects in the extracellular matrix include skeletal muscle alterations (e.g., atrophy,
decreased tone, loss of contractility), cataracts, diverticula, hernias, and rupture of
intervertebral disks.
Free radicals of oxygen that result from oxidative cellular metabolism, oxidative
stress (e.g., respiratory chain, phagocytosis, prostaglandin synthesis), damage
tissues during the aging process. The oxygen radicals produced include superoxide
radical, hydroxyl radical, and hydrogen peroxide (see p. 81). These oxygen
products are extremely reactive and can damage nucleic acids, destroy
polysaccharides, oxidize proteins, peroxidize unsaturated fatty acids, and kill and
lyse cells. Oxidant effects on target cells can lead to malignant transformation,
presumably through DNA damage. That progressive and cumulative damage from
oxygen radicals may lead to harmful alterations in cellular function is consistent
with those alterations of aging. This hypothesis is founded on the wear-and-tear
theory of aging, which states that damages accumulate with time, decreasing the
organism’s ability to maintain a steady state. Because these oxygen-reactive species
not only can permanently damage cells but also may lead to cell death, there is new
support for their role in the aging process.
Of much interest is the relationship between aging and the disappearance or
alteration of extracellular substances important for vessel integrity. With aging,
lipid, calcium, and plasma proteins are deposited in vessel walls. These depositions
cause serious basement membrane thickening and alterations in smooth muscle
functioning, resulting in arteriosclerosis (a progressive disease that causes such
problems as stroke, myocardial infarction, renal disease, and peripheral vascular
disease).
Cellular Aging
Cellular changes characteristic of aging include atrophy, decreased function, and
loss of cells, possibly caused by apoptosis (Figure 4-35). Loss of cellular function
from any of these causes initiates the compensatory mechanisms of hypertrophy and
hyperplasia of the remaining cells, which can lead to metaplasia, dysplasia, and
neoplasia. All of these changes can alter receptor placement and function, nutrient
pathways, secretion of cellular products, and neuroendocrine control mechanisms.
In the aged cell, DNA, RNA, cellular proteins, and membranes are most susceptible
to injurious stimuli. DNA is particularly vulnerable to such injuries as breaks,
deletions, and additions. Lack of DNA repair increases the cell’s susceptibility to
mutations that may be lethal or may promote the development of neoplasia (see
Chapter 10).
FIGURE 4-35 Some Biologic Changes Associated with Aging. Insets show the proportion of
remaining functions in the organs of a person in late adulthood compared with those of a 20-
year-old.
Mitochondria are the organelles responsible for the generation of most of the
energy used by eukaryotic cells. Mitochondrial DNA (mtDNA) encodes some of
the proteins of the electron-transfer chain, the system necessary for the conversion
of adenosine diphosphate (ADP) to ATP. Mutations in mtDNA can deprive the cell of
ATP, and mutations are correlated with the aging process. The accumulation of
mutations could be caused by errors in replication or by unrepaired damage.99,100
The most common age-related mtDNA mutation in humans is a large
rearrangement called the 4977 deletion, or common deletion, and is found in humans
older than 40 years. It is a deletion that removes all or part of 7 of the 13 protein-
encoding mtDNA genes and 5 of the 22 tRNA genes. Individual cells containing this
deletion have a condition known as heteroplasmy. Heteroplasmy levels rise with
aging. Cumulative damage of mtDNA is implicated in the progression of such
common diseases as diabetes, cancer, heart failure, and neurodegenerative
disorders.
Tissue and Systemic Aging
It is probably safe to say that every physiologic process functions less efficiently
with increasing age. The most characteristic tissue change with age is a progressive
stiffness or rigidity that affects many systems, including the arterial, pulmonary, and
musculoskeletal systems. A consequence of blood vessel and organ stiffness is a
progressive increase in peripheral resistance to blood flow. The movement of
intracellular and extracellular substances also decreases with age, as does the
diffusion capacity of the lung. Blood flow through organs also decreases.
Changes in the endocrine and immune systems include thymus atrophy. Although
this occurs at puberty, causing a decreased immune response to T-dependent
antigens (foreign proteins), increased formation of autoantibodies and immune
complexes (antibodies that are bound to antigens) and an overall decrease in the
immunologic tolerance for the host’s own cells further diminish the effectiveness of
the immune system later in life. In women the reproductive system loses ova, and in
men spermatogenesis decreases. Responsiveness to hormones decreases in the
breast and endometrium.
The stomach experiences decreases in the rate of emptying and secretion of
hormones and hydrochloric acid. Muscular atrophy diminishes mobility by
decreasing motor tone and contractility. Sarcopenia, loss of muscle mass and
strength, can occur into old age. The skin of the aged individual is affected by
atrophy and wrinkling of the epidermis and by alterations in the underlying dermis,
fat, and muscle.
Total body changes include a decrease in height; a reduction in circumference of
the neck, thighs, and arms; widening of the pelvis; and lengthening of the nose and
ears. Several of these changes are the result of tissue atrophy and of decreased bone
mass caused by osteoporosis and osteoarthritis. Some body composition changes
include an increase in body weight, which begins in middle age (men gain until 50
years of age and women until 70 years), and an increase fat mass followed by a
decrease in stature, weight, fat-free mass, and body cell mass at older ages. Fat-free
mass (FFM) includes all minerals, proteins, and water plus all other constituents
except lipids. As the amount of fat increases, the percentage of total body water
decreases. Increased body fat and centralized fat distribution (abdominal area) are
associated with non–insulin-dependent diabetes and heart disease. Total body
potassium concentration also decreases because of decreased cellular mass. An
increased sodium/potassium ratio suggests that the decreased cellular mass is
accompanied by an increased extracellular compartment.
Although some of these alterations are probably inherent in aging, others
represent consequences of the process. Advanced age increases susceptibility to
disease, and death occurs after an injury or insult because of diminished cellular,
tissue, and organ function.
Frailty
Frailty is a common clinical syndrome in older adults, leaving a person vulnerable
to falls, functional decline, disability, disease, and death. With an increasing aged
population worldwide efforts to promote independence and decrease frailty are
challenging and needed. Sarcopenia and cachexia are common as a consequence of
aging and many acute and chronic illnesses.101 Investigators are grappling with a
common nomenclature to develop consensus for definitions of sarcopenia and
cachexia. One proposal has been to define it simply as “muscle wasting disease,”
which can be applied in both acute and chronic settings.101 An acceptable vocabulary
and classification system is yet to be developed.
The determinants of sarcopenia include environmental and genetic factors, which
presently are poorly understood.102 Common themes of mechanisms for sarcopenia
include the following: (1) decrease in the number of skeletal muscle fibers, mainly
type II fibers; (2) decline in muscle protein synthesis with age; (3) decline in muscle
fractions, such as myofibrillar and mitochondrial, with age; (4) reduction in protein
turnover adversely affecting muscle function by inducing protein loss and protein
accumulation; (5) loss of alpha motor neurons in the spinal column; (6)
dysregulation of anabolic hormones; (7) cytokine productions and inflammation;
(8) inadequate nutrition; and (9) sedentary history.102,103 For research and clinical
purposes, the criteria indicating compromised energetics include low grip strength,
slowed walking speed, low physical activity, and unintentional weight loss.104 The
syndrome is complex and involves other alterations such as osteopenia, cognitive
impairment, anemia, and gender differences.
Somatic Death
Somatic death is death of the entire person. Unlike the changes that follow cellular
death in a live body, postmortem change is diffuse and does not involve
components of the inflammatory response. Within minutes after death, postmortem
changes appear, eliminating any difficulty in determining that death has occurred.
The most notable manifestations are complete cessation of respiration and
circulation. The surface of the skin usually becomes pale and yellowish; however,
the lifelike color of the cheeks and lips may persist after death that is caused by
carbon monoxide poisoning, drowning, or chloroform poisoning.105
Body temperature falls gradually immediately after death and then more rapidly
(approximately 1.0° to 1.5° F/hour) until, after 24 hours, body temperature equals
that of the environment.106 After death caused by certain infective diseases, body
temperature may continue to rise for a short time. Postmortem reduction of body
temperature is called algor mortis.
Blood pressure within the retinal vessels decreases, causing muscle tension to
decrease and the pupils to dilate. The face, nose, and chin become sharp or peaked-
looking as blood and fluids drain from these areas.105 Gravity causes blood to settle
in the most dependent, or lowest, tissues, which develop a purple discoloration
called livor mortis. Incisions made at this time usually fail to cause bleeding. The
skin loses its elasticity and transparency.
Within 6 hours after death, acidic compounds accumulate within the muscles
because of the breakdown of carbohydrates and the depletion of ATP. This interferes
with ATP-dependent detachment of myosin from actin (contractile proteins), and
muscle stiffening, or rigor mortis, develops. The smaller muscles are usually
affected first, particularly the muscles of the jaw. Within 12 to 14 hours, rigor
mortis usually affects the entire body.
Signs of putrefaction are generally obvious about 24 to 48 hours after death.
Rigor mortis gradually diminishes, and the body becomes flaccid at 36 to 62 hours.
Putrefactive changes vary depending on the temperature of the environment. The
most visible is greenish discoloration of the skin, particularly on the abdomen. The
discoloration is thought to be related to the diffusion of hemolyzed blood into the
tissues and the production of sulfhemoglobin, choleglobin, and other denatured
hemoglobin derivatives.106,107 Slippage or loosening of the skin from underlying
tissues occurs at the same time. After this, swelling or bloating of the body and
liquefactive changes occur, sometimes causing opening of the body cavities. At a
microscopic level, putrefactive changes are associated with the release of enzymes
and lytic dissolution called postmortem autolysis.
Quick Check 4-5
1. Aging is a complex process, discuss the multitude of mechanisms of aging.
2. What are the body composition changes that occur with aging?
3. Define frailty and possible endocrine-immune system involvement.
Did You Understand?
Cellular Adaptation
1. Cellular adaptation is a reversible, structural, or functional response both to
normal or physiologic conditions and to adverse or pathologic conditions. Cells
can adapt to physiologic demands or stress to maintain a steady state called
homeostasis.
2. The most significant adaptive changes include atrophy, hypertrophy, hyperplasia,
and metaplasia.
3. Atrophy is a decrease in cellular size caused by aging, disuse, or reduced/absent
blood supply, hormonal stimulation, or neural stimulation. The amounts of ER,
mitochondria, and microfilaments decrease. The mechanisms of atrophy probably
include decreased protein synthesis, increased protein catabolism, or both. A new
hypothesis called ribosome biogenesis involves the role of mRNA and protein
translation.
4. Hypertrophy is an increase in the size of cells in response to mechanical stimuli
and consequently increases the size of the affected organ. The amounts of protein in
the plasma membrane, ER, microfilaments, and mitochondria increase. Hypertrophy
can be classified as physiologic or pathologic.
5. Hyperplasia is an increase in the number of cells caused by an increased rate of
cellular division. Hyperplasia is classified as physiologic (compensatory and
hormonal) and pathologic.
6. Metaplasia is the reversible replacement of one mature cell type by another less
mature cell type.
7. Dysplasia, or atypical hyperplasia, is an abnormal change in the size, shape, and
organization of mature tissue cells. It is considered atypical rather than a true
adaptational change.
Cellular Injury
1. Injury to cells and to the extracellular matrix (ECM) leads to injury of tissues and
organs and ultimately determining the structural patterns of disease. Cellular injury
occurs if the cell is unable to maintain homeostasis—a normal or adaptive steady
state—in the face of injurious stimuli or stress. Injured cells may recover
(reversible injury) or die (irreversible injury).
2. Injury is caused by lack of oxygen (hypoxia), free radicals, caustic or toxic
chemicals, infectious agents, inflammatory and immune responses, genetic factors,
insufficient nutrients, or physical and mechanical trauma from many causes.
3. Four biochemical themes are important to cell injury: (1) ATP depletion, resulting
in mitochondrial damage; (2) accumulation of oxygen and oxygen-derived free
radicals, causing membrane damage; (3) protein folding defects; and (4) increased
intracellular calcium concentration and loss of calcium steady state.
4. The sequence of events leading to cell death is commonly decreased ATP
production, failure of active transport mechanisms (the sodium-potassium pump),
cellular swelling, detachment of ribosomes from the ER, cessation of protein
synthesis, mitochondrial swelling as a result of calcium accumulation, vacuolation,
leakage of digestive enzymes from lysosomes, autodigestion of intracellular
structures, lysis of the plasma membrane, and death.
5. The initial insult in hypoxic injury is usually ischemia (the cessation of blood
flow into vessels that supply the cell with oxygen and nutrients).
6. Free radicals cause cellular injury because they have an unpaired electron that
makes the molecule unstable. To stabilize itself, the molecule either donates or
accepts an electron from another molecule. Therefore it forms injurious chemical
bonds with proteins, lipids, and carbohydrates—key molecules in membranes and
nucleic acids.
7. The damaging effects of free radicals, especially activated oxygen species such as
, OH•, and H2O2, called oxidative stress, include (1) peroxidation of lipids, (2)
alteration of ion pumps and transport mechanisms, (3) fragmentation of DNA, and
(4) damage to mitochondria, releasing calcium into the cytosol.
8. Restoration of oxygen, however, can cause additional injury, called reperfusion
injury. The mechanisms discussed for reperfusion-injury include oxidative stress,
increased intracellular calcium concentration, inflammation, and complement
activation.
9. Humans are exposed to thousands of chemicals that have inadequate toxicologic
data. A systems biology approach is now being used to investigate toxicity pathways
that include oxidative stress, heat shock proteins, DNA damage response, hypoxia,
ER stress, mental stress, inflammation, and osmotic stress.
10. Unintentional and intentional injuries are an important health problem in the
United States. Death as a result of these injuries is more common for men than
women and higher among blacks than whites and other racial groups.
11. Injuries by blunt force are the result of the application of mechanical energy to
the body, resulting in tearing, shearing, or crushing of tissues. The most common
types of blunt-force injuries include motor vehicle accidents and falls.
12. A contusion is bleeding into the skin or underlying tissues as a consequence of a
blow. A collection of blood in soft tissues or an enclosed space may be referred to
as a hematoma.
13. An abrasion (scrape) results from removal of the superficial layers of the skin
caused by friction between the skin and injuring object. Abrasions and contusions
may have a patterned appearance that mirrors the shape and features of the injuring
object.
14. A laceration is a tear or rip resulting when the tensile strength of the skin or
tissue is exceeded.
15. An incised wound is a cut that is longer than it is deep. A stab wound is a
penetrating sharp-force injury that is deeper than it is long.
16. Gunshot wounds may be either penetrating (bullet retained in the body) or
perforating (bullet exits the body). The most important factors determining the
appearance of a gunshot injury are whether it is an entrance or an exit wound and
the range of fire.
17. Asphyxial injuries are caused by a failure of cells to receive or utilize oxygen.
These injuries can be grouped into four general categories: suffocation,
strangulation, chemical, and drowning.
18. Activation of inflammation and immunity, which occurs after cellular injury or
infection, involves powerful biochemicals and proteins capable of damaging
normal (uninjured and uninfected) cells.
19. Genetic disorders injure cells by altering the nucleus and the plasma membrane’s
structure, shape, receptors, or transport mechanisms.
20. Deprivation of essential nutrients (proteins, carbohydrates, lipids, vitamins) can
cause cellular injury by altering cellular structure and function, particularly of
transport mechanisms, chromosomes, the nucleus, and DNA.
21. Injurious physical agents include temperature extremes, changes in atmospheric
pressure, ionizing radiation, illumination, mechanical stresses, and noise.
22. Errors in health care are a leading cause of injury or death in the United States.
Errors involve medicines, surgery, diagnosis, equipment, and laboratory reports.
They can occur anywhere in the healthcare system including hospitals, clinics,
outpatient surgery centers, physicians’ and nurse practitioners’ offices, pharmacies,
and the individual’s home.
Manifestations of Cellular Injury
1. An important manifestation of cell injury is the resultant metabolic disturbances
of intracellular accumulation (infiltration) of abnormal amounts of various
substances. Two categories of accumulations are (1) normal cellular substances,
such as water, proteins, lipids, and carbohydrate excesses; and (2) abnormal
substances, either endogenous (e.g., from abnormal metabolism) or exogenous
(e.g., a virus).
2. Most accumulations are attributed to four types of mechanisms, all abnormal: (1)
An endogenous substance is produced in excess or at an increased rate; (2) an
abnormal substance, often the result of a mutated gene, accumulates; (3) an
endogenous substance is not effectively catabolized; and (4) a harmful exogenous
substance accumulates because of inhalation, ingestion, or infection.
3. Accumulations harm cells by “crowding” the organelles and by causing excessive
(and sometimes harmful) metabolites to be produced during their catabolism. The
metabolites are released into the cytoplasm or expelled into the extracellular matrix.
4. Cellular swelling, the accumulation of excessive water in the cell, is caused by the
failure of transport mechanisms and is a sign of many types of cellular injury.
Oncosis is a type of cellular death resulting from cellular swelling.
5. Accumulations of organic substances—lipids, carbohydrates, glycogen, proteins,
pigments—are caused by disorders in which (1) cellular uptake of the substance
exceeds the cell’s capacity to catabolize (digest) or use it or (2) cellular anabolism
(synthesis) of the substance exceeds the cell’s capacity to use or secrete it.
6. Dystrophic calcification (accumulation of calcium salts) is always a sign of
pathologic change because it occurs only in injured or dead cells. Metastatic
calcification, however, can occur in uninjured cells in individuals with
hypercalcemia.
7. Disturbances in urate metabolism can result in hyperuricemia and deposition of
sodium urate crystals in tissue—leading to a painful disorder called gout.
8. Systemic manifestations of cellular injury include fever, leukocytosis, increased
heart rate, pain, and serum elevations of enzymes in the plasma.
Cellular Death
1. Cellular death has historically been classified as necrosis and apoptosis. Necrosis
is characterized by rapid loss of the plasma membrane structure, organelle
swelling, mitochondrial dysfunction, and the lack of features of apoptosis.
Apoptosis is known as regulated or programmed cell death and is characterized by
“dropping off” of cellular fragments, called apoptotic bodies. It is now understood
that under certain conditions necrosis is regulated or programmed, hence the new
term programmed necrosis, or necroptosis.
2. There are four major types of necrosis: coagulative, liquefactive, caseous, and
fatty. Different types of necrosis occur in different tissues.
3. Structural signs that indicate irreversible injury and progression to necrosis are
the dense clumping and disruption of genetic material and the disruption of the
plasma and organelle membranes.
4. Apoptosis, a distinct type of sublethal injury, is a process of selective cellular
self-destruction that occurs in both normal and pathologic tissue changes.
5. Death by apoptosis causes loss of cells in many pathologic states including (1)
severe cell injury, (2) accumulation of misfolded proteins, (3) infections, and (4)
obstruction in tissue ducts.
6. Excessive accumulation of misfolded proteins in the ER leads to a condition
known as endoplasmic reticulum stress. ER stress results in apoptotic cell death and
this mechanism has been linked to several degenerative diseases of the CNS and
other organs.
7. Excessive or insufficient apoptosis is known as dysregulated apoptosis.
8. Autophagy means “eating of self,” and as a recycling factory it is a self-
destructive process and a survival mechanism. When cells are starved or nutrient
deprived, the autophagic process institutes cannibalization and recycles the digested
contents. Autophagy can maintain cellular metabolism under starvation conditions
and remove damaged organelles under stress conditions, improving the survival of
cells. Autophagy declines and becomes less efficient as the cell ages, thus
contributing to the aging process.
9. Gangrenous necrosis, or gangrene, is tissue necrosis caused by hypoxia and the
subsequent bacterial invasion.
Aging and Altered Cellular and Tissue Biology
1. It is difficult to determine the physiologic (normal) from the pathologic changes
of aging. Investigators are focused on genetic, epigenetic, inflammatory, oxidative
stress, and metabolic origins of aging.
2. Important factors in aging include increased damage to the cell, reduced capacity
to divide, reduced ability to repair damaged DNA, and increased likelihood of
defective protein balance or homeostasis.
3. Frailty is a common clinical syndrome in older adults, leaving a person
vulnerable to falls, functional decline, disability, disease, and death. Sarcopenia and
cachexia are common as a consequence of aging.
Somatic Death
1. Somatic death is death of the entire organism. Postmortem change is diffuse and
does not involve the inflammatory response.
2. Manifestations of somatic death include cessation of respiration and circulation,
gradual lowering of body temperature, dilation of the pupils, loss of elasticity and
transparency in the skin, stiffening of the muscles (rigor mortis), and discoloration
of the skin (livor mortis). Signs of putrefaction are obvious about 24 to 48 hours
after death.
Key Terms
Adaptation, 73
Aging, 107
Algor mortis, 109
Anoxia, 80
Anthropogenic, 93
Apoptosis, 104
Asphyxial injury, 94
Atrophy, 74
Autolysis, 102
Autophagic vacuole, 74
Autophagy, 105
Bilirubin, 100
Carbon monoxide (CO), 90
Carboxyhemoglobin, 90
Caseous necrosis, 103
Caspase, 105
Cellular accumulations (infiltrations), 96
Cellular swelling, 97
Chemical asphyxiant, 96
Choking asphyxiation, 94
Coagulative necrosis, 102
Compensatory hyperplasia, 76
Cyanide, 96
Cytochrome, 100
Disuse atrophy, 74
Drowning, 96
Dry-lung drowning, 96
Dysplasia (atypical hyperplasia), 77
Dystrophic calcification, 100
Electrophile, 84
Endoplasmic reticulum stress (ER stress), 104
Ethanol, 90
Fat-free mass (FFM), 109
Fatty change (steatosis), 98
Fatty necrosis, 103
Fetal alcohol syndrome, 92
Frailty, 109
Free radical, 81
Gangrenous necrosis, 104
Gas gangrene, 104
Hanging strangulation, 95
Hemoprotein, 100
Hemosiderin, 100
Hemosiderosis, 100
Hormonal hyperplasia, 76
Hydrogen sulfide, 96
Hyperplasia, 76
Hypertrophy, 75
Hypoxia, 78
Hypoxia-inducible factor (HIF), 79
Infarct, 103
Irreversible injury, 78
Ischemia, 79
Ischemia-reperfusion injury, 81
Karyolysis, 102
Karyorrhexis, 102
Lead, 87
Life expectancy, 108
Life span, 107
Ligature strangulation, 96
Lipid peroxidation, 82
Lipofuscin, 75
Liquefactive necrosis, 103
Livor mortis, 109
Manual strangulation, 96
Maximal life span, 108
Melanin, 99
Mesenchymal (tissue from embryonic mesoderm) cell, 77
Metaplasia, 77
Metastatic calcification, 100
Mitochondrial DNA (mtDNA), 109
Necrosis, 102
Nucleophile, 84
Oncosis (vacuolar degeneration), 97
Oxidative stress, 81
Pathologic atrophy, 74
Pathologic hyperplasia, 76
Physiologic atrophy, 74
Postmortem autolysis, 110
Postmortem change, 109
Programmed necrosis (necroptosis), 101
Protein adduct, 85
Proteasome, 74
Psammoma body, 100
Pyknosis, 102
Reperfusion injury, 81
Reversible injury, 78
Rigor mortis, 110
Sarcopenia, 109
Somatic death, 109
Strangulation, 95
Suffocation, 94
Toxicophore, 84
Ubiquitin, 74
Ubiquitin-proteasome pathway, 74
Urate, 101
Vacuolation, 81
Xenobiotic, 84
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5
Fluids and Electrolytes, Acids and
Bases
Sue E. Huether
CHAPTER OUTLINE
Distribution of Body Fluids and Electrolytes, 114
Water Movement Between Plasma and Interstitial
Fluid, 115
Water Movement Between ICF and ECF, 115
Alterations in Water Movement, 115
Edema, 115
Sodium, Chloride, and Water Balance, 116
Alterations in Sodium, Chloride, and Water Balance, 119
Isotonic Alterations, 119
Hypertonic Alterations, 119
Hypotonic Alterations, 121
Alterations in Potassium and Other Electrolytes, 122
Potassium, 122
Other Electrolytes—Calcium, Phosphate, and
Magnesium, 125
Acid-Base Balance, 125
Hydrogen Ion and pH, 125
Buffer Systems, 125
Acid-Base Imbalances, 127
PEDIATRIC CONSIDERATIONS: Distribution of Body Fluids, 131
GERIATRIC CONSIDERATIONS: Distribution of Body Fluids, 131
The cells of the body live in a fluid environment with electrolyte and acid-base
concentrations maintained within a narrow range. Changes in electrolyte
concentration affect the electrical activity of nerve and muscle cells and cause shifts
of fluid from one compartment to another. Alterations in acid-base balance disrupt
cellular functions. Fluid fluctuations also affect blood volume and cellular function.
Disturbances in these functions are common and can be life-threatening.
Understanding how alterations occur and how the body compensates or corrects the
disturbance is important for comprehending many pathophysiologic conditions.
Distribution of Body Fluids and Electrolytes
The sum of fluids within all body compartments constitutes total body water
(TBW)—about 60% of body weight in adults (Table 5-1). The volume of TBW is
usually expressed as a percentage of body weight in kilograms. One liter of water
weighs 2.2 lb (1 kg). The rest of the body weight is composed of fat and fat-free
solids, particularly bone.
TABLE 5-1
Total Body Water (%) in Relation to Body Weight*
Body Build Adult Male Adult Female Child (1-10 yr) Infant (1 mo to 1 yr) Newborn (Up to 1 mo)
Normal 60 50 65 70 70-80
Lean 70 60 50-60 80
Obese 50 42 50 60
*NOTE: Total body water is a percentage of body weight.
Body fluids are distributed among functional compartments, or spaces, and
provide a transport medium for cellular and tissue function. Intracellular fluid
(ICF) comprises all the fluid within cells, about two thirds of TBW. Extracellular
fluid (ECF) is all the fluid outside the cells (about one third of TBW) and includes
the interstitial fluid (the space between cells and outside the blood vessels) and the
intravascular fluid (blood plasma) (Table 5-2). The total volume of body water for
a 70-kg person is about 42 liters. Other ECF compartments include lymph and
transcellular fluids, such as synovial, intestinal, and cerebrospinal fluid; sweat;
urine; and pleural, peritoneal, pericardial, and intraocular fluids.
TABLE 5-2
Distribution of Body Water (70-kg Man)
Fluid Compartment % of Body Weight Volume (L)
Intracellular fluid (ICF) 40 28
Extracellular fluid (ECF) 20 14
Interstitial 15 11
Intravascular 5 3
Total body water (TBW) 60 42
Electrolytes and other solutes are distributed throughout the intracellular and
extracellular fluid (Table 5-3). Note that the extracellular fluid contains a large
amount of sodium and chloride and a small amount of potassium, whereas the
opposite is true of the intracellular fluid. The concentrations of phosphates and
magnesium are greater in the intracellular fluid and the concentration of calcium is
greater in the extracellular fluid. These differences are important for the
maintenance of electroneutrality between the extracellular and intracellular
compartments, the transmission of electrical impulses, and the movement of water
among body compartments (see Chapter 1).
TABLE 5-3
Representative Distribution of Electrolytes in Body Compartments
Electrolytes ECF (mEq/L) ICF (mEq/L)
Cations
Sodium 142 12
Potassium 4.2 150
Calcium 5 0
Magnesium 2 24
TOTAL 153.2 186
Anions
Bicarbonate 24 12
Chloride 103 4
Phosphate 2 100
Proteins 16 65
Other anions 8 6
TOTAL 153 187
ECF, Extracellular fluid; ICF, intracellular fluid.
Although the amount of fluid within the various compartments is relatively
constant, solutes (e.g., salts) and water are exchanged between compartments to
maintain their unique compositions. The percentage of TBW varies with the amount
of body fat and age. Because fat is water repelling (hydrophobic), very little water is
contained in adipose (fat) cells. Individuals with more body fat have proportionately
less TBW and tend to be more susceptible to dehydration.
The distribution and the amount of TBW change with age (see the Pediatric
Considerations and Geriatric Considerations boxes), and although daily fluid intake
may fluctuate widely, the body regulates water volume within a relatively narrow
range. Water obtained by drinking, water ingested in food, and water derived from
oxidative metabolism are the primary sources of body water. Normally, the largest
amounts of water are lost through renal excretion, with lesser amounts lost through
the stool and vaporization from the skin and lungs (insensible water loss) (Table 5-
4).
TABLE 5-4
Normal Water Gains and Losses (70-kg Man)
Daily Intake (mL) Daily Output (mL)
Drinking 1400-1800 Urine 1400-1800
Water in food 700-1000 Stool 100
Water of oxidation 300-400 Skin 300-500
Lungs 600-800
TOTAL 2400-3200 TOTAL 2400-3200
Water Movement Between Plasma and Interstitial
Fluid
The distribution of water and the movement of nutrients and waste products between
the capillary and interstitial spaces occur as a result of changes in hydrostatic
pressure (pushes water) and osmotic/oncotic pressure (pulls water) at the arterial
and venous ends of the capillary (see Figure 1-24). Water, sodium, and glucose
readily move across the capillary membrane. The plasma proteins normally do not
cross the capillary membrane and maintain effective osmolality by generating
plasma oncotic pressure (particularly albumin).
As plasma flows from the arterial to the venous end of the capillary, four forces
determine if fluid moves out of the capillary and into the interstitial space
(filtration) or if fluid moves back into the capillary from the interstitial space
(reabsorption). These forces acting together are described as net filtration or
Starling forces:
1. Capillary hydrostatic pressure (blood pressure) facilitates the outward
movement of water from the capillary to the interstitial space.
2. Capillary (plasma) oncotic pressure osmotically attracts water from the
interstitial space back into the capillary.
3. Interstitial hydrostatic pressure facilitates the inward movement of water from
the interstitial space into the capillary.
4. Interstitial oncotic pressure osmotically attracts water from the capillary into
the interstitial space.
The forces moving fluid back and forth across the capillary wall are summarized
below:
At the arterial end of the capillary, hydrostatic pressure exceeds capillary oncotic
pressure and fluid moves into the interstitial space (filtration). At the venous end of
the capillary, capillary oncotic pressure exceeds capillary hydrostatic pressure and
fluids are attracted back into the circulation (reabsorption). Interstitial hydrostatic
pressure promotes the movement of about 10% of the interstitial fluid along with
small amounts of protein into the lymphatics, which then returns to the circulation.
Because albumin does not normally cross the capillary membrane, interstitial
oncotic pressure is normally minimal. Figure 5-1 illustrates net filtration.
FIGURE 5-1 Net Filtration—Fluid Movement between Plasma and Interstitial Space. The
movement of fluid between the vascular, interstitial spaces and the lymphatics is the result of
net filtration of fluid across the semipermeable capillary membrane. Capillary hydrostatic
pressure is the primary force for fluid movement out of the arteriolar end of the capillary and
into the interstitial space. At the venous end, capillary oncotic pressure (from plasma proteins)
attracts water back into the vascular space. Interstitial hydrostatic pressure promotes the
movement of fluid and proteins into the lymphatics. Osmotic pressure accounts for the
movement of fluid between the interstitial space and the intracellular space. Normally,
intracellular and extracellular fluid osmotic pressures are equal (280 to 294 mOsm) and water
is equally distributed between the interstitial and intracellular compartments.
Water Movement Between ICF and ECF
Water moves between ICF and ECF compartments primarily as a function of
osmotic forces (see Chapter 1 for definitions). Water moves freely by diffusion
through the lipid bilayer cell membrane and through aquaporins, a family of water
channel proteins that provide permeability to water.1 Sodium is responsible for the
ECF osmotic balance, and potassium maintains the ICF osmotic balance. The
osmotic force of ICF proteins and other nondiffusible substances is balanced by the
active transport of ions out of the cell. Water crosses cell membranes freely, so the
osmolality of TBW is normally at equilibrium. Normally the ICF is not subject to
rapid changes in osmolality, but when ECF osmolality changes, water moves from
one compartment to another until osmotic equilibrium is reestablished (see Figure
5-7, p. 120).
Alterations in Water Movement
Edema
Edema is excessive accumulation of fluid within the interstitial spaces. The forces
favoring fluid movement from the capillaries or lymphatic channels into the tissues
are increased capillary hydrostatic pressure, decreased plasma oncotic pressure,
increased capillary membrane permeability, and lymphatic channel obstruction2
(Figure 5-2).
FIGURE 5-2 Mechanisms of Edema Formation.
Pathophysiology
Capillary hydrostatic pressure increases as a result of venous obstruction or salt and
water retention. Venous obstruction causes hydrostatic pressure to increase behind
the obstruction, pushing fluid from the capillaries into the interstitial spaces.
Thrombophlebitis (inflammation of veins), hepatic obstruction, tight clothing
around the extremities, and prolonged standing are common causes of venous
obstruction. Congestive heart failure, renal failure, and cirrhosis of the liver are
associated with excessive salt and water retention, which cause plasma volume
overload, increased capillary hydrostatic pressure, and edema.
Since plasma albumin acts like a magnet to attract water, the loss or diminished
production (e.g., from liver disease or protein malnutrition) contributes to
decreased plasma oncotic pressure. Plasma proteins are lost in glomerular diseases
of the kidney, serous drainage from open wounds, hemorrhage, burns, and cirrhosis
of the liver. The decreased oncotic attraction of fluid within the capillary causes
filtered capillary fluid to remain in the interstitial space, resulting in edema.
Capillaries become more permeable with inflammation and immune responses,
especially with trauma such as burns or crushing injuries, neoplastic disease, and
allergic reactions. Proteins escape from the vascular space and produce edema
through decreased capillary oncotic pressure and interstitial fluid protein
accumulation.
The lymphatic system normally absorbs interstitial fluid and a small amount of
proteins. When lymphatic channels are blocked or surgically removed, proteins and
fluid accumulate in the interstitial space, causing lymphedema.3 For example,
lymphedema of the arm or leg occurs after surgical removal of axillary or femoral
lymph nodes, respectively, for treatment of carcinoma. Inflammation or tumors may
cause lymphatic obstruction, leading to edema of the involved tissues.
Clinical manifestations
Edema may be localized or generalized. Localized edema is usually limited to a site
of trauma, as in a sprained finger. Another kind of localized edema occurs within
particular organ systems and includes cerebral, pulmonary, and laryngeal edema;
pleural effusion (fluid accumulation in the pleural space); pericardial effusion (fluid
accumulation within the membrane around the heart); and ascites (accumulation of
fluid in the peritoneal space). Edema of specific organs, such as the brain, lung, or
larynx, can be life-threatening. Generalized edema is manifested by a more uniform
distribution of fluid in interstitial spaces. Dependent edema, in which fluid
accumulates in gravity-dependent areas of the body, might signal more generalized
edema. Dependent edema appears in the feet and legs when standing and in the sacral
area and buttocks when supine (lying on back). It can be identified by pressing on
tissues overlying bony prominences. A pit left in the skin indicates edema (hence the
term pitting edema) (Figure 5-3).
FIGURE 5-3 Pitting Edema. (From Bloom A, Ireland J: Color atlas of diabetes, ed 2, St Louis, 1992, Mosby.)
Edema usually is associated with weight gain, swelling and puffiness, tight-fitting
clothes and shoes, limited movement of affected joints, and symptoms associated
with the underlying pathologic condition. Fluid accumulations increase the distance
required for nutrients and waste products to move between capillaries and tissues.
Blood flow may be impaired also. Therefore wounds heal more slowly, and with
prolonged edema the risks of infection and pressure sores over bony prominences
increase. As edematous fluid accumulates, it is trapped in a “third space” (i.e., the
interstitial space, pleural space, pericardial space) and is unavailable for metabolic
processes or perfusion. Dehydration can develop as a result of this sequestering.
Such sequestration occurs with severe burns, where large amounts of vascular fluid
are lost to the interstitial spaces, reducing plasma volume and causing shock (see
Chapter 24).
Evaluation and treatment
Specific conditions causing edema require diagnosis. Edema may be treated
symptomatically until the underlying disorder is corrected. Supportive measures
include elevating edematous limbs, using compression stockings, avoiding
prolonged standing, restricting salt intake, and taking diuretics. Administration of
IV albumin can be required in severe cases.
Quick Check 5-1
1. How does an increase in capillary hydrostatic pressure cause edema?
2. How does a decrease in capillary oncotic pressure cause edema?
Sodium, Chloride, and Water Balance
The kidneys and hormones have a central role in maintaining sodium and water
balance. Because water follows the osmotic gradients established by changes in salt
concentration, sodium concentration and water balance are intimately related.
Sodium concentration is regulated by renal effects of aldosterone (see Figure 18-
18). Water balance is regulated primarily by antidiuretic hormone (ADH; also
known as vasopressin).
Sodium (Na+) accounts for 90% of the ECF cations (positively charged ions) (see
Table 5-3). Along with its constituent anions (negatively charged ions) chloride and
bicarbonate, sodium regulates extracellular osmotic forces and therefore regulates
water balance. Sodium is important in other functions, including maintenance of
neuromuscular irritability for conduction of nerve impulses (in conjunction with
potassium and calcium; see Figure 1-29), regulation of acid-base balance (using
sodium bicarbonate and sodium phosphate), participation in cellular chemical
reactions, and transport of substances across the cellular membrane.
The kidney, in conjunction with neural and hormonal mediators, maintains
normal serum sodium concentration within a narrow range (135 to 145 mEq/L)
primarily through renal tubular reabsorption. Hormonal regulation of sodium (and
potassium) balance is mediated by aldosterone, a mineralocorticoid synthesized
and secreted from the adrenal cortex as a component of the renin-angiotensin-
aldosterone system. Aldosterone secretion is influenced by circulating blood
volume, by blood pressure, and by plasma concentrations of sodium and potassium.
When circulating blood volume or blood pressure is reduced, or sodium levels are
depressed or potassium levels are increased, renin, an enzyme secreted by the
juxtaglomerular cells of the kidney, is released. Renin stimulates the formation of
angiotensin I, an inactive polypeptide. Angiotensin-converting enzyme (ACE) in
pulmonary vessels converts angiotensin I to angiotensin II, which stimulates the
secretion of aldosterone and antidiuretic hormone (see below) and also causes
vasoconstriction. The aldosterone promotes renal sodium and water reabsorption
and excretion of potassium, increasing blood volume (Figure 5-4; also see Figure
29-9). Vasoconstriction elevates the systemic blood pressure and restores renal
perfusion (blood flow). This restoration inhibits the further release of renin.
FIGURE 5-4 The Renin-Angiotensin-Aldosterone System. ADH, Antidiuretic hormone; BP, blood
pressure; ECF, extracellular fluid; Na, sodium. (Modified from Herlihy B, Maebius N: The human body in health and
disease, ed 4, Philadelphia, 2011, Saunders. Borrowed from Lewis et al: Medical-surgical nursing: and management of clinical problems, ed 9, St
Louis, 2014, Mosby.)
Natriuretic peptides are hormones primarily produced by the myocardium.
Atrial natriuretic hormone (ANH) is produced by the atria. B-type natriuretic
peptide (BNP) is produced by the ventricles. Urodilatin (an ANP analog) is
synthesized within the kidney. Natriuretic peptides are released when there is an
increase in transmural atrial pressure (increased volume), which may occur with
congestive heart failure or when there is an increase in mean arterial pressure4
(Figure 5-5). They are natural antagonists to the renin-angiotensin-aldosterone
system. Natriuretic peptides cause vasodilation and increase sodium and water
excretion, decreasing blood pressure. Natriuretic peptides are sometimes called a
“third factor” in sodium regulation. (Increased glomerular filtration rate is thus the
first factor and aldosterone the second factor.)
FIGURE 5-5 The Natriuretic Peptide System. ANH, Atrial natriuretic hormone; BNP, brain
natriuretic peptide; GFR, glomerular filtration rate; Na+, sodium ion.
Chloride (Cl−) is the major anion in the ECF and provides electroneutrality,
particularly in relation to sodium. Chloride transport is generally passive and
follows the active transport of sodium so that increases or decreases in chloride
concentration are proportional to changes in sodium concentration. Chloride
concentration tends to vary inversely with changes in the concentration of
bicarbonate ( ), the other major anion.
Water balance is regulated by the secretion of ADH (also known as vasopressin).
ADH is secreted when plasma osmolality increases or circulating blood volume
decreases and blood pressure drops (Figure 5-6). Increased plasma osmolality
occurs with water deficit or sodium excess in relation to total body water. The
increased osmolality stimulates hypothalamic osmoreceptors. In addition to
causing thirst, these osmoreceptors signal the posterior pituitary gland to release
ADH. Thirst stimulates water drinking and ADH increases water reabsorption into
the plasma from the distal tubules and collecting ducts of the kidney (see Chapter
29). The reabsorbed water decreases plasma osmolality, returning it toward normal,
and urine concentration increases.
FIGURE 5-6 The Antidiuretic Hormone (ADH) System.
With fluid loss (dehydration) from vomiting, diarrhea, or excessive sweating, a
decrease in blood volume and blood pressure often occurs. Volume-sensitive
receptors and baroreceptors (nerve endings that are sensitive to changes in
volume and pressure) also stimulate the release of ADH from the pituitary gland
and stimulate thirst. The volume receptors are located in the right and left atria and
thoracic vessels; baroreceptors are found in the aorta, pulmonary arteries, and
carotid sinus. ADH secretion also occurs when atrial pressure drops, as occurs with
decreased blood volume and with the release of angiotensin II (see Figure 29-9).
The reabsorption of water mediated by ADH then promotes the restoration of
plasma volume and blood pressure (see Figure 5-6).
Quick Check 5-2
1. What forces promote net filtration?
2. How do hormones regulate salt and water balance?
3. What are aquaporins?
Alterations in Sodium, Chloride, and Water
Balance
Alterations in sodium and water balance are closely related. Sodium imbalances
occur with gains or losses of body water. Water imbalances develop with gains or
losses of salt. In general, these alterations can be classified as changes in tonicity,
the change in the concentration of solutes in relation to water: isotonic, hypertonic,
or hypotonic (Table 5-5 and Figure 5-7; also see Figure 1-25). Changes in tonicity
also alter the volume of water in the intracellular and extracellular compartments,
resulting in isovolemia, hypervolemia, or hypovolemia.
TABLE 5-5
Water and Solute Imbalances
Tonicity Mechanism
Isotonic (isoosmolar) imbalance
Serum osmolality = 280-
294 mOsm/kg
Gain or loss of ECF resulting in concentration equivalent to 0.9% sodium chloride solution (normal saline); no
shrinking or swelling of cells
Hypertonic (hyperosmolar)
imbalance
Serum osmolality >294 mOsm/kg
Imbalances that result in ECF concentration >0.9% salt solution (i.e., water loss or solute gain); cells shrink in
hypertonic fluid
Hypotonic (hypoosmolar)
imbalance
Serum osmolality <280 mOsm/kg
Imbalance that results in ECF <0.9% salt solution (i.e., water gain or solute loss); cells swell in hypotonic fluid
Formula for calculating serum
osmolarity
(2 × [Na] + [Glu])/18 + BUN/2.8
BUN, Blood serum urea nitrogen level (mg/dl); ECF, extracellular fluid; [Glu], serum glucose concentration
(mg/dl); [Na], serum sodium concentration (mEq/dl).
FIGURE 5-7 Effects of Alterations in Extracellular Sodium Concentration in RBC, Body Cell, and
Neuron. A, Hypotonic alteration: Decrease in ECF sodium (Na+) concentration (hyponatremia)
results in ICF osmotic attraction of water with swelling and potential bursting of cells. B, Isotonic
alteration: Normal concentration of sodium in the ECF and no change in shifts of fluid in or out of
cells. C, Hypertonic alteration: An increase in ECF sodium concentration (hypernatremia) results
in osmotic attraction of water out of cells with cell shrinkage. RBC, Red blood cell.
Isotonic Alterations
Isotonic alterations are the most common and occur when TBW changes are
accompanied by proportional changes in the concentrations of electrolytes (see
Figure 5-7). Isotonic fluid loss causes dehydration and hypovolemia. For example,
if an individual loses pure plasma or ECF, fluid volume is depleted but the
concentration and type of electrolytes and the osmolality remain in the normal
range (280 to 294 milliosmoles [mOsm]). Causes include hemorrhage, severe
wound drainage, excessive diaphoresis (sweating), and inadequate fluid intake.
There is loss of extracellular fluid volume with weight loss, dryness of skin and
mucous membranes, decreased urine output, and symptoms of hypovolemia.
Indicators of hypovolemia include a rapid heart rate, flattened neck veins, and
normal or decreased blood pressure. In severe states, hypovolemic shock can occur
(see Chapter 24). Isotonic fluids containing electrolytes and glucose are given
orally, intravenously (i.e., 0.9% saline solution or 5% dextrose in 0.225% saline
solution), or, in some cases, subcutaneously (hypodermoclysis).
Isotonic fluid excess causes hypervolemia. Common causes include excessive
administration of intravenous fluids, hypersecretion of aldosterone, or the effects of
drugs such as cortisone (which causes renal reabsorption of sodium and water). As
plasma volume expands, hypervolemia develops with weight gain. The diluting
effect of excess plasma volume leads to decreased hematocrit and decreased plasma
protein concentration. The neck veins may distend, and the blood pressure increases.
Increased capillary hydrostatic pressure leads to edema formation. Ultimately,
pulmonary edema and heart failure may develop. Diuretics are commonly used for
treatment.
Hypertonic Alterations
Hypertonic fluid alterations develop when the osmolality of the ECF is elevated
above normal (greater than 294 mOsm). The most common causes are increased
concentration of ECF sodium (hypernatremia) or deficit of ECF water, or both. In
both instances, ECF hypertonicity attracts water from the intracellular space,
causing ICF dehydration (see Figure 5-7).
Hypernatremia
Pathophysiology
Hypernatremia occurs when serum sodium levels exceed 145 mEq/L. Increased
levels of serum sodium cause hypertonicity. Hypernatremia can be isovolemic,
hypovolemic, or hypervolemic depending on the accompanying ECF water volume.
Isovolemic hypernatremia is the most common and occurs when there is a loss of
free water with a near normal body sodium concentration. Causes include
inadequate water intake; excessive sweating (sweat is hypotonic), fever, or
respiratory tract infections, which increase the respiratory rate and enhance water
loss from the lungs; burns; vomiting; diarrhea; and central or nephrogenic diabetes
insipidus (lack of ADH or inadequate renal response to ADH). Infants with severe
diarrhea are vulnerable and have increased risk because they cannot communicate
thirst. Insufficient water intake occurs particularly in individuals who are comatose,
confused, or immobilized or are receiving gastric feedings. Dehydration refers to
water deficit but also is commonly used to indicate both sodium and water loss
(isotonic or isoosmolar dehydration).5
Hypovolemic hypernatremia occurs where there is loss of sodium accompanied
by a relatively greater loss of body water. Causes include use of loop diuretics,
osmotic diuresis (i.e., from hyperglycemia related to uncontrolled diabetes mellitus
or use of mannitol), or failure of the kidneys to concentrate urine.
Hypervolemic hypernatremia is rare and occurs when there is increased total
body water and a greater increase in total body sodium level, resulting in
hypervolemia. Causes include infusion of hypertonic saline solutions (e.g., as
sodium replacement for treatment of salt depletion, which can occur with renal
impairment, heart failure, or gastrointestinal [GI] losses); oversecretion of
adrenocorticotropic hormone (ACTH) or aldosterone (e.g., Cushing syndrome,
adrenal hyperplasia); and near salt water drowning.6 High amounts of dietary
sodium rarely cause hypernatremia in a healthy individual because the sodium is
eliminated by the kidneys.
Because chloride follows sodium, hyperchloremia (elevation of serum chloride
concentration greater than 105 mEq/L) often accompanies hypernatremia, as well as
plasma bicarbonate deficits (such as in metabolic acidosis)7 (see p. 127). There are
no specific symptoms or treatment for chloride excess.
Clinical manifestations
When there is excessive sodium intake or decreased sodium loss in relation to
water, water is osmotically redistributed to the hypertonic extracellular space,
resulting in hypervolemia, and intracellular dehydration ensues. Clinical
manifestations include thirst, weight gain, bounding pulse, and increased blood
pressure. Central nervous system signs are the most serious and are related to
alterations in membrane potentials and shrinking of brain cells (sodium cannot
cross brain capillaries because of their tight endothelial junctions). Signs include
muscle twitching and hyperreflexia (hyperactive reflexes), confusion, coma,
convulsions, and cerebral hemorrhage from stretching of veins. Hypernatremia
with marked water deficit is manifested by signs and symptoms of intracellular and
extracellular dehydration with volume depletion (Box 5-1).
Box 5-1
Signs and Symptoms of Dehydration
Increased serum sodium concentration
Thirst
Headache
Weight loss
Oliguria and concentrated urine
Hard stools
Decreased skin turgor
Dry mucous membranes
Decreased sweating and tears
Elevated temperature
Soft eyeballs
Sunken fontanels in infants
Prolonged capillary refill time
Tachycardia
Weak pulses
Low blood pressure
Postural hypotension
Hypovolemic shock
Confusion
Coma
Evaluation and treatment
Serum sodium levels are greater than 147 mEq/L and urine specific gravity will be
greater than 1.030. The history and physical examination provide information about
underlying disorders and events. The treatment of hypernatremia and water deficit is
to give oral fluids or isotonic salt-free fluid (5% dextrose in water) until the serum
sodium level returns to normal. Fluid replacement must be given slowly to prevent
cerebral edema. Serum sodium levels need to be monitored. Hypervolemia or
hypovolemia requires treatment of the underlying clinical condition.
Hypotonic Alterations
Hypotonic fluid imbalances occur when the osmolality of the ECF is less than
280 mOsm (see Figure 5-7). The most common causes are sodium deficit or water
excess. Either leads to intracellular overhydration (cellular edema) and cell
swelling. When there is a sodium deficit, the osmotic pressure of the ECF decreases
and water moves into the cell where the osmotic pressure is greater. The plasma
volume then decreases, leading to symptoms of hypovolemia. With water excess,
increases in both the ICF and ECF volume occur, causing symptoms of
hypervolemia and water intoxication with cerebral and pulmonary edema.
Hyponatremia
Pathophysiology
Hyponatremia develops when the serum sodium concentration falls below
135 mEq/L. Hyponatremia occurs when there is loss of sodium, inadequate intake of
sodium, or dilution of sodium by water excess.8 Sodium depletion usually causes
hypoosmolality with movement of water into cells with rupture of cell membranes.
Isovolemic hyponatremia occurs when there is loss of sodium without a
significant loss of water (pure sodium deficit). Causes can include syndrome of
inappropriate antidiuretic hormone9 (SIADH [see Chapter 19], which enhances water
retention), hypothyroidism, pneumonia, and glucocorticoid deficiency. Inadequate
intake of dietary sodium is rare but possible in individuals on low-sodium diets,
particularly with use of diuretics.
Hypervolemic hyponatremia occurs when total body sodium level increases. The
increased sodium leads to an increase in total body water and dilution of sodium in
the extracellular space. Causes include congestive heart failure, cirrhosis of the
liver, and nephrotic syndrome. Edema is present.
Hypovolemic hyponatremia occurs with a loss of total body water, but there is a
greater loss of body sodium. The extracelluar volume is decreased. Causes include
prolonged vomiting, severe diarrhea, inadequate secretion of aldosterone (e.g.,
adrenal insufficiency), and renal losses from diuretics.
Dilutional hyponatremia (water intoxication) occurs when there is intake of
large amounts of free water or replacement of fluid loss with intravenous 5%
dextrose in water, which dilutes sodium. The glucose is metabolized to carbon
dioxide and water, leaving a hypotonic solution with a diluting effect. Excessive
sweating stimulates thirst and intake of large amounts of free water (as can occur in
endurance athletes), which dilutes sodium. Some individuals with psychogenic
disorders develop water intoxication from compulsive water drinking. Other causes
can include tap water enemas, near fresh water drowning, and use of selective
serotonin reuptake inhibitors (SSRIs). When the body is functioning normally, it is
almost impossible to produce an excess of TBW because water balance is regulated
by the kidneys.
Hypochloremia, a low level of serum chloride (less than 97 mEq/L), usually
occurs with hyponatremia or an elevated bicarbonate concentration, as in metabolic
alkalosis (see p. 127). Sodium deficit related to restricted intake, use of diuretics,
vomiting, or nasogastric suction is accompanied by chloride deficiency. Cystic
fibrosis is characterized by hypochloremia (see Chapter 28). Treatment of the
underlying cause is required.
Clinical manifestations
The serum sodium concentration will be less than 135 mEq/L. Sodium depletion
usually causes hypoosmolality with movement of water into cells. The hematocrit is
reduced from the dilutional effect of water excess in dilutional hyponatremia. The
high amount of intracellular solutes compared to the low amount of extracellular
solutes as a result of the hyponatremia causes an intracellular osmotic shift of water,
resulting in cell swelling. The most life-threatening consequence is cerebral edema
and increased intracranial pressure. Neurologic changes include lethargy,
confusion, apprehension, seizures, and coma. A decrease in sodium concentration
changes the cell's ability to depolarize and repolarize normally, altering the action
potential in neurons and muscle (see Chapter 1). Muscle twitching, depressed
reflexes, and weakness are common. Nausea and vomiting are more common with
less severe hyponatremia (i.e., decreases between 120 and 130 mEq/L).
Hypovolemic hyponatremia has signs of hypotension, tachycardia, and decreased
urine output. Hypervolemic hyponatremia is accompanied by weight gain, edema,
ascites, and jugular vein distention. Hyponatremia is a major cause of morbidity and
mortality in intensive care units and in the elderly (see Health Alert: Hyponatremia
and the Elderly).
Health Alert
Hyponatremia and the Elderly
Hyponatremia is the most common of the electrolyte disorders and prevalence is
highest among elderly hospitalized individuals. Isovolemic hyponatremia caused
by SIADH is thought to be the most common cause and can occur with central
nervous system injury, pulmonary disease, malignancies, nausea, pain, and aging
changes. Other contributing factors include use of thiazide diuretics, proton pump
inhibitors, age-related decrease in thirst with dehydration, and diminished urine
concentrating ability. Hyponatremia contributes to cognitive deficits, gait
disturbances, falls, fractures, long-term hospitalization, the need for long-term
care, and death. The elderly need to be assessed for risk, implementation of
preventive strategies, and early intervention.
SIADH, Syndrome of inappropriate antidiuretic hormone.
From Ayus JC et al: Nephrol Dial Transplant 27(10):3725-3731, 2012; Berl T: Clin J Am Soc Nephrol
8(3):469-475; Cowen LE et al: Endocrinol Metab Clin North Am 42(2):349-370, 2013; Cumming K et al: PLoS
One 9(2):e88272, 2014; Mannesse CK et al: Ageing Res Rev 12(1):165-173, 2013; Schrier RW et al: Nat Rev
Nephrol 9(1):37-50, 2013 (Erratum in: Nat Rev Nephrol 9[3]:124, 2013).
Evaluation and treatment
The cause of hyponatremia must be determined and treatment planned accordingly.
Small amounts of intravenous hypertonic sodium chloride (i.e., 3% sodium
chloride) can be given when neurologic manifestations are severe but must be given
slowly to prevent osmotic demyelination syndrome in the brain.9 Restriction of
water intake is required in most cases of dilutional hyponatremia because body
sodium levels may be normal or increased even though serum sodium levels are
low. Arginine vasopressin (ADH) receptor antagonists (vaptans) are a class of drugs
used for the treatment of hypervolemic and euvolemic hyponatremia.10 Serum
sodium concentration must be monitored.8
Quick Check 5-3
1. What causes isotonic imbalance?
2. What are some causes of hypernatremia?
3. What is the most severe complication of hyponatremia?
Alterations in Potassium and Other
Electrolytes
Potassium
Potassium (K+) is the major intracellular electrolyte and is essential for normal
cellular functions. Total body potassium content is about 4000 mEq, with most of it
(98%) located in the cells. The ICF concentration of potassium is 150 to 160 mEq/L;
the ECF potassium concentration is 3.5 to 5.0 mEq/L. The difference in
concentration is maintained by a sodium-potassium adenosinetriphosphatase active
transport system (Na+-K+ ATPase pump) (see Figure 1-26).
As the predominant ICF ion, potassium exerts a major influence on the regulation
of ICF osmolality and fluid balance as well as on intracellular electrical neutrality in
relation to hydrogen (H+) and sodium. Potassium is required for glycogen and
glucose deposition in liver and skeletal muscle cells. It also maintains the resting
membrane potential, as reflected in the transmission and conduction of nerve
impulses (see Figure 1-29), the maintenance of normal cardiac rhythms, and the
contraction of skeletal muscle and smooth muscle.
Dietary potassium moves rapidly into cells after ingestion. However, the
distribution of potassium between intracellular and extracellular fluids is influenced
by several factors. Insulin, aldosterone, epinephrine, and alkalosis facilitate the shift
of potassium into cells. Insulin deficiency, aldosterone deficiency, acidosis, cell
lysis, and strenuous exercise facilitate the shift of potassium out of cells. Glucagon
blocks entry of potassium into cells, and glucocorticoids promote potassium
excretion. Potassium also will move out of cells along with water when there is
increased ECF osmolarity.
Although potassium is found in most body fluids, the kidney is the most efficient
regulator of potassium balance. Potassium is freely filtered by the renal glomerulus,
and 90% is reabsorbed by the proximal tubule and loop of Henle. In the distal
tubules, principal cells secrete potassium and intercalated cells reabsorb potassium.
These cells determine the amount of potassium excreted from the body. The gut may
also sense the amount of K+ ingested and stimulate renal K+ excretion independent
of aldosterone.11
The potassium concentration in the distal tubular cells is determined primarily by
the plasma concentration in the peritubular capillaries. When plasma potassium
concentration increases from increased dietary intake or shifts of potassium from
the ICF to the ECF occur, potassium is secreted into the urine by the distal tubules.
Decreased levels of plasma potassium result in decreased distal tubular secretion,
although approximately 5 to 15 mEq per day will continue to be lost. Changes in the
rate of filtrate (urine) flow through the distal tubule also influence the concentration
gradient for potassium secretion. When the urine flow rate is high, as with the use of
diuretics, potassium concentration in the distal tubular urine is lower, leading to the
secretion of potassium into the urine.
Changes in pH and thus in hydrogen ion concentration also affect potassium
balance. During acute acidosis, hydrogen ions accumulate in the ICF and potassium
shifts out of the cell to the ECF to maintain a balance of cations across the cell
membrane. This occurs in part because of a decrease in sodium-potassium ATPase
pump activity. Decreased ICF potassium results in decreased secretion of potassium
by the distal tubular cells, contributing to hyperkalemia. In acute alkalosis,
intracellular fluid levels of hydrogen diminish and potassium shifts into the cell; in
addition, the distal tubular cells increase their secretion of potassium, further
contributing to hypokalemia.12
Besides conserving sodium, aldosterone also regulates potassium concentration.
Elevated plasma potassium concentration causes the release of renin by renal
juxtaglomerular cells and the adrenal secretion of aldosterone through the renin-
angiotensin-aldosterone system. Aldosterone then stimulates the release of
potassium into the urine by the distal renal tubules. Aldosterone also increases the
secretion of potassium from sweat glands.
Insulin helps regulate plasma potassium levels by stimulating the sodium-
potassium ATPase pump, thus promoting the movement of potassium into liver and
muscle cells, particularly after eating. Insulin can also be used to treat hyperkalemia.
Dangerously low levels of plasma potassium can result when insulin is given while
potassium levels are depressed. Potassium balance is especially significant in the
treatment of conditions requiring insulin administration, such as insulin-dependent
diabetes mellitus.
Potassium adaptation is the ability of the body to adapt to increased levels of
potassium intake over time. A sudden increase in potassium may be fatal, but if the
intake of potassium is slowly increased by amounts of more than 120 mEq per day,
the kidney can increase the urinary excretion of potassium and maintain potassium
balance.
Hypokalemia
Pathophysiology
Potassium deficiency, or hypokalemia, develops when the serum potassium
concentration falls to less than 3.5 mEq/L. Because cellular and total body stores of
potassium are difficult to measure, changes in potassium balance are described,
although not always accurately, by the plasma concentration. Generally, lowered
serum potassium level indicates loss of total body potassium. With potassium loss
from the ECF, the concentration gradient change favors movement of potassium
from the cell to the ECF. The ICF/ECF concentration ratio is maintained, but the
amount of total body potassium is depleted.
Factors contributing to the development of hypokalemia include reduced intake
of potassium, increased entry of potassium into cells, and increased losses of body
potassium. Dietary deficiency of potassium is more common in elderly individuals
with both low protein intake and inadequate intake of fruits and vegetables and in
individuals with alcoholism or anorexia nervosa (see Health Alert: Potassium
Intake: Hypertension and Stroke). Reduced potassium intake generally becomes a
problem when combined with other causes of potassium depletion.
Health Alert
Potassium Intake: Hypertension and Stroke
Enriched dietary intake of potassium is associated with lower risk of hypertension
and stroke. The American diet often exceeds recommendations for sodium intake
and a deficiency in potassium intake. There is increased risk of high blood
pressure, cardiovascular disease, and mortality when the plasma ratio of sodium
concentration to potassium concentration is high. Potassium attenuates the effects of
high dietary salt with reduction in blood pressure, stroke rates, and cardiovascular
disease risk. The exact mechanism of how potassium affects blood pressure is
unknown but is thought to be related to renal handling of sodium, endothelial cell
function, decreased vascular resistance, and reduced oxidative stress. A large
prospective study of older women showed they were found to have lower risk of
ischemic but not hemorrhagic stroke associated with higher intakes of potassium,
especially in women without hypertension. Lower risk of mortality was found in all
women with higher intakes of potassium. Increased dietary intake of potassium is
recommended for most individuals without impaired renal handling of potassium.
Data from Aaron KJ, Sanders PW: Mayo Clin Proc 88(9):987-995, 2013; Arjun S et al: Stroke September 4,
2014 [Epub ahead of print]; Castro H, Raij L: Semin Nephrol 33(3):277-289, 2013; Whelton PK, He J: Curr
Opin Lipidol 25(1):75-79, 2014.
ECF hypokalemia can develop without losses of total body potassium. For
example, potassium shifts from the ECF to the ICF in exchange for hydrogen to
maintain plasma acid-base balance during respiratory or metabolic alkalosis.
Insulin promotes cellular uptake of potassium and insulin administration may cause
an ECF potassium deficit.
Potassium shifts from the ICF to the ECF in conditions such as diabetic
ketoacidosis, in which the increased hydrogen ion concentration in the ECF causes
H+ to shift into the cell in exchange for potassium. A normal level of potassium is
maintained in the plasma, but potassium continues to be lost in the urine, causing a
deficit in the amount of total body potassium. Severe, even fatal, hypokalemia may
occur if insulin is administered without also providing potassium supplements. Thus
total body potassium depletion becomes evident when insulin treatment and
rehydration therapy are initiated. Potassium replacement is instituted cautiously to
prevent hyperkalemia.
Losses of potassium from body stores are usually caused by gastrointestinal and
renal disorders. Diarrhea, intestinal drainage tubes or fistulae, and laxative abuse
also result in hypokalemia. Normally, only 5 to 10 mEq of potassium and 100 to
150 ml of water are excreted in the stool each day. With diarrhea, fluid and
electrolyte losses can be voluminous, with several liters of fluid and 100 to
200 mEq of potassium lost per day. Vomiting or continuous nasogastric suctioning
often is associated with potassium depletion, partly because of the potassium lost
from the gastric fluid but principally because of renal compensation for volume
depletion and the metabolic alkalosis (elevated bicarbonate levels) that occurs from
sodium, chloride, and hydrogen ion losses. The loss of fluid and sodium stimulates
the secretion of aldosterone, which in turn causes renal losses of potassium.
Renal potassium losses occur with increased secretion of potassium by the distal
tubule. Use of potassium-wasting diuretics, excessive aldosterone secretion,
increased distal tubular flow rate, and low plasma magnesium concentration all may
contribute to urinary losses of potassium. The elevated flow of bicarbonate at the
distal tubule during alkalosis also contributes to renal excretion of potassium
because the increased tubular lumen electronegativity attracts potassium. Many
diuretics inhibit the reabsorption of sodium chloride, causing the diuretic effect.
The distal tubular flow rate then increases, promoting potassium excretion. If
sodium loss is severe, the compensating aldosterone secretion may further deplete
potassium stores. Primary hyperaldosteronism with excessive secretion of
aldosterone from an adrenal adenoma (tumor) also causes potassium wasting. Many
kidney diseases reduce the ability to conserve sodium. The disordered sodium
reabsorption produces a diuretic effect, and the increased distal tubule flow rate
favors the secretion of potassium. Magnesium deficits increase renal potassium
secretion and promote hypokalemia. Certain antibiotics (i.e., carbenicillin disodium
and amphotericin B) are known to cause hypokalemia by increasing the rate of
potassium excretion. Rare hereditary defects in renal potassium transport (e.g.,
Bartter and Gitelman syndromes) also can cause hypokalemia.
Clinical manifestations
Mild losses of potassium are usually asymptomatic. Severe loss of potassium results
in neuromuscular and cardiac manifestations. Neuromuscular excitability decreases,
causing skeletal muscle weakness, smooth muscle atony, cardiac dysrhythmias,
glucose intolerance, and impaired urinary concentrating ability.13
Symptoms occur in relation to the rate of potassium depletion. Because the body
can accommodate slow losses of potassium, the decrease in ECF concentration may
allow potassium to shift from the intracellular space, restoring the potassium
concentration gradient toward normal, with less severe neuromuscular changes.
With acute and severe losses of potassium, changes in neuromuscular excitability
are more profound. Skeletal muscle weakness occurs initially in the larger muscles
of the legs and arms and ultimately affects the diaphragm and depresses ventilation.
Paralysis and respiratory arrest can occur with severe losses. Loss of smooth
muscle tone is manifested by constipation, intestinal distention, anorexia, nausea,
vomiting, and paralytic ileus (paralysis of the intestinal muscles).
The cardiac effects of hypokalemia are related also to changes in membrane
excitability. As ECF potassium concentration decreases, the resting membrane
potential becomes more negative (i.e., from −90 millivolts to −100 millivolts
[hypopolarization]). Because potassium contributes to the repolarization phase of
the action potential, hypokalemia delays ventricular repolarization. Various
dysrhythmias may occur, including sinus bradycardia, atrioventricular block, and
paroxysmal atrial tachycardia. The characteristic changes in the electrocardiogram
(ECG) reflect delayed repolarization. For instance, the amplitude of the T wave
decreases, the amplitude of the U wave increases, and the ST segment is depressed
(Figure 5-8). In severe states of hypokalemia, P waves peak, the QT interval is
prolonged, and T wave inversions may be seen. Hypokalemia enhances the
therapeutic effect of digitalis and increases the risk of digitalis toxicity.
FIGURE 5-8 Electrocardiogram Changes with Potassium Imbalance.
A wide range of metabolic dysfunctions may result from potassium deficiency
(Table 5-6). Carbohydrate metabolism is affected because hypokalemia depresses
insulin secretion and alters hepatic and skeletal muscle glycogen synthesis. Renal
function is impaired, with a decreased ability to concentrate urine. Polyuria
(increased urine) and polydipsia (increased thirst) are associated with decreased
responsiveness to ADH. Long-term potassium deficits lasting more than 1 month
may damage renal tissue, with interstitial fibrosis and tubular atrophy.
TABLE 5-6
Clinical Manifestations of Potassium Level Alterations
Organ System Hypokalemia Hyperkalemia
Cardiovascular Dysrhythmias
ECG changes (flattened T waves, U waves, ST depression, peaked
P wave, prolonged QT interval)
Cardiac arrest
Weak, irregular pulse rate
Postural hypotension
Dysrhythmias
ECG changes (peaked T waves, prolonged PR interval, absent P
wave with widened QRS complex)
Bradycardia
Heart block
Cardiac arrest
Nervous Lethargy
Fatigue
Confusion
Paresthesias
Anxiety
Tingling
Numbness
Gastrointestinal Nausea and vomiting
Decreased motility
Distention
Decreased bowel sounds
Ileus
Nausea and vomiting
Diarrhea
Colicky pain
Kidney Water loss
Thirst
Inability to concentrate urine
Increased tubular production of ammonia and ammonium
Kidney damage
Oliguria
Kidney damage
Skeletal and
smooth muscle
Weakness
Flaccid paralysis
Respiratory arrest
Constipation
Bladder dysfunction
Early: hyperactive muscles
Late: weakness and flaccid paralysis
Evaluation and treatment
The diagnosis of hypokalemia is significantly related to the medical history and the
identification of disorders associated with potassium loss or shifts of extracellular
potassium to the intracellular space. Treatment involves an estimation of total body
potassium losses and correction of acid-base imbalances. Further losses of
potassium should be prevented and the individual should be encouraged to eat foods
rich in potassium. The maximal rate of oral replacement is 40 to 80 mEq/day if
renal function is normal. A maximal safe rate of intravenous replacement is
20 mEq/hr. Because potassium is irritating to blood vessels, a maximal
concentration of 40 mEq/L should be used. Serum potassium values are monitored
until normokalemia is achieved.
Hyperkalemia
Pathophysiology
Elevation of ECF potassium concentration greater than 5.5 mEq/L constitutes
hyperkalemia.14 Because of efficient renal excretion, increases in total body
potassium level are relatively rare. Acute increases in serum potassium level are
handled quickly through increased cellular uptake and renal excretion of body
potassium excesses.
Hyperkalemia may be caused by increased intake, a shift of potassium from cells
to the ECF, decreased renal excretion, or drugs that decrease renal potassium
excretion (i.e., ACE inhibitors, angiotensin receptor blockers, and aldosterone
antagonists). If renal function is normal, slow, long-term increases in potassium
intake are usually well tolerated through potassium adaptation, although short-term
potassium loading can exceed renal excretion rates. Dietary excesses of potassium
are uncommon but accidental ingestion of potassium salt substitutes can cause
toxicity. Use of stored whole blood and intravenous boluses of potassium penicillin
G or replacement potassium can precipitate hyperkalemia, particularly with
impaired renal function. Potassium moves from the ICF to the ECF with cell trauma
or a change in cell membrane permeability, acidosis, insulin deficiency, or cell
hypoxia. Burns, massive crushing injuries, and extensive surgeries can cause
release of potassium to the ECF as a result of cell trauma. If renal function is
sustained, potassium is excreted. As cell repair begins, hypokalemia develops
without an adequate replacement of potassium.
In acidosis, ECF hydrogen ions shift into cells in exchange for ICF potassium and
sodium; hyperkalemia and acidosis therefore often occur simultaneously. Because
insulin promotes cellular entry of potassium, insulin deficits, which occur with such
conditions as diabetic ketoacidosis, are accompanied by hyperkalemia. Hypoxia can
lead to hyperkalemia by diminishing the efficiency of cell membrane active
transport, resulting in the escape of potassium to the ECF. Digitalis overdose
(toxicity) may cause hyperkalemia by inhibiting the Na+-K+ ATPase pump, and thus
allowing potassium to remain outside the cell,
Decreased renal excretion of potassium commonly is associated with
hyperkalemia. Renal failure that results in oliguria (urine output of 30 ml/hr or less)
is accompanied by elevations of serum potassium level. The severity of
hyperkalemia is related to the amount of potassium intake, the degree of acidosis,
and the rate of renal cell damage. Decreases in the secretion or renal effects of
aldosterone also can cause decreases in the urinary excretion of potassium. For
example, Addison disease (a disease of adrenal cortical insufficiency) results in
decreased production and secretion of aldosterone (and other steroids) and thus
contributes to hyperkalemia.
Clinical manifestations
Symptoms vary with the severity of hyperkalemia. During mild attacks, increased
neuromuscular irritability may be manifested as restlessness, intestinal cramping,
and diarrhea. Severe hyperkalemia decreases the resting membrane potential (i.e.,
from −90 millivolts to −70 millivolts [hyperpolarization]) and causes muscle
weakness, loss of muscle tone, and paralysis. In mild states of hyperkalemia, there is
more rapid repolarization, reflected in the ECG as narrow and taller T waves with a
shortened QT interval. Severe hyperkalemia causes delayed cardiac conduction and
prevents repolarization of heart muscle. Severe hyperkalemia depresses the ST
segment, prolongs the PR interval, and widens the QRS complex because of
decreased conduction velocity from inactivated sodium channels (see Figure 5-8).
Bradydysrhythmias and delayed conduction are common in hyperkalemia; severe
hyperkalemia can cause ventricular fibrillation or cardiac arrest.15
As with hypokalemia, changes in the ratio of intracellular to extracellular
potassium concentration contribute to the symptoms of hyperkalemia (see Table 5-
6). The neuromuscular effects of hyperkalemia are related to the increase in rate of
repolarization and the presence of other contributing factors, such as acidosis and
calcium balance. Long-term increases in ECF potassium concentration result in
shifts of potassium into the cell, because the tendency is to maintain a normal ratio
of ICF to ECF potassium concentrations. Acute elevations of extracellular potassium
concentration affect neuromuscular irritability because this ratio is disrupted.
Increases in extracellular fluid calcium concentration can override the
neuromuscular effects of hyperkalemia because calcium is also a cation and affects
the threshold potential (see Chapter 1).
Evaluation and treatment
Hyperkalemia should be investigated when there is a history of renal disease,
massive trauma, insulin deficiency, Addison disease, use of potassium salt
substitutes, or metabolic acidosis. The acuity of the onset of symptoms may be
related to the underlying cause.
Management of hyperkalemia includes treating the contributing causes and
correcting the potassium excess. When serum potassium levels are dangerously
high, calcium gluconate can be administered to restore normal neuromuscular
irritability and to stabilize the resting cardiac membrane potential by making the
threshold potential less negative. Administration of glucose (which readily
stimulates insulin secretion) or administration of both glucose and insulin for
diabetic individuals facilitates cellular entry of potassium. Sodium bicarbonate
corrects metabolic acidosis and lowers serum potassium concentration. Oral or
rectal administration of cation exchange resins, which exchange sodium for
potassium in the intestine, can be effective. Dialysis effectively removes potassium
when renal failure has occurred.
Quick Check 5-4
1. What role does potassium play in the body? What metabolic dysfunctions occur
in potassium deficiency? In potassium excess?
2. Explain how a person can have normal total body potassium levels but still
exhibit hypokalemia.
3. What is the most prominent ECG change associated with hyperkalemia? With
hypokalemia?
Other Electrolytes—Calcium, Phosphate, and
Magnesium
The specifics of balance for the other body electrolytes—calcium (Ca++), phosphate
(PO4
3−), and magnesium (Mg++)—are summarized in Table 5-7. Parathyroid
hormone and vitamin D are important for the regulation of these minerals16 (see
Chapter 18).
TABLE 5-7
Alterations in Calcium, Phosphate, and Magnesium
Parameter Calcium Phosphate Magnesium
Normal
values
Serum: 8.8-10.5 mg/dl (total), 4.5-5.6 mg/dl
(ionized); 99% in bone as hydroxyapatite; remainder
in plasma and body cells with 50% bound to plasma
proteins; 40% free or ionized; ionized form most
important physiologically
Serum: 2.5-5.0 mg/dl, but may be as high as
6.0-7.0 mg/dl in infants and young children;
mainly in bone with some in ICF and ECF;
exists as phospholipids, phosphate esters, and
inorganic phosphate (ionized form)
Serum: 1.8-3.0 mEq/L; 40-60%
stored in bone, 33% bound to plasma
proteins; primary intracellular
divalent cation
Function Needed for fundamental metabolic processes; major
cation for structure of bone and teeth; enzymatic
cofactor for blood clotting; required for hormone
secretion and function of cell receptors; directly related
to plasma membrane stability and permeability,
transmission of nerve impulses, and contraction of
muscles; parathyroid hormone, vitamin D3, and
calcitonin act together to control calcium absorption
and excretion (see Chapter 18)
Intracellular and extracellular anion buffer in
regulation of acid-base balance; provides energy
for muscle contraction (as ATP); parathyroid
hormone, vitamin D3, and calcitonin act
together to control phosphate absorption and
excretion (see Chapter 18)
Cofactor in intracellular enzymatic
reactions and causes neuromuscular
excitability; often interacts with
calcium and potassium in reactions
at cellular level and has important
role in smooth muscle contraction
and relaxation; magnesium is
absorbed in the intestine and
eliminated by the kidney
Excess Hypercalcemia (serum concentrations >10-
12 mg/dl)
Hyperphosphatemia (serum concentrations
>4.7 mg/dl)
Hypermagnesemia (serum
concentrations >3.0 mEq/L)
Causes Hyperparathyroidism; bone metastases with calcium
resorption from breast, prostate, renal, and cervical
cancer; sarcoidosis; excess vitamin D; many tumors
that produce PTH
Acute or chronic renal failure with significant
loss of glomerular filtration; treatment of
metastatic tumors with chemotherapy that
releases large amounts of phosphate into serum;
long-term use of laxatives or enemas containing
phosphates; hypoparathyroidism
Usually renal insufficiency or
failure; also excessive intake of
magnesium-containing antacids,
adrenal insufficiency
Effects Many nonspecific; fatigue, weakness, lethargy,
anorexia, nausea, constipation; impaired renal
function, kidney stones; dysrhythmias, bradycardia,
cardiac arrest; bone pain, osteoporosis
Symptoms primarily related to low serum
calcium levels (caused by high phosphate levels)
similar to results of hypocalcemia; when
prolonged, calcification of soft tissues in lungs,
kidneys, joints
Skeletal smooth muscle contraction;
excess nerve function; loss of deep
tendon reflexes; nausea and
vomiting; muscle weakness;
hypotension; bradycardia;
respiratory distress
Deficit Hypocalcemia (serum calcium concentration
<8.5 mg/dl)
Hypophosphatemia (serum phosphate
concentration <2.0 mg/dl)
Hypomagnesemia (serum
magnesium concentration
<1.5 mEq/L)
Causes Related to inadequate intestinal absorption, deposition
of ionized calcium into bone or soft tissue, blood
administration, or decreases in PTH and vitamin D;
nutritional deficiencies occur with inadequate sources
of dairy products or green leafy vegetables
Most commonly by intestinal malabsorption
related to vitamin D deficiency, use of
magnesium- and aluminum-containing
antacids, long-term alcohol abuse, and
malabsorption syndromes; respiratory alkalosis;
increased renal excretion of phosphate associated
with hyperparathyroidism
Malnutrition, malabsorption
syndromes, alcoholism, urinary
losses (renal tubular dysfunction,
loop diuretics)
Effects Increased neuromuscular excitability; tingling, muscle
spasm (particularly in hands, feet, and facial muscles),
intestinal cramping, hyperactive bowel sounds; severe
cases show convulsions and tetany; prolonged QT
interval, cardiac arrest
Conditions related to reduced capacity for
oxygen transport by red blood cells and
disturbed energy metabolism; leukocyte and
platelet dysfunction; deranged nerve and muscle
function; in severe cases, irritability, confusion,
numbness, coma, convulsions; possibly
respiratory failure (because of muscle
weakness), cardiomyopathies, bone resorption
(leading to rickets or osteomalacia)
Behavioral changes, irritability,
increased reflexes, muscle cramps,
ataxia, nystagmus, tetany,
convulsions, tachycardia,
hypotension
ATP, Adenosine triphosphate; PTH, parathyroid hormone.
Acid-Base Balance
Acid-base balance must be regulated within a narrow range for the body to function
normally. Slight changes in amounts of hydrogen and changes in pH can
significantly alter biologic processes in cells and tissues.17 Hydrogen ion is needed
to maintain membrane integrity and the speed of metabolic enzyme reactions. Most
pathologic conditions disturb acid-base balance, producing circumstances possibly
more harmful than the disease process itself.
Hydrogen Ion and pH
The concentration of hydrogen ions in body fluids is very small—approximately
0.0000001 mg/L. This number, which may be expressed as 10−7 mg/L, is indicated as
pH 7.0. The symbol pH represents the acidity or alkalinity of a solution. As the pH
changes 1 unit (e.g., from pH 7.0 to pH 6.0), the [H+] ([H+] = hydrogen ion
concentration) changes tenfold. The greater the [H+], the more acidic the solution
and the lower the pH. The lower the [H+], the more alkaline or basic the solution and
the higher the pH. In biologic fluids, a pH of less than 7.4 is defined as acidic and a
pH greater than 7.4 is defined as alkaline or basic (Table 5-8).
TABLE 5-8
pH of Body Fluids
Body Fluid pH Factors Affecting pH
Gastric juices 1.0-3.0 Hydrochloric acid production
Urine 5.0-6.0 H+ ion excretion from waste products
Arterial blood 7.35-7.45 pH is slightly higher because there is less carbonic acid (H2CO3)
Venous blood 7.37 pH is slightly lower because there is more carbonic acid
Cerebrospinal fluid 7.32 Decreased bicarbonate and higher carbon dioxide content decrease pH
Pancreatic fluid 7.8-8.0 Contains bicarbonate produced by exocrine cells
Bile 7.0-8.0 Contains bicarbonate
Small intestine fluid 6.5-7.5 Contains alkaline fluid from pancreas, liver, and gallbladder
Body acids are formed as end products of protein, carbohydrate, and fat
metabolism and acids can release hydrogen ion. Acids must be balanced by the
amount of basic substances in the body to maintain normal pH. The lungs, kidneys,
and bones are the major organs involved in regulating acid-base balance. The
systems work together to regulate short- and long-term changes in acid-base status.
Body acids exist in two forms: volatile (can be eliminated as CO2 gas) and
nonvolatile (can be eliminated by the kidney). The volatile acid is carbonic acid
(H2CO3), a weak acid (i.e., it does not release its hydrogen easily). In the presence of
the enzyme carbonic anhydrase, it readily dissociates into carbon dioxide (CO2) and
water (H2O). The carbon dioxide is then eliminated by pulmonary ventilation.
Nonvolitile acids are sulfuric, phosphoric, and other organic acids. They are
strong acids (readily release their hydrogens). Nonvolatile acids are secreted into
the urine by the renal tubules in amounts of about 60 to 100 mEq of hydrogen per
day or about 1 mEq per kilogram of body weight.
Buffer Systems
Buffering occurs in response to changes in acid-base status. Buffers can absorb
excessive hydrogen ion (H+) (acid) or hydroxyl ion (OH−) (base) and prevent a
significant change in pH. The buffer systems are located in both the ICF and the ECF
compartments, and they function at different rates (Table 5-9). The most important
plasma buffer systems are carbonic acid–bicarbonate and the protein hemoglobin
(Figure 5-9). Phosphate and protein are the most important intracellular buffers and
provide a first line of defense. Ammonia and phosphate can attach hydrogen ions
and are important renal buffers.
TABLE 5-9
Buffer Systems
Buffer Pairs Buffer System Chemical Reaction Rate
Bicarbonate Instantaneously
Hb−/HHb Hemoglobin HHb ⇌ H+ + Hb− Instantaneously
Phosphate Instantaneously
Pr−/HPr Plasma proteins HPr ⇌ H+ + Pr− Instantaneously
Organs Physiologic Mechanism Rate
Lung ventilation Regulates retention or elimination of CO2 and therefore H2CO3 concentration Minutes to hours
Ionic shifts Exchange of intracellular potassium and sodium for hydrogen 2-4 hours
Kidney tubules Bicarbonate reabsorption and regeneration, ammonia formation, phosphate buffering Hours to days
Bone Exchanges of calcium and phosphate and release of carbonate Hours to days
CO2, Carbon dioxide; Hb
−, hemoglobin; , bicarbonate; H2CO3, carbonic acid; HHb, hydrogenated
hemoglobin; , dibasic phosphate; , monobasic phosphate; HPr, hydrogenated protein; Pr−,
protein.
FIGURE 5-9 Integration of pH Control Mechanisms (example for acidosis). CO2 is produced in
tissue cells and diffuses to plasma, where it is transported as dissolved CO2, or it combines
with water to form carbonic acid (H2CO3), or it combines with protein from which hydrogen has
been released. Most of the CO2 diffuses into the red blood cells and combines with water to
form H2CO3. The H2CO3 dissociates to form hydrogen ion (H
+) and bicarbonate ( ).
Hydrogen combines with hemoglobin that has released its oxygen to form HHb, which buffers
the hydrogen and makes venous blood slightly more acidic than arterial blood. The increase in
H+ coupled with elevated CO2 levels results in HHbCO3 and an increase in the respiratory rate
and secretion of H+ by the kidneys.
Carbonic Acid–Bicarbonate Buffering
The carbonic acid–bicarbonate buffer pair operates in both the lung and the
kidney and is a major extracellular buffer. The lungs are a second line of defense
and can relatively quickly (within seconds to minutes) decrease the amount of
carbonic acid by blowing off carbon dioxide and leaving water. The kidneys are a
third line of defense (hours to days) and can reabsorb bicarbonate (a type of base)
or regenerate new bicarbonate from carbon dioxide and water. The relationship
between bicarbonate ( ) and carbonic acid (H2CO3) is usually expressed as a
ratio. Normal bicarbonate level is about 24 mEq/L, and normal carbonic acid level
is about 1.2 mEq/L (when the arterial CO2 partial pressure [PaCO2] is 40 mm Hg),
producing a 20 : 1 (24/1.2) ratio and the normal pH of 7.4 (Figure 5-10). These two
systems are very effective together because the lungs can adjust acid concentration
rapidly by ventilation and bicarbonate is easily reabsorbed or regenerated by the
kidney tubules, although more slowly.
FIGURE 5-10 Ratio of Carbonic Acid and Bicarbonate Concentration in Maintaining pH Within
Normal Limits. An increase in H2CO3 or decrease in concentration causes acidosis. A
decrease in H2CO3 or increase in concentration causes alkalosis. H2CO3, Carbonic acid;
, bicarbonate. (From Monahan FD: Medical-surgical nursing: health and illness perspectives, ed 8, St Louis, 2007, Mosby.)
Renal and respiratory adjustments to primary changes in pH are known as
compensation. The respiratory system compensates for changes in pH by
increasing or decreasing the concentration of carbon dioxide (carbonic acid) by
changing ventilation. The renal system compensates by producing more acidic or
more alkaline urine. The values for PaCO2 and bicarbonate will vary from normal
levels in an attempt to maintain a ratio of 20 : 1. Correction occurs when the values
for both components of the buffer pair (carbonic acid and bicarbonate) return to
normal levels.
Protein Buffering
Both intracellular and extracellular proteins have negative charges and can serve as
buffers for hydrogen, but because most proteins are inside cells, they are primarily
an intracellular buffer system. Hemoglobin (Hb) is an excellent intracellular blood
buffer because it can bind with hydrogen ion (H+) (forming HHb) and carbon
dioxide (forming HHbCO2). Hemoglobin bound to hydrogen ion becomes a weak
acid. Hemoglobin not saturated with oxygen (venous blood) is a better buffer than
hemoglobin saturated with oxygen (arterial blood). The pH control mechanism is
illustrated in Figure 5-9.
Renal Buffering
The distal tubule of the kidney regulates acid-base balance by secreting hydrogen
into the urine and reabsorbing bicarbonate into the plasma. Dibasic phosphate (
) and ammonia (NH3) are two important renal buffers because they can attach
hydrogen ions and be secreted into the urine. The renal buffering of hydrogen ions
requires the use of carbon dioxide (CO2) and water (H2O) to form carbonic acid
(H2CO3). The enzyme carbonic anhydrase catalyzes the reaction. The hydrogen in
the carbonic acid is then secreted from the tubular cell and buffered in the lumen by
phosphate and ammonia (i.e., forms and ). The remaining bicarbonate is
reabsorbed. The end effect is the addition of new bicarbonate to the plasma, which
contributes to the alkalinity of the plasma because the hydrogen ion is excreted from
the body (Figure 5-11).
FIGURE 5-11 Renal Excretion of Acid. 1, Conservation of filtered bicarbonate. Filtered
bicarbonate combines with secreted hydrogen ion in the presence of carbon anhydrase (CA) to
form carbonic acid (H2CO3), which then dissociates to water (H2O) and carbon dioxide (CO2);
both diffuse into the epithelial cell. The CO2 and H2O combine to form H2CO3 in the presence of
CA, and the resulting bicarbonate ion ( ) is reabsorbed into the capillary. 2, Formation of
titratable acid. Hydrogen ion is secreted and combines with dibasic phosphate ( ) to form
monobasic phosphate ( ). The secreted hydrogen ion is formed from the dissociation of
H2CO3, and the remaining is reabsorbed into the capillary. 3, Formation of ammonium.
Ammonia (NH3) is produced from glutamine in the epithelial cell and diffuses to the tubular
lumen, where it combines with H+ to form ammonium ion ( ). Once has been formed, it
cannot return to the epithelial cell (diffusional trapping), and the bicarbonate remaining in the
epithelial cell is reabsorbed into the capillary.
Acid-Base Imbalances
Pathophysiologic changes in the concentration of hydrogen ion in the blood lead to
acid-base imbalances.18,19 In acidemia the pH of arterial blood is less than 7.4. A
systemic increase in hydrogen ion concentration or a loss of base is termed
acidosis. In alkalemia the pH of arterial blood is greater than 7.4. A systemic
decrease in hydrogen ion concentration or an excess of base is termed alkalosis.
These changes may be caused by metabolic or respiratory processes. Figure 5-10
summarizes the relationship among pH, the partial pressure of carbon dioxide
(respiratory regulation), and the concentration of bicarbonate (renal regulation)
during alkalosis and acidosis. Acid-base imbalances are assessed using
measurement of arterial blood gases, which includes the reporting of pH, PaCO2, and
. The medical history and clinical symptoms are important in determining the
cause of the disorder. Figure 5-12 summarizes the relationships among pH, PCO2,
and bicarbonate during different acid-base alterations.
FIGURE 5-12 Primary and Compensatory Acid-Base Changes. A systematic approach can be
used to interpret the cause of an acid-base imbalance. 1, Is the pH low or high? 2, If the pH is
low (acidemia), is the cause respiratory (high PaCO2) or metabolic (low )? 3, If the pH is
high (alkalemia), is the cause respiratory (low PaCO2) or metabolic (high )? 4, Is there
compensation for the primary acid-base disorder? (a) will be ≥24 mEq/L if there is renal
compensation for a primary respiratory acidosis; (b) PaCO2 will be <40 mm Hg if there is
respiratory compensation of a primary metabolic acidosis; (c) will be <24 mEq/L if there
is renal compensation for primary respiratory alkalosis; (d) PaCO2 will be >40 mm Hg if there is
respiratory compensation for primary metabolic alkalosis. NOTE: Examine the pH first to
determine if there is acidemia or alkalemia. Then examine the changes in and PaCO2. 1,
will be elevated when there is primary metabolic alkalosis or renal compensation for
primary respiratory acidosis. 2, will be decreased when there is primary metabolic
acidosis or renal compensation for primary respiratory alkalosis. 3, PaCO2 will be elevated
when there is primary respiratory acidosis or respiratory compensation for primary metabolic
alkalosis. 4, PaCO2 will be decreased when there is primary respiratory alkalosis or respiratory
compensation for metabolic acidosis. H2CO3, Carbonic acid; , bicarbonate; PaCO2,
arterial partial pressure of carbon dioxide.
Metabolic Acidosis
In metabolic acidosis the concentrations of non–carbonic acids increase or
bicarbonate is lost from extracellular fluid or cannot be regenerated by the kidney
(Table 5-10). This can occur either quickly, as in lactic acidosis caused by poor
perfusion or hypoxemia, or slowly over an extended time, as in renal failure,
diabetic ketoacidosis, or starvation (anion gap acidosis).20 There is a decrease in the
20 : 1 ratio of to H2CO3.
TABLE 5-10
Causes of Metabolic Acidosis
Increased Non–Carbonic Acids (Elevated Anion Gap*) Bicarbonate Loss or Hyperchloremic Acidosis (Normal Anion
Gap)
Increased H+ load Diarrhea
Ketoacidosis (e.g., diabetes mellitus, starvation)
Lactic acidosis (e.g., shock, hypoxemia)
Ingestion (e.g., ammonium chloride, ethylene glycol, methanol, salicylates,
paraldehyde)
Ureterosigmoidoscopy (chloride absorbed in excess of sodium in small
intestine)
Renal failure (loss of bicarbonate)
Proximal renal tubular acidosis (loss of more renal sodium in relation to
chloride)Decreased renal H+ excretion
Uremia
Distal renal tubule acidosis
*Anion gap refers to anions not usually measured in laboratory reports (e.g., sulfate, phosphate, and
lactate). The anions usually measured are chloride (Cl−) and bicarbonate ( ). When the sum of the
concentrations of measured anions (e.g., chloride and bicarbonate) is subtracted from the sum of the
concentrations of measured cations (e.g., sodium and potassium), there is a “gap” of approximately 10 to
12 mEq/L; this is the normal anion gap. An elevated anion gap provides clues to the cause of the acidosis
(i.e., to the addition of endogenously or exogenously generated acids). In a normal anion gap acidosis,
chloride is retained to replace lost bicarbonate.
The buffering systems normally compensate for excess acid and maintain arterial
pH within normal range. When acidosis is severe, buffers become depleted and
cannot compensate, and the ratio of the concentrations of bicarbonate to carbonic
acid decreases to less than 20 : 1 (see Figure 5-10). An increase in the plasma
concentration of chloride out of proportion of sodium causes hyperchloremic
acidosis (nonainion gap acidosis). The specific type of acidosis can be determined
by examining the serum anion gap (see Table 5-10).
Metabolic acidosis is manifested by changes in the function of the neurologic,
respiratory, gastrointestinal, and cardiovascular systems. Early symptoms include
headache and lethargy, which progress to confusion and coma in severe acidosis.
The respiratory system’s efforts to compensate for the increase in metabolic acids
result in what are termed Kussmaul respirations (a form of hyperventilation), which
are deep and rapid. This represents the body’s attempt to increase pH by expelling
carbon dioxide, which decreases carbonic acid concentration. Other symptoms
include anorexia, nausea, vomiting, diarrhea, and abdominal discomfort. Death can
result in the most severe and prolonged cases preceded by dysrhythmias and
hypotension. The underlying condition must be diagnosed to establish effective
treatment.
Metabolic Alkalosis
When excessive loss of metabolic acids occurs, bicarbonate concentration
increases, causing metabolic alkalosis21 (see Figure 5-12). When acid loss is caused
by vomiting, renal compensation is not very effective because loss of chloride (an
anion) in hydrochloric acid (HCl) stimulates renal retention of bicarbonate (an
anion). The result is known as hypochloremic metabolic alkalosis.21
Hyperaldosteronism also can lead to alkalosis as a result of sodium bicarbonate
retention and loss of hydrogen and potassium. Diuretics may produce a mild
alkalosis because they promote greater excretion of sodium, potassium, and
chloride than of bicarbonate.
Some common signs and symptoms of metabolic alkalosis are weakness, muscle
cramps, hyperactive reflexes, tetany, confusion, convulsions, and atrial tachycardia.
Respirations may be shallow and slow ventilation as the lungs attempt to
compensate by increasing carbon dioxide retention. The manifestations vary with
the cause and severity of the alkalosis. The symptoms of hyperactive reflexes and
tetany occur because alkalosis increases binding of Ca++ to plasma proteins, thus
decreasing ionized calcium concentration. The decreased ionized calcium
concentration causes excitable cells to become hypopolarized, initiating an action
potential more easily and causing muscle contraction.
Treatments are related to the underlying cause of the condition. With
hypochloremic alkalosis or contraction alkalosis with volume depletion, a sodium
chloride solution is required for correction because chloride must be replaced
before bicarbonate can be excreted by the kidney.
Respiratory Acidosis
Respiratory acidosis occurs when there is alveolar hypoventilation, resulting in an
excess of carbon dioxide in the blood (hypercapnia). The arterial carbon dioxide
tension (or pressure) (PaCO2) is >45 mm Hg and the pH is less than 7.35 (see Figure
5-12). A decrease in alveolar ventilation in relation to the metabolic production of
carbon dioxide produces respiratory acidosis by an increase in the concentration of
carbonic acid. Respiratory acidosis can be acute or chronic.22 Common causes
include depression of the respiratory center (e.g., from drugs or head injury),
paralysis of the respiratory muscles, disorders of the chest wall (e.g.,
kyphoscoliosis or broken ribs), and disorders of the lung parenchyma (e.g.,
pneumonia, pulmonary edema, emphysema, asthma, bronchitis). Renal
compensation occurs by elimination of hydrogen ion and retention of bicarbonate.
The signs and symptoms seen often include headache, blurred vision,
breathlessness, restlessness, and apprehension followed by lethargy, disorientation,
muscle twitching, tremors, convulsions, and coma. Respiratory rate is rapid at first
and gradually becomes depressed as the respiratory center adapts to increasing
levels of carbon dioxide. The skin may be warm and flushed because the elevated
carbon dioxide concentration causes vasodilation. The restoration of adequate
alveolar ventilation is necessary to remove the excess CO2 (↓H2CO3).
Respiratory Alkalosis
Respiratory alkalosis occurs when there is alveolar hyperventilation (deep, rapid
respirations). Excessive reduction in plasma carbon dioxide levels (hypocapnia)
decrease carbonic acid concentration.23,24 The PaCO2 is <35 mm Hg and the pH is
greater than normal (see Figure 5-12). Respiratory alkalosis can be chronic or
acute. Hypoxemia (caused by pulmonary disease, congestive heart failure, or high
altitudes), hypermetabolic states (e.g., fever, anemia, thyrotoxicosis), early salicylate
intoxication, hysteria, cirrhosis, and gram-negative sepsis stimulate
hyperventilation. Improper use of mechanical ventilators also can cause iatrogenic
(treatment-related) respiratory alkalosis, and secondary alkalosis may develop as a
result of hyperventilation stimulated by metabolic or respiratory acidosis. The
kidneys compensate by decreasing hydrogen excretion and bicarbonate
reabsorption.
The central and peripheral nervous systems are stimulated by respiratory
alkalosis, causing dizziness, confusion, tingling of extremities (paresthesias),
convulsions, and coma. Cerebral vasoconstriction reduces cerebral blood flow.
Carpopedal spasm (spasm of muscles in the fingers and toes), tetany, and other
symptoms of hypocalcemia (see Table 5-7, p. 126) are similar to those of metabolic
alkalosis. The underlying disturbance must be treated, particularly hypoxemia.
Quick Check 5-5
1. What is the difference between compensation and correction of acid-base
disturbances?
2. What two chemicals are altered in metabolic acid-base disturbances?
3. How do alterations in carbon dioxide concentration influence acid-base status?
Pediatric Considerations
Distribution of Body Fluids
Newborn Infants
At birth TBW represents about 75% to 80% of body weight and decreases to about
67% during the first year of life. Physiologic loss of body water amounting to 5%
of body weight occurs as an infant adjusts to a new environment. Infants are
particularly susceptible to significant changes in TBW because of a high metabolic
rate and greater body surface area, as compared to adults. Consequently, they have
a greater fluid intake and output in relation to their body size. Renal mechanisms of
fluid and electrolyte conservation may not be mature enough to counter abnormal
losses related to vomiting or diarrhea, thereby allowing dehydration to occur.
Symptoms of dehydration include increased thirst, decreased urine output,
decreased body weight, decreased skin elasticity, sunken fontanels, absent tears, dry
mucous membranes, increased heart rate, and irritability.
Children and Adolescents
TBW slowly decreases to 60% to 65% of body weight. At adolescence the
percentage of TBW approaches adult levels and differences according to gender
appear. Males have a greater percentage of body water because of increased muscle
mass, and females have more body fat because of the influence of estrogen and thus
less water.
Geriatric Considerations
Distribution of Body Fluids
The further decline in the percentage of TBW in the elderly is in part the result of a
decreased free fat mass and decreased muscle mass, as well as a reduced ability to
regulate sodium and water balance. Kidneys are less efficient in producing either a
concentrated or a diluted urine, and sodium-conserving responses are sluggish.
Thirst perception also may decline and loss of cognitive function can influence
access to beverages. Healthy older adults can adequately maintain their hydration
status. When disease is present, a decrease in TBW, dehydration, and hypernatremia
can become life-threatening.
Did You Understand?
Distribution of Body Fluids
1. Body fluids are distributed among functional compartments and are classified as
intracellular fluid (ICF) and extracellular fluid (ECF).
2. The sum of all fluids is the total body water (TBW), which varies with age and
amount of body fat.
3. Water moves between the ICF and ECF compartments principally by osmosis.
4. Water moves between the plasma and interstitial fluid by osmosis (pulling of
water) and hydrostatic pressure (pushing of water), which occur across the capillary
membrane.
5. Movement across the capillary wall is called net filtration and is described
according to Starling law (the balance between hydrostatic and osmotic forces).
Alterations in Water Movement
1. Edema is a problem of fluid distribution that results in accumulation of fluid
within the interstitial spaces.
2. The pathophysiologic process that leads to edema is related to an increase in
forces favoring fluid filtration from the capillaries or lymphatic channels into the
tissues.
3. Edema is caused by arterial dilation, venous or lymphatic obstruction, increased
vascular volume, loss of plasma proteins, or increased capillary permeability.
4. Edema may be localized or generalized and usually is associated with weight
gain, swelling and puffiness, tighter-fitting clothes and shoes, and limited movement
of the affected area.
Sodium, Chloride, and Water Balance
1. There is an intimate relationship between the balance of sodium and water levels;
chloride levels are generally proportional to changes in sodium levels.
2. Water balance is regulated by the sensation of thirst and by antidiuretic hormone
(ADH), which is secreted in response to an increase in plasma osmolality or a
decrease in circulating blood volume.
3. Sodium balance is regulated by aldosterone, which increases reabsorption of
sodium from the urine into the blood by the distal tubule of the kidney.
4. Renin and angiotensin are enzymes that promote secretion of aldosterone and
thus regulate sodium and water balance.
5. Natriuretic hormones are involved in decreasing tubular reabsorption and
promoting urinary excretion of sodium.
Alterations in Sodium, Water, and Chloride Balance
1. Alterations in sodium and water balance may be classified as isotonic, hypertonic,
or hypotonic.
2. Isotonic alterations occur when changes in TBW are accompanied by
proportional changes in electrolytes.
3. Hypertonic alterations develop when the osmolality of the ECF is elevated above
normal, usually because of an increased concentration of ECF sodium or a deficit
of ECF water.
4. Hypernatremia (sodium levels more than 145 mEq/L) may be caused by an acute
increase in sodium level or a loss of water.
5. Hypernatremia can be isovolemic, hypovolemic, or hypervolemic depending on
accompanying changes in the level of body water.
6. Hypernatremia with marked water deficit is manifested by hypovolemia and
dehydration.
7. Hyperchloremia is caused by an excess of sodium or a deficit of bicarbonate.
8. Hypotonic alterations occur when the osmolality of the ECF is less than normal.
9. Hyponatremia (serum sodium concentration less than 135 mEq/L) usually causes
movement of water into cells.
10. Hyponatremia may be caused by sodium loss, inadequate sodium intake, or
dilution of the body's sodium level with excess water.
11. Hyponatremia can be isovolemic, hypervolemic or hypovolemic, or dilutional
depending on accompanying changes in the amount of body water.
12. Hypochloremia usually is the result of hyponatremia or elevated bicarbonate
concentrations.
Alterations in Potassium and Other Electrolytes
1. Potassium is the predominant ICF ion; it regulates ICF osmolality, maintains the
resting membrane potential, and is required for deposition of glycogen in liver and
skeletal muscle cells.
2. Potassium balance is regulated by the kidney, by aldosterone and insulin
secretion, and by changes in pH.
3. Potassium adaptation allows the body to accommodate slowly to increased levels
of potassium intake.
4. Hypokalemia (serum potassium concentration less than 3.5 mEq/L) indicates loss
of total body potassium, although ECF hypokalemia can develop without losses of
total body potassium, and plasma potassium levels may be normal or elevated when
total body potassium is depleted.
5. Hypokalemia may be caused by reduced potassium intake, a shift of potassium
from the ECF to the ICF, increased aldosterone secretion, increased renal excretion,
and alkalosis.
6. Hyperkalemia (potassium levels that are greater than 5.5 mEq/L) may be caused
by increased potassium intake, a shift of potassium from the ICF to the ECF, or
decreased renal excretion.
7. Calcium is an ion necessary for bone and teeth formation, blood coagulation,
hormone secretion and cell receptor function, and membrane stability.
8. Phosphate acts as a buffer in acid-base regulation and provides energy for muscle
contraction.
9. Calcium and phosphate concentrations are rigidly controlled by parathyroid
hormone (PTH), vitamin D, and calcitonin.
10. Hypocalcemia (serum calcium concentration less than 8.5 mg/dl) is related to
inadequate intestinal absorption, deposition of calcium into bone or soft tissue,
blood administration, or decreased PTH and vitamin D levels.
11. Hypercalcemia (serum calcium concentration greater than 12 mg/dl) can be
caused by a number of diseases, including hyperparathyroidism, bone metastases,
sarcoidosis, and excess vitamin D.
12. Hypophosphatemia is usually caused by intestinal malabsorption and increased
renal excretion of phosphate.
13. Hyperphosphatemia develops with acute or chronic renal failure when there is
significant loss of glomerular filtration.
14. Magnesium is a major intracellular cation and is regulated principally by PTH.
15. Magnesium functions in enzymatic reactions and often interacts with calcium at
the cellular level.
16. Hypomagnesemia (serum magnesium concentrations less than 1.5 mEq/L) may
be caused by malabsorption syndromes.
17. Hypermagnesemia (serum magnesium concentrations greater than 2.5 mEq/L) is
rare and usually is caused by renal failure.
Acid-Base Balance
1. Hydrogen ions, which maintain membrane integrity and the speed of enzymatic
reactions, must be concentrated within a narrow range if the body is to function
normally.
2. Hydrogen ion concentration, [H+], is expressed as pH, which represents the
negative logarithm (i.e., 10−7) of hydrogen ions in solution (i.e., 0.0000001 mg/L).
3. Different body fluids have different pH values; values less than 7.4 are more
acidic and values greater than 7.4 are more basic.
4. The renal and respiratory systems, together with the body's buffer systems, are
the principal regulators of acid-base balance.
5. Buffers are substances that can absorb excessive acid or base without a significant
change in pH.
6. Buffers exist as acid-base pairs; the principal plasma buffers are carbonic acid
(H2CO3), bicarbonate ( ), protein (hemoglobin), and phosphate.
7. The lungs and kidneys act to compensate for primary changes in pH by
increasing or decreasing ventilation and by producing more acidic or more alkaline
urine.
8. Correction is a process different from compensation; correction occurs when the
values for both components of the buffer pair return to normal as the primary
disorder is treated or resolves.
9. Acid-base imbalances are caused by changes in the concentration of hydrogen ion
in the blood; an increase causes acidosis, and a decrease causes alkalosis.
10. An abnormal increase or decrease in bicarbonate concentration causes
metabolic alkalosis or metabolic acidosis; changes in the rate of alveolar ventilation
and removal of carbon dioxide produce respiratory acidosis or respiratory
alkalosis.
11. Metabolic acidosis is caused by an increase in the levels of non–carbonic acids
or by the loss of bicarbonate from the extracellular fluid.
12. Metabolic alkalosis occurs with an increase in bicarbonate concentration, which
is usually caused by loss of metabolic acids from conditions such as vomiting or
gastrointestinal suctioning or by excessive bicarbonate intake, hyperaldosteronism,
and diuretic therapy.
13. Respiratory acidosis occurs with decreased alveolar ventilation, which in turn
causes hypercapnia (an increase in carbon dioxide concentration) and increased
carbonic acid concentration.
14. Respiratory alkalosis occurs with alveolar hyperventilation and excessive
reduction of carbon dioxide level, or hypocapnia with decreases in carbonic acid
concentration.
Key Terms
Acidemia, 127
Acidosis, 127
Aldosterone, 117
Alkalemia, 127
Alkalosis, 127
Angiotensin I, 117
Angiotensin II, 117
Anion gap, 127
Aquaporin, 115
Antidiuretic hormone (ADH), 117
Baroreceptor, 119
Buffer, 125
Buffering, 125
Capillary hydrostatic pressure (blood pressure), 115
Capillary (plasma) oncotic pressure, 115
Carbonic acid–bicarbonate buffer, 125
Chloride (Cl−), 117
Compensation, 125
Correction, 125
Dehydration, 119
Dilutional hyponatremia (water intoxication), 121
Edema, 115
Extracellular fluid (ECF), 114
Hypercapnia, 128
Hyperchloremia, 120
Hyperkalemia, 124
Hypernatremia, 119
Hypertonic fluid alterations, 119
Hypervolemic hypernatremia, 119
Hypervolemic hyponatremia, 121
Hypocapnia, 130
Hypochloremia, 121
Hypochloremic metabolic alkalosis, 127
Hypokalemia, 122
Hyponatremia, 121
Hypotonic fluid imbalance, 121
Hypovolemic hypernatremia, 119
Hypovolemic hyponatremia, 121
Interstitial fluid, 114
Interstitial hydrostatic pressure, 115
Interstitial oncotic pressure, 115
Intracellular fluid (ICF), 114
Intravascular fluid, 114
Isotonic alteration, 119
Isotonic fluid excess, 119
Isotonic fluid loss, 119
Isovolemic hypernatremia, 119
Isovolemic hyponatremia, 121
Lymphedema, 116
Metabolic acidosis, 127
Metabolic alkalosis, 127
Natriuretic peptide, 117
Net filtration, 115
Nonvolatile, 125
Osmoreceptor, 118
Potassium (K+), 122
Potassium adaptation, 122
Renin, 117
Renin-angiotensin-aldosterone system, 117
Respiratory acidosis, 128
Respiratory alkalosis, 130
Sodium (Na+), 117
Starling forces, 115
Total body water (TBW), 114
Volatile, 125
Volume-sensitive receptor, 119
Water balance, 118
References
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Biochim Biophys Acta. 2014;1840(5):1492–1506.
2. Trayes KP, et al. Edema: diagnosis and management. Am Fam Physician.
2013;88(2):102–110.
3. Ridner SH. Pathophysiology of lymphedema. Semin Oncol Nurs.
2013;29(1):4–11.
4. Motiwala SR, Januzzi JL Jr. The role of natriuretic peptides as biomarkers
for guiding the management of chronic heart failure. Clin Pharmacol Ther.
2013;93(1):57–67.
5. Cheuvront SN, et al. Physiologic basis for understanding quantitative
dehydration assessment. Am J Clin Nutr. 2013;97(3):455–462.
6. Sam R, Feizi I. Understanding hypernatremia. Am J Nephrol. 2012;36(1):97–
104.
7. Berend K, van Hulsteijn LH, Gans RO. Chloride: the queen of electrolytes?
Eur J Intern Med. 2012;23(3):203–211.
8. Sterns RH. Disorders of plasma sodium—causes, consequences, and
correction. N Engl J Med. 2015;372(1):55–65.
9. Gross P. Clinical management of SIADH. Ther Adv Endocrinol Metab.
2012;3(2):61–73.
10. Lehrich RW, et al. Role of vaptans in the management of hyponatremia. Am J
Kidney Dis. 2013;62(2):364–376.
11. Youn JH. Gut sensing of potassium intake and its role in potassium
homeostasis. Semin Nephrol. 2013;33(3):248–256.
12. Lee Hamm L, et al. Acid-base and potassium homeostasis. Semin Nephrol.
2013;33(3):257–264.
13. Pepin J, Shields C. Advances in diagnosis and management of hypokalemic
and hyperkalemic emergencies. Emerg Med Pract. 2012;14(2):1–17.
14. Lim S. Approach to hyperkalemia. Acta Med Indones. 2007;39(2):99–103.
15. Maxwell AP, et al. Management of hyperkalaemia. J R Coll Physicians
Edinb. 2013;43(3):246–251.
16. Moe SM. Disorders involving calcium, phosphorus and magnesium. Prim
Care. 2008;35(2):215–237.
17. Adeva-Andany MM, et al. The importance of the ionic product for water to
understand the physiology of the acid-base balance in humans. Biomed Res
Int. 2014;2014:695281.
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acid-base disturbances. N Engl J Med. 2014;371(15):1434–1445 [Erratum
in: N Engl J Med 371(20):1948].
19. Carmody JB, Norwood VF. A clinical approach to paediatric acid-base
disorders. Postgrad Med J. 2012;88(1037):143–151.
20. Kraut JA, Madias NE. Differential diagnosis of nongap metabolic acidosis:
value of a systematic approach. Clin J Am Soc Nephrol. 2012;7(4):671–679.
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Am. 2014;32(2):453–463.
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J Nephrol. 2010;16(Suppl 16):S85–S91.
UNIT 2
Mechanisms of Self-Defense
OUTLINE
6 Innate Immunity: Inflammation and Wound Healing
7 Adaptive Immunity
8 Infection and Defects in Mechanisms of Defense
9 Stress and Disease
6
Innate Immunity
Inflammation and Wound Healing
Neal S. Rote
CHAPTER OUTLINE
Human Defense Mechanisms, 134
Innate Immunity, 134
First Line of Defense: Physical and Biochemical
Barriers and the Human Microbiome, 135
Second Line of Defense: Inflammation, 137
Plasma Protein Systems and Inflammation, 138
Cellular Components of Inflammation, 141
Acute and Chronic Inflammation, 147
Local Manifestations of Acute Inflammation, 149
Systemic Manifestations of Acute Inflammation,
149
Chronic Inflammation, 149
Wound Healing, 151
Phase I: Inflammation, 152
Phase II: Proliferation and New Tissue Formation,
152
Phase III: Remodeling and Maturation, 153
Dysfunctional Wound Healing, 153
PEDIATRIC CONSIDERATIONS: Age-Related Factors Affecting Innate
Immunity in the Newborn Child, 154
GERIATRIC CONSIDERATIONS: Age-Related Factors Affecting Innate
Immunity in the Elderly, 154
The human body is continually exposed to a large variety of conditions that result in
damage, such as sunlight, pollutants, agents that can cause physical trauma, and
infectious agents (viruses, bacteria, fungi, parasites). Damage can also arise from
within, such as cancers. The damage may be at the level of a single cell, which can
be easily repaired, or may be at the level of multiple cells or tissues or organs,
which can result in disease and potentially the death of the individual. To protect us
from these conditions, the body has developed a highly sophisticated, multilevel
system of interactive defense mechanisms.
Human Defense Mechanisms
The human body has developed several means of protecting itself from injury and
infection. Innate immunity, also known as natural or native immunity, includes
natural barriers (physical, mechanical, and biochemical) and inflammation. Innate
barriers form the first line of defense at the body's surfaces and are in place at birth
to prevent damage by substances in the environment and thwart infection by
pathogenic microorganisms. Surface barriers may also harbor a group of
microorganisms known as the “normal flora” that can protect us from pathogens. If
the surface barriers are breached, the second line of defense, the inflammatory
response, is activated to protect the body from further injury, prevent infection of
the injured tissue, and promote healing. The inflammatory response is a rapid
activation of biochemical and cellular mechanisms that are relatively nonspecific,
with similar responses being initiated against a wide variety of causes of tissue
damage. The third line of defense, adaptive immunity (also known as acquired or
specific immunity), is induced in a relatively slower and more specific process and
targets particular invading microorganisms for the purpose of eradicating them.
Adaptive immunity also involves “memory,” which results in a more rapid response
during future exposure to the same microorganism. Comparisons among defense
mechanisms are described in Table 6-1. The information presented in this chapter
introduces the components and processes of innate immunity and sets the stage for
Chapter 7, which presents an overview of adaptive immunity, and Chapter 8, which
discusses processes of infection and alterations in immune defenses.
TABLE 6-1
Overview of Human Defenses
Characteristics Barriers Innate Immunity Adaptive (Acquired)
Immunity
Level of defense First line of defense against infection and tissue
injury
Second line of defense; occurs as response to tissue
injury or infection (inflammatory response)
Third line of defense; initiated
when innate immune system
signals cells of adaptive
immunity
Timing of
defense
Constant Immediate response Delay between primary exposure
to antigen and maximal response;
immediate against secondary
exposure to antigen
Specificity Broadly specific Broadly specific Response is very specific toward
“antigen”
Cells Epithelial cells
Microbiome
Mast cells, granulocytes (neutrophils, eosinophils,
basophils), monocytes/ macrophages, natural killer
(NK) cells, platelets, endothelial cells
T lymphocytes, B lymphocytes,
macrophages, dendritic cells
Memory No memory involved No memory involved Specific immunologic memory
by T and B lymphocytes
Active molecules Defensins, cathelicidins, collectins, lactoferrin,
bacterial toxins
Complement, clotting factors, kinins, cytokines Antibodies, complement,
cytokines
Protection Protection includes anatomic barriers (i.e., skin
and mucous membranes), cells and secretory
molecules (e.g., lysozymes, low pH of stomach
and urine), and ciliary activity
Protection includes vascular responses, cellular
components (e.g., mast cells, neutrophils,
macrophages), secretory molecules or cytokines, and
activation of plasma protein systems
Protection includes activated T
and B lymphocytes, cytokines,
and antibodies
Innate Immunity
Innate immunity includes natural barriers (physical, mechanical, and biochemical)
that form the first line of defense at the body's surfaces and are in place at birth.
Surface barriers also may harbor a group of frequently benign microorganisms
known as the “normal microbiome” that can protect us from pathogenic
microorganisms. Innate immunity in the newborn and changes associated with aging
are reviewed in the Pediatric and Geriatric Considerations boxes.
First Line of Defense: Physical and Biochemical
Barriers and the Human Microbiome
Physical Barriers
The physical barriers that cover the external parts of the human body offer
considerable protection from damage and infection. These barriers are composed
of tightly associated epithelial cells of the skin and of the linings of the
gastrointestinal, genitourinary, and respiratory tracts (Figure 6-1). When pathogens
attempt to penetrate this physical barrier, they may be removed by mechanical
means—sloughed off with dead skin cells as they are routinely replaced, expelled
by coughing or sneezing, vomited from the stomach, or flushed from the urinary
tract by urine. Epithelial cells of the upper respiratory tract also produce mucus and
have hair-like cilia that trap and move pathogens upward to be expelled by coughing
or sneezing. Additionally, the low temperature (such as on the skin) and the low pH
(such as of the skin and stomach) generally inhibit microorganisms, most of which
routinely require temperatures near 37° C (98.6° F) and pH near neutral for efficient
growth.
FIGURE 6-1 The Closed Barrier. The digestive, respiratory, and genitourinary tracts and skin
form closed barriers between the internal organs and the environment. (From Grimes DE: Infectious
diseases, St. Louis, 1991, Mosby.)
Epithelial Cell–Derived Chemicals
Epithelial cells secrete an array of substances that protect against infection,
including mucus, perspiration (or sweat), saliva, tears, and earwax. These can trap
potential invaders and contain substances that will kill microorganisms.
Perspiration, tears, and saliva contain an enzyme (lysozyme) that attacks the cell
walls of gram-positive bacteria. Sebaceous glands in the skin also secrete fatty acids
and lactic acid that kill bacteria and fungi. These glandular secretions create an
acidic (pH 3 to 5) and inhospitable environment for most bacteria.
Epithelial cell secretions also contain small-molecular-weight antimicrobial
peptides that kill or inhibit the growth of disease-causing bacteria, fungi, and
viruses.1 These are generally positively charged polypeptides of approximately 15
to 95 amino acids. More than a thousand antimicrobial peptides have been found, but
the best studied are cathelicidins and defensins.
Several cathelicidins have been discovered in other species, but only one is
currently known to function in humans. Bacteria have cholesterol-free cell
membranes into which cathelicidin can insert and disrupt the membrane, killing the
bacteria. Cathelicidin is produced by epithelial cells of the skin, gut, urinary tract,
and respiratory tract, and is stored in neutrophils, mast cells, and monocytes and can
be released during inflammation.
In contrast, many different human defensins have been identified. Defensin
molecules can be further subdivided into α (at least six identified in humans) and β
types (at least six identified, but perhaps up to 40 different molecules). The α-
defensins often require activation by proteolytic enzymes, whereas the β-defensins
are synthesized in active forms. Given the similarity in their chemical charges,
defensins may kill bacteria in the same way as cathelicidin. The α-defensins are
particularly rich in the granules of neutrophils and may contribute to the killing of
bacteria by those cells. They are also found in Paneth cells lining the small intestine,
where they protect against a variety of disease-causing microorganisms. The β-
defensins are found in epithelial cells lining the respiratory, urinary, and intestinal
tracts, as well as in the skin. In addition to antibacterial properties, β-defensins may
also help protect epithelial surfaces from infection with adenovirus (one of the
causes of the common cold) and human immunodeficiency virus (HIV). Both
classes of antimicrobial peptides also can activate cells of the next levels of defense:
innate and acquired immunity.
The lung also produces and secretes a family of glycoproteins, collectins, which
includes surfactant proteins A through D and mannose-binding lectin. Collectins
react with carbohydrates on the surface of a wide array of pathogenic
microorganisms and help cells of the innate immune system (macrophages) to
recognize and kill the microorganism. Mannose-binding lectin (MBL) recognizes
a sugar commonly found on the surface of microbes and is a powerful activator of
a plasma protein system (complement) resulting in damage to bacteria or increased
recognition by macrophages.
The Normal Microbiome
The body's surfaces are colonized with an array of microorganisms, the normal
microbiome previously known as normal flora. Each surface (the skin and the
mucous membranes of the eyes, upper and lower gastrointestinal tracts, upper
respiratory tract, urethra, and vagina) is colonized by a combination of bacteria and
fungi that is unique to the particular location and individual2 (Table 6-2). The
microorganisms in the microbiome do not normally cause disease, and although
their relationship with humans has been referred to as commensal (to the benefit of
one organism without affecting the other), the relationship may be more mutualistic
(to the benefit of both organisms). Using the colon for an example, at birth the
lower gut is relatively sterile but colonization with bacteria begins quickly, with the
number, diversity, and concentration increasing progressively during the first year
of life.
TABLE 6-2
The Human Microbiome
Location Microorganisms
Skin Predominantly gram-positive cocci and rods; Staphylococcus epidermidis, corynebacteria, mycobacteria, and streptococci are primary
inhabitants; Staphylococcus aureus in some people; also yeasts (Candida, Pityrosporum) in some areas of skin
Numerous transient microorganisms may become temporary residents
In moist areas, gram-negative bacteria
Around sebaceous glands, Propionibacterium and Brevibacterium
Mite Demodex folliculorum lives in hair follicles and sebaceous glands around face
Nose Predominantly gram-positive cocci and rods, especially S. epidermidis
Some people are nasal carriers of pathogenic bacteria, including S. aureus, β-hemolytic streptococci, and Corynebacterium diphtheria
Mouth Complex of bacteria that includes several species of streptococci, Actinomyces, lactobacilli, and Haemophilus
Anaerobic bacteria and spirochetes colonize gingival crevices
Pharynx Similar to flora in mouth plus staphylococci, Neisseria, and diphtheroids
Some asymptomatic persons also harbor pathogens: pneumococcus, Haemophilus influenzae, Neisseria meningitidis, and C. diphtheria
Distal
small
intestine
Enterobacteria, streptococci, lactobacilli, anaerobic bacteria, and C. albicans
Colon Bacteroides, lactobacilli, clostridia, Salmonella, Shigella, Klebsiella, Proteus, Pseudomonas, enterococci, and other streptococci, bacilli, and
Escherichia coli
Distal
urethra
Typical bacteria found on skin, especially S. epidermidis and diphtheroids; also lactobacilli and nonpathogenic streptococci
Vagina Birth to 1 month: similar to adult
1 month to puberty: S. epidermidis, diphtheroids, E. coli, and streptococci
Puberty to menopause: Lactobacillus acidophilus, diphtheroids, staphylococci, streptococci, and variety of anaerobes
Postmenopause: similar to prepubescence
Adapted from Bennett JE et al, editors: Mandell, Douglas, and Bennett's principles and practice of
infectious diseases, ed 8, Philadelphia, 2015, Saunders.
The normal microbiome benefits us in many ways; bacteria in the gastrointestinal
(GI) tract produce (1) enzymes that facilitate the digestion and utilization of many
molecules in the human diet, such as fatty acids and large polysaccharides; (2)
usable metabolites (e.g., vitamin K, B vitamins); and (3) antibacterial factors that
prevent colonization by pathogenic microorganisms (see Chapter 8) For instance,
members of the normal microbiome in the colon produce chemicals (ammonia,
phenols, indoles, and other toxic materials) and proteins (bacteriocins) that are toxic
to more pathogenic microorganisms. They also compete with pathogens for
nutrients and block attachment to the epithelium, which is an obligatory first step in
the infectious process by most pathogens. Additionally, the normal microbiome of
the gut helps train the adaptive immune system by inducing growth of gut-associated
lymphoid tissue (where most cells of the adaptive immune system reside) and the
development of both local and systemic adaptive immunity. Bidirectional
communication between the brain and GI tract (brain-gut axis) is influenced by GI
bacteria with importance for cognitive function, behavior, pain modulation, and
stress responses.3
Prolonged treatment with broad-spectrum antibiotics can alter the normal
microbiome, decreasing its protective activity, and lead to an overgrowth of
pathogenic microorganisms. In the intestine, overgrowth of the yeast Candida
albicans or the bacteria Clostridium difficile (a cause of pseudomembranous colitis,
an infection of the colon) may occur. The bacterium Lactobacillus is a major
constituent of the normal gastrointestinal and vaginal microbiome in healthy
women.4 This microorganism produces a variety of chemicals (e.g., hydrogen
peroxide, lactic acid, bacteriocins) that help prevent infections of the vagina and
urinary tract by other bacteria and yeast. Prolonged antibiotic treatment can
diminish colonization with Lactobacillus and increase the risk for urologic or
vaginal infections, such as vaginosis.
The mutualistic relationship with the microbiome is maintained through the
physical integrity of the skin and mucosal epithelium and other mechanisms that
protect the microbiome from the immune and inflammatory systems. Some
members of the normal bacterial microbiome are opportunistic; opportunistic
microorganisms can cause disease if the individual's defenses are compromised.
These microorganisms are normally controlled by the innate and adaptive immune
systems and contribute to our defenses. For example, Pseudomonas aeruginosa is a
member of the normal microbiome of the skin and produces a toxin that protects
against infections with staphylococcal and other bacteria. However, severe burns
compromise the integrity of the skin and may lead to life-threatening systemic
infections with Pseudomonas.
Quick Check 6-1
1. How do physical and mechanical barriers contribute to defense mechanisms?
2. What are antimicrobial peptides?
3. What two types of defensins contribute to the biochemical barrier?
4. What is the normal bacterial flora? What is its role in defense?
5. What are opportunistic microorganisms?
Second Line of Defense: Inflammation
Whereas the physical and chemical barriers of the innate immune system are
relatively static, inflammation is programmed to respond to cellular or tissue
damage, whether the damaged tissue is septic or sterile. The response is a rapid
initiation of an interactive system of humoral (soluble in the blood) and cellular
systems designed to limit the extent of tissue damage, destroy contaminating
infectious microorganisms, initiate the adaptive immune response, and begin the
healing process.
The inflammatory response (1) occurs in tissues with a blood supply
(vascularized); (2) is activated rapidly (within seconds) after damage occurs; (3)
depends on the activity of both cellular and chemical components; and (4) is
nonspecific, meaning that it takes place in approximately the same way regardless of
the type of stimulus or whether exposure to the same stimulus has occurred in the
past.
Inflammation will be activated by virtually any injury to vascularized tissues,
including infection or tissue necrosis (e.g., ischemia, trauma, physical or chemical
injury, foreign bodies, immune reactions). The classic or cardinal signs of acute
inflammation were described in the first century by a Roman named Celsus and
included rubor (redness), calor (heat), tumor (swelling), and dolor (pain). A fifth
sign, functio laesa (loss of function), was added later. Microscopic inflammatory
changes occur within seconds in the microcirculation (arterioles, capillaries, and
venules) near the site of an injury and include the following processes (Figure 6-2):
1. Vasodilation (increased size of the blood vessels), which causes slower blood
velocity and increases blood flow to the injured site
2. Increased vascular permeability (the blood vessels become porous from
contraction of endothelial cells) and leakage of fluid out of the vessel (exudation),
causing swelling (edema) at the site of injury; as plasma moves outward, blood in
the microcirculation becomes more viscous and flows more slowly, and the
increased blood flow and increasing concentration of red cells at the site of
inflammation cause locally increased redness (erythema) and warmth
3. White blood cell adherence to the inner walls of vessels and their migration
through enlarged junctions between the endothelial cells lining the vessels into the
surrounding tissue
FIGURE 6-2 The Major Local Changes in the Process of Inflammation. Compared with the
normal circulation, inflammation is characterized by (1) dilation of the blood vessels and
increased blood flow, leading to erythema and warmth; (2) increased vascular permeability with
leakage of plasma from the vessels, leading to edema; and (3) movement of leukocytes from
the vessels into the site of injury. (From Kumar V et al: Robbins and Cotran pathological basis of disease, ed 8, Philadelphia, 2009,
Saunders.)
Each of the characteristic changes associated with inflammation is the direct
result of the activation and interactions of a host of chemicals and cellular
components found in the blood and tissues. The vascular changes deliver leukocytes
(particularly neutrophils), plasma proteins, and other biochemical mediators to the
site of injury, where they act in concert. Some of these chemical mediators activate
pain fibers. The tissue injury, pain, and swelling contribute to loss of function.
Figure 6-3 summarizes the process of inflammation. The lymphatic vessels drain
the extravascular fluid to the lymph nodes and may, themselves, become secondarily
inflamed; lymphangitis of the lymph vessels and lymphadenitis of the nodes, which
become hyperplastic, enlarged, and frequently painful.
FIGURE 6-3 Acute Inflammatory Response. Inflammation is usually initiated by cellular injury
and may be complicated by infection. Mast cell degranulation, the activation of three plasma
systems, and the release of subcellular components from the damaged cells occur as a
consequence. These systems are interdependent, so that induction of one (e.g., mast cell
degranulation) can result in the induction of the other two. The result is the development of the
characteristic microscopic and clinical hallmarks of inflammation. The figure numbers refer to
additional figures in which more detailed information may be found on that portion of the
response.
There are several benefits of inflammation, including the following:
1. Prevents infection and further damage by invading microorganisms. The
inflammatory exudate dilutes toxins produced by bacteria and released from dying
cells. The activation of plasma protein systems (e.g., complement and clotting
systems) helps contain and destroy bacteria. The influx of phagocytes (e.g.,
neutrophils, macrophages) destroys cellular debris and microorganisms.
2. Limits and controls the inflammatory process. The influx of plasma protein
systems (e.g., clotting system), plasma enzymes, and cells (e.g., eosinophils)
prevents the inflammatory response from spreading to areas of healthy tissue.
3. Interacts with components of the adaptive immune system to elicit a more specific
response to contaminating pathogen(s) through the influx of macrophages and
lymphocytes that destroy pathogens.
4. Prepares the area of injury for healing and repair through removal of bacterial
products, dead cells, and other products of inflammation (e.g., by way of channels
through the epithelium or drainage by lymphatic vessels).
Fluid and debris that accumulate at an inflamed site are drained by lymphatic
vessels. This process also facilitates the development of acquired immunity because
microbial antigens in lymphatic fluid pass through the lymph nodes, where they
encounter lymphocytes.
Quick Check 6-2
1. Why are innate immunity and inflammation described as “nonspecific”?
2. How are the five classic superficial symptoms of inflammation related to the
process of inflammation?
3. Describe the basic steps in acute inflammation.
4. What are the benefits of inflammation?
Plasma Protein Systems and Inflammation
Three key plasma protein systems are essential to an effective inflammatory
response (Figure 6-4). These are the complement system, the clotting system, and
the kinin system. Although each system has a unique role in inflammation, they have
many similarities. Each system consists of multiple proteins found in the blood,
usually in inactive forms; several are enzymes that circulate as proenzymes. Each
system contains a few proteins that can be activated early in inflammation.
Activation of the first components results in sequential activation of other
components of the system, leading to a biologic function that helps protect the
individual. This sequential activation is referred to as a cascade. Thus, we
occasionally refer to the complement cascade, the clotting cascade, or the kinin
cascade. In some cases, activation of a particular protein in the system may require
that it be enzymatically cut into two pieces of different size. Usually the larger
fragment continues the cascade by activating the next component, and the smaller
fragment frequently has potent proinflammatory activities.
FIGURE 6-4 Plasma Protein Systems in Inflammation: Complement, Clotting, and Kinin
Systems. Each plasma protein system consists of a family of proteins that are activated in
sequence to create potent biologic effects. The complement system can be activated by three
mechanisms, each of which results in proteolytic activation of C3. The fragments of C3
activation, C3a and C3b, are major components of inflammation. C3a is a potent anaphylatoxin,
which induces degranulation of mast cells. C3b can bind to the surface or cells, such as
bacteria, and either serve as an opsonin for phagocytosis or proteolytically activate the next
component of the complement cascade, C5. The smaller fragment of C5 activation is C5a, a
powerful anaphylatoxin, and is also chemotactic for neutrophils, attracting them to the site of
inflammation. The larger fragment, C5b, activates the components of the membrane attack
complex (C5-C9), which damage the bacterial membrane and kill the bacteria. The clotting
system can be activated by the tissue factor (extrinsic) pathway and the contact activation
(intrinsic) pathway. All routes of clotting initiation lead to activation of factor X and thrombin.
Thrombin is an enzyme that proteolytically activates fibrinogen to form fibrin and small
fibrinopeptides (FPs). Fibrin polymerizes to form a clot, and the FPs are highly active as
chemotactic factors and cause increased vascular permeability. The XIIa produced by the
clotting system can also be activated by kallikrein of the kinin system (red arrow). Prekallikrein
is enzymatically converted to kininogen, which activates bradykinin. Bradykinin functions
similar to histamine and increases vascular permeability. Bradykinin can also stimulate nerve
endings to cause pain. FP, Fibrinopeptide; TF, tissue factor.
Complement System
The complement system consists of a large number of proteins (sometimes called
complement factors) that together constitute about 10% of the total circulating
serum protein. Activation of the complement system produces several factors that
can destroy pathogens directly or can activate or increase the activity of many other
components of the inflammatory and adaptive immune response. Factors produced
during activation of the complement system are among the body's most potent
defenders, particularly against bacterial infection.
The most important function of the complement cascade is activation of C3 and
C5, which results in a variety of molecules that are (1) opsonins, (2) chemotactic
factors, or (3) anaphylatoxins.5 Opsonins coat the surface of bacteria and increase
their susceptibility to being phagocytized (eaten) and killed by inflammatory cells,
such as neutrophils and macrophages. Chemotactic factors diffuse from a site of
inflammation and attract phagocytic cells to that site. Anaphylatoxins induce rapid
degranulation of mast cells (i.e., release of histamine that induces vasodilation and
increased capillary permeability), a major cellular component of inflammation. The
most potent complement products are C3b (opsonin), C3a (anaphylatoxin), and C5a
(anaphylatoxin, chemotactic factor). Activation of terminal complement components
C5b through C9 (membrane attack complex, or MAC) results in a complex that
creates pores in the outer membranes of cells or bacteria. The pores disrupt the
cell's membrane and permit water to enter, causing the death of the cell.
Three major pathways control the activation of complement (see Figure 6-4). The
classical pathway is primarily activated by antibodies, which are proteins of the
acquired immune system. Antibodies must first bind to their targets, called antigens,
which can be proteins or carbohydrates from bacteria or other infectious agents.
Antibodies activate the first component of complement, C1, which leads to
activation of other complement components, leading to activation of C3 and C5.
Thus, antibodies of the acquired immune response can use the complement system
to kill bacteria and activate inflammation.
The alternative pathway is activated by several substances found on the surface
of infectious organisms (e.g., lipopolysaccharides [endotoxin] on the bacterial
surface or yeast cell wall carbohydrates [zymosan]). This pathway uses unique
proteins (factor B, factor D, and properdin) to form a complex that activates C3. C3
activation leads to C5 activation and convergence with the classical pathway. Thus,
the complement system can be directly activated by certain infectious organisms
without antibody being present.
The lectin pathway is similar to the classical pathway but is independent of
antibody. It is activated by several plasma proteins, particularly mannose-binding
lectin (MBL). MBL binds to bacterial polysaccharides containing the carbohydrate
mannose and activates complement through two proteins that are similar to C1—
MASP-1 (mannose-binding lectin-associated serine protease) and MASP-2.6 Thus,
infectious agents that do not activate the alternative pathway may be susceptible to
complement through the lectin pathway.
In summary, the complement cascade can be activated by at least three different
means, and its products have four functions: (1) opsonization (C3b), (2)
anaphylatoxic activity resulting in mast cell degranulation (C3a, C5a), (3) leukocyte
chemotaxis (C5a), and (4) cell lysis (C5b-C9; membrane attack complex [MAC]).
Clotting System
The clotting (coagulation) system is a group of plasma proteins that, when
activated sequentially, form a blood clot. A blood clot is a meshwork of protein
(fibrin) strands that contains platelets (the primary cellular initiator of clotting) and
traps other cells, such as erythrocytes, phagocytes, and microorganisms. Clots (1)
plug damaged vessels and stop bleeding, (2) trap microorganisms and prevent their
spread to adjacent tissues, and (3) provide a framework for future repair and
healing. Specific details and illustrations of the clotting system are presented in
Chapter 20 (also see Figure 20-18) and only the relationship between clotting and
inflammation is presented here.
The clotting system can be activated by many substances that are released during
tissue injury and infection, including collagen, proteinases, kallikrein, and plasmin,
as well as by bacterial products such as endotoxins. Like the complement cascade,
the coagulation cascade can be activated through different pathways that converge
and result in the formation of a clot (see Figure 6-4). The tissue factor (extrinsic)
pathway is activated by tissue factor (TF) (also called tissue thromboplastin) that
is released by damaged endothelial cells in blood vessels and reacts with activated
factor VII (VIIa). The contact activation (intrinsic) pathway is activated when the
vessel wall is damaged and Hageman factor (factor XII) in plasma contacts
negatively-charged subendothelial substances. The pathways converge at factor X.
Activation of factor X begins a common pathway leading to activation of fibrin that
polymerizes to form a fibrin clot.
As with the complement system, activation of the clotting system produces
protein fragments known as fibrinopeptides (FPs) A and B that enhance the
inflammatory response. Fibrinopeptides are released from fibrinogen when fibrin is
produced. Both fibrinopeptides (especially fibrinopeptide B) are chemotactic for
neutrophils and increase vascular permeability by enhancing the effects of
bradykinin (formed from the kinin system) on endothelial cells.
Kinin System
The third plasma protein system, the kinin system (see Figure 6-4), interacts closely
with the coagulation system. Both the clotting and kinin systems can be initiated
through activation of Hageman factor (factor XII) to factor XIIa. Another name
for factor XIIa is prekallikrein activator because it enzymatically activates the first
component of the kinin system, prekallikrein. The final product of the kinin system
is a small-molecular-weight molecule, bradykinin, which is produced from a
larger precursor molecule, kininogen. Bradykinin causes dilation of blood vessels,
acts with prostaglandins to induce pain, causes smooth muscle cell contraction, and
increases vascular permeability.
Control and Interaction of Plasma Protein Systems
The three plasma protein systems are highly interactive so that activation of one
results in production of a large number of very potent, biologically active
substances that further activate the other systems. Very tight regulation of these
processes is essential for the following two reasons.
1. The inflammatory process is critical for an individual's survival; thus efficient
activation must be guaranteed regardless of the cause of tissue injury. Interaction
among the plasma systems may result in activation of the entire inflammatory
response regardless of which system is activated initially.
2. The biochemical mediators generated during these processes are potent and
potentially detrimental to the individual, and their actions must be strictly confined
to injured or infected tissues.
Therefore, multiple mechanisms are available to either activate or inactivate
(regulate) these plasma protein systems. For instance, the plasma that enters the
tissues during inflammation (edema) contains enzymes that destroy mediators of
inflammation. Carboxypeptidase inactivates the anaphylatoxic activities of C3a and
C5a, and kininases degrade kinins. Histaminase degrades histamine and kallikrein
and down-regulates the inflammatory response.
The formation of clots also activates a fibrinolytic system that is designed to
limit the size of the clot and remove the clot after bleeding has ceased. Thrombin of
the clotting system activates plasminogen in the blood to form the enzyme plasmin.
The primary activity of plasmin is to degrade fibrin polymers in clots. However,
plasmin can also activate the complement cascade through components C1, C3, and
C5 and the kinin cascade by activating factor XII and producing prekallikrein
activator.
Another example of a common regulator is C1 esterase inhibitor (C1 inh). C1
inh inhibits complement activation through C1 (classical pathway), MASP-2 (lectin
pathway), and C3b (alternative pathway). It is also a major inhibitor of the clotting
and kinin pathway components (e.g., kallikrein, factor XIIa). A genetic defect in C1
inh (C1 inh deficiency) results in hereditary angioedema, which is a self-limiting
edema of cutaneous and mucosal layers resulting from stress, illness, or relative
minor or unapparent trauma. The disease is characterized by hyperactivation of all
three plasma protein systems, although excessive production of bradykinin appears
to be the principal cause of increased vascular permeability.
Many cells are protected from inadvertent complement system damage by factors
linked to the external surface of the plasma membrane. Two examples are decay
accelerating factor (DAF) and CD59; DAF prevents activation of C3 and CD59
inhibits the membrane attack complex.
Quick Check 6-3
1. What are the three most important products of the complement system?
2. How is the coagulation cascade activated? How is it related to the plasma kinin
cascade?
3. What factors control the plasma protein systems of inflammation?
Cellular Components of Inflammation
Inflammation is a process in vascular tissue; thus the cellular components can be
found in the blood or in tissue surrounding the blood vessels. The blood vessels are
lined with endothelial cells, which under normal conditions actively maintain blood
flow. During inflammation the vascular endothelium becomes a principal
coordinator of blood clotting and the passage of cells and fluid into the tissue. The
tissues close to the vessels contain mast cells, which are probably the most
important activators of inflammation, and dendritic cells, which connect the innate
and acquired immune responses. The blood contains a complex mixture of cells
(Figure 6-5 and see Chapter 20). Blood cells are divided into erythrocytes (red
blood cells), platelets, and leukocytes (white blood cells). Erythrocytes carry
oxygen to the tissues and platelets are small cell fragments involved in blood
clotting. Leukocytes are subdivided into granulocytes (containing many enzyme-
filled cytoplasmic granules), monocytes, and lymphocytes. Granulocytes are the
most common leukocytes and are classified by the type of stains needed to visualize
enzyme-containing granules in their cytoplasm (basophils, eosinophils, and
neutrophils). Monocytes are precursors of macrophages that are found in the tissue.
Various forms of lymphocytes participate in the innate immune response (e.g.,
natural killer [NK] cells) and the acquired immune response (B and T cells).
FIGURE 6-5 Cellular Components of the Blood. Cells in the blood can be classified as red blood
cells (erythrocytes), cellular fragments (platelets), or white blood cells (leukocytes). Leukocytes
consist of lymphocytes, monocytes, and granulocytes (neutrophils, eosinophils,
basophils). (Erythrocyte plate from Goldman L, Schafer AI, editors: Goldman's Cecil medicine, ed 24, Philadelphia, 2012, Saunders; rest of
plates from McPherson RA, editor: Henry's clinical diagnosis and management by laboratory methods, ed 22, Philadelphia, 2012, Saunders.)
Cells of both innate and acquired immune systems respond to molecules
produced at a site of cellular damage and are recruited to that site to augment the
protective response. These molecules originate from destroyed or damaged cells,
contaminating microbes, activation of the plasma protein systems, or secretions by
other cells of the innate or acquired immune systems. Each cell has a set of cell
surface receptors that specifically bind these molecules, resulting in activation of
intracellular signaling pathways and activation of the cell itself. Activation may
result in the cell gaining a function critical to the inflammatory response or the
induction of the release of additional cellular products that increase inflammation,
or both. Most of these inflammatory cells and protein systems, along with the
substances they produce, act at the site of tissue injury to confine the extent of
damage; kill microorganisms; remove the cellular debris; and activate healing,
tissue regeneration (a process known as resolution), or repair.
Cellular Receptors
As will be discussed in Chapter 7, B and T lymphocytes of the adaptive immune
system have evolved surface receptors (i.e., the T-cell receptor, or TCR, and the B-
cell receptor, or BCR) that bind a large spectrum of antigens. Cells involved in
innate resistance have evolved a different set of receptors that recognize a much
more limited array of specific molecules (ligands). These are referred to as pattern
recognition receptors (PRRs). PRRs recognize two types of molecular patterns:
molecules that are expressed by infectious agents, either found on their surface or
released as soluble molecules (pathogen-associated molecular patterns, or
PAMPs); or products of cellular damage (damage-associated molecular patterns,
or DAMPs). Thus cells of the innate immune system can respond to both sterile
(through DAMPs) and septic (through PAMPs and DAMPs) tissue damage. It is
estimated that at least 100 different PRRs are expressed that recognize more than
1000 different molecules.
PRRs are generally expressed on cells in tissues near the body's surface (i.e., skin,
respiratory tract, gastrointestinal tract, genitourinary tract) where they monitor the
environment for products of cellular damage and potentially infectious
microorganisms. Classes of cellular PRRs primarily differ in the specificity of
ligands they bind. PRRs can be found as cell surface receptors that bind extracellular
ligands, in endosomes in contact with ingested microbes and other materials, in the
cytosol where they bind intracellular materials resulting from cellular damage, or
secreted into the extracellular environment. An example of a secreted PRR is
mannose-binding lectin of the lectin pathway of complement activation (see p. 139).
Toll-like receptors (TLRs) primarily recognize a large variety of PAMPs
located on the microorganism's cell wall or surface (e.g., bacterial
lipopolysaccharide [LPS], peptidoglycans, lipoproteins, yeast zymosan, viral coat
proteins), other surface structures (e.g., bacterial flagellin), or microbial nucleic
acid (e.g., bacterial DNA, viral double-stranded RNA).7 Ten different TLRs have
been described in humans (Table 6-3). They are expressed on the surface of many
cells that have direct and early contact with potential pathogenic microorganisms,
including mucosal epithelial cells, mast cells, neutrophils, macrophages, dendritic
cells, and some subpopulations of lymphocytes. TLRs are linked to pathways that
produce two groups of transcription factors: NF-κB, which controls synthesis and
release of cytokines; and interferon regulatory factors (IRFs), which control the
production of anti-viral type I interferons.8
TABLE 6-3
Cellular Source and Microbial Target for Each Toll-like Receptor (TLR)
Receptor Cellular Expression Pattern PAMP Recognition
TLR1 Cell surface (ubiquitous): neutrophils, monocytes/macrophages,
dendritic cells, T cells, B cells, NK cells
Fungal, bacterial, viral; forms heterodimer with TLR2 (see TLR2
recognition)
TLR2 Cell surface: neutrophils, monocytes/macrophages, dendritic cells Fungal (yeast zymosan), bacterial (gram-positive bacterial
peptidoglycan, lipoproteins), viral (lipoproteins)
TLR3 Intracellular: monocytes/macrophages, dendritic cells, T cells, NK cells,
epithelial cells
Double-stranded RNA produced by many viruses
TLR4 Cell surface: granulocytes, monocytes/macrophages, dendritic cells, T
cells, B cells, epithelial cells
Bacterial (primarily gram-negative bacterial LPS, lipoteichoic acids),
viral (RSV F protein, hepatitis C)
TLR5 Cell surface: granulocytes, monocytes/macrophages, dendritic cells,
NK cells, epithelial cells
Bacterial (flagellin); forms heterodimer with TLR4
TLR6 Cell surface: monocytes/macrophages, dendritic cells, B cells, NK cells Fungal, bacterial, viral; forms heterodimer with TLR2 (see TLR2
recognition)
TLR7 Intracellular: monocytes/macrophages, dendritic cells, B cells Natural ligand uncertain; may bind viral single-strand RNA
TLR8 Cell surface: monocytes/macrophages, dendritic cells, NK cells Natural ligand uncertain; may bind fungal PAMPs or viral single-
stranded RNA
TLR9 Intracellular: monocytes/macrophages, dendritic cells, B cells Bacterial (unmethylated DNA [CpG dinucleotides])
TLR10 Cell surface: monocytes/macrophages, dendritic cells, B cells Natural ligand uncertain; may form heterodimers with TLR2
TLR11 TLR11 gene does not code a full-length protein in humans No known immune response
Complement receptors are found on many cells of the innate and acquired
immune responses (e.g., granulocytes, monocytes/macrophages, lymphocytes, mast
cells, erythrocytes, platelets), as well as some epithelial cells. They recognize
several fragments produced through activation of the complement system,
particularly C3a, C5a, and C3b.
Scavenger receptors are primarily expressed on macrophages and facilitate
recognition and phagocytosis of bacterial pathogens, as well as damaged cells and
altered soluble lipoproteins associated with vascular damage (e.g., high-density
lipoprotein [HDL], acetylated low-density lipoprotein [LDL], oxidized LDL).9 More
than eight receptors have been identified. Some scavenger receptors (e.g., SR-
PSOX) recognize the cell membrane phospholipid phosphatidylserine (PS). PS is
normally sequestered on the cytoplasmic surface of the cell membrane, but is
externalized under a very limited variety of conditions, including erythrocyte
senescence and cellular apoptosis. Thus macrophages, through this receptor, can
identify and remove old red blood cells and cells undergoing apoptosis.
NOD-like receptors (NLRs) are cytoplasmic receptors that recognize products
of microbes and damaged cells. At least 22 NLRs have been identified in humans.
NOD-1 and NOD-2 are cytoplasmic and recognize fragments of peptidoglycans
from intracellular bacteria and initiate production of proinflammatory mediators,
such as tumor necrosis factor (TNF) and interleukin-6 (IL-6).10 Other NLRs
associate with intracellular multiprotein complexes called inflammasomes.
Inflammasomes primarily bind cellular stress-related molecules, a type of DAMP,
and control the production of the inflammatory cytokines interleukin-1β (IL-1β) and
IL-18.11
Cellular Products
To elicit an effective inflammatory (or adaptive immune) response, intercellular
communication and cooperation are necessary. Cytokines constitute a large family
of small-molecular-weight soluble intercellular-signaling molecules that are
secreted, bind to specific cell membrane receptors, and regulate innate or adaptive
immunity (Figure 6-6). Cytokines may be either proinflammatory or anti-
inflammatory in nature, depending on whether they tend to induce or inhibit the
inflammatory response. These molecules usually diffuse over short distances, but
some effects occur over long distances, such as the systemic induction of fever by
some cytokines (i.e., endogenous pyrogens) that are produced at an inflammatory
site. Binding of cytokines to a target cell often induces synthesis of additional
cellular products. For example, binding of the cytokine TNF-α to a cell may result
in synthesis and release of IL-1.
FIGURE 6-6 Principal Mediators of Inflammatory Processes. C3b, Large fragment produced
from complement component C3; C5a, small fragment produced from complement component
C5; ECF-A, eosinophil chemotactic factor of anaphylaxis; FGF, fibroblast growth factor; IFN,
interferon; IgG, immunoglobulin G (predominant class of antibody in the blood); IL, interleukin;
MCF, monocyte chemotactic factor; NCF, neutrophil chemotactic factor; PAF, platelet-activating
factor; TGF, T-cell growth factor; TNF, tumor necrosis factor; VEGF, vascular endothelial growth
factor.
A large number of cytokines have been described and are classified into several
families.12 The terms lymphokines and monokines refer respectively to cytokines
secreted from lymphocytes or monocytes, although cytokines are secreted by many
different types of cells. Chemokines are members of a special family of cytokines
that are chemotactic and primarily attract leukocytes to sites of inflammation.13
Chemokines are synthesized by many cell types, including macrophages,
fibroblasts, and endothelial cells, in response to proinflammatory cytokines, such as
TNF-α. To date, more than 50 different human chemokines have been described.
Examples include those that primarily attract macrophages (e.g.,
monocyte/macrophage chemotactic proteins [MCP-1, MCP-2, and MCP-3]),
macrophage inflammatory proteins ([MIP-α and MIP-1β]), or neutrophils (e.g.,
interleukin-8 [IL-8]).
Interleukins (ILs) are produced predominantly by macrophages and lymphocytes
in response to stimulation of PRRs or by other cytokines.14 More than 30
interleukins have been identified. Their effects include the following:
1. Alteration of adhesion molecule expression on many types of cells
2. Attraction of leukocytes to a site of inflammation (chemotaxis)
3. Induction of proliferation and maturation of leukocytes in the bone marrow
4. General enhancement or suppression of inflammation
5. Development of the acquired immune response
Two major proinflammatory ILs are interleukin-1 and interleukin-6, which
cooperate closely with another cytokine, tumor necrosis factor-alpha. Interleukin-1
(IL-1) is produced in two forms, IL-1α and IL-1β, mainly by macrophages.15 IL-1
activates monocytes, other macrophages, and lymphocytes, thereby enhancing both
innate and acquired immunity, and acts as a growth factor for many cells. It has
several effects on neutrophils, including induction of proliferation (resulting in an
increase in the number of circulating neutrophils), attraction to an inflammatory site
(chemotaxis), and increased cellular respiration and lysosomal enzyme activity
(both effects resulting in increased cellular killing of bacteria). IL-1 is an
endogenous pyrogen (i.e., fever-causing cytokine) that reacts with receptors on cells
of the hypothalamus and affects the body's thermostat, resulting in fever.
Interleukin-6 (IL-6) is produced by macrophages, lymphocytes, fibroblasts, and
other cells. IL-6 directly induces hepatocytes (liver cells) to produce many of the
proteins needed in inflammation (acute-phase reactants, discussed later in this
chapter). IL-6 also stimulates growth and differentiation of blood cells in the bone
marrow and the growth of fibroblasts (required for wound healing).
Although not classified as an interleukin, tumor necrosis factor-alpha (TNF-α)
is secreted by macrophages and other cells (e.g., mast cells) in response to
stimulation of TLRs. TNF-α induces a multitude of proinflammatory effects,
particularly on the vascular endothelium and macrophages. When secreted in large
amounts, TNF-α has systemic effects that include the following:
1. Inducing fever by acting as an endogenous pyrogen
2. Causing increased synthesis of inflammation-related serum proteins by the liver
3. Causing muscle wasting (cachexia) and intravascular thrombosis in cases of
severe infection and cancer.
Very high levels of TNF-α can be lethal and are probably responsible for
fatalities from shock caused by gram-negative bacterial infections.
Some cytokines are anti-inflammatory and diminish the inflammatory response.
The most important are interleukin-10 and transforming growth factor-beta.
Interleukin-10 (IL-10) is primarily produced by lymphocytes and suppresses the
growth of other lymphocytes and the production of proinflammatory cytokines by
macrophages, leading to down-regulation of both inflammatory and acquired
immune responses. Transforming growth factors, including transforming
growth factor-beta (TGF-β), are produced by many cells in response to
inflammation and induce cell division and differentiation of other cell types, such as
immature blood cells.
Interferons (IFNs) are members of a family of cytokines that protect against
viral infections and modulate the inflammatory response. (Mechanisms of viral
infection are described in Chapter 8.) Type I interferons (primarily IFN-α, IFN-β)
are produced and released by virally infected cells in response to viral double-
stranded RNA and other viral PAMPs. These IFNs do not kill viruses directly but
instead are secreted and induce antiviral proteins and protection in neighboring
healthy cells. Type II interferon (IFN-γ) is produced primarily by lymphocytes; it
activates macrophages, resulting in increased capacity to kill infectious agents
(including viruses and bacteria), and enhances the development of acquired immune
responses against viruses.
Mast Cells and Basophils
The mast cell is probably the most important cellular activator of the inflammatory
response. Mast cells are filled with granules and located in the loose connective
tissues close to blood vessels near the body's outer surfaces (i.e., in the skin and
lining the gastrointestinal and respiratory tracts). Basophils are found in the blood
and probably function in the same way as tissue mast cells.16 A great number of
stimuli activate mast cells to release potent soluble inducers of inflammation. These
are released by (1) degranulation (the release of the contents of mast cell granules)
and (2) synthesis (the new production and release of mediators in response to a
stimulus) (Figure 6-7).
FIGURE 6-7 Mast Cell and Mast Cell Degranulation and Synthesis of Biologic Mediators During
Inflammation. A, Colorized photomicrograph of mast cell; dense red granules contain histamine
and other biologically active substances. Among these are histamine, which is a major initiator
of vascular changes, and a variety of chemotactic factors. B, Mast cell degranulation (left) and
synthesis (right). Histamine and other biologically active substances are released immediately
after stimulation of mast cells. (A from Roitt IM et al: Immunology, ed 3, St Louis, 1993, Mosby.)
Degranulation.
In response to a stimulus, biochemical mediators in the mast cell granules,
including histamine, chemotactic factors, and cytokines (e.g., tumor necrosis factor-
alpha [TNF-α], IL-4), are released within seconds and exert their effects
immediately. Histamine is a small-molecular-weight molecule with potent effects
on many other cells, particularly those that control the circulation. Histamine, along
with serotonin (found in many cells, but not human mast cells), is called a
vasoactive amine. These molecules cause temporary, rapid constriction of smooth
muscle and dilation of the postcapillary venules, which results in increased blood
flow into the microcirculation. Histamine also causes increased vascular
permeability resulting from retraction of endothelial cells lining the capillaries and
increased adherence of leukocytes to the endothelium. Histamine affects cells by
binding to histamine H1 and H2 receptors on the target cell surface (Figure 6-8).
Antihistamines are drugs that block the binding of histamine to its receptors,
resulting in decreased inflammation.
FIGURE 6-8 Effects of Histamine Through H1 and H2 Receptors. The effects depend on (1) the
density and affinity of H1 or H2 receptors on the target cell and (2) the identity of the target cell.
ATP, Adenosine triphosphate; cAMP, cyclic adenosine monophosphate; cGMP, cyclic guanosine
monophosphate; GTP, guanosine triphosphate.
Binding of histamine to the H1 receptor is essentially proinflammatory; that is, it
promotes inflammation. On the other hand, binding to the H2 receptor is generally
anti-inflammatory because it results in suppression of leukocyte function. The H1
receptor is present on smooth muscle cells, especially those of the bronchi, and
causes bronchial smooth muscle to contract (bronchoconstriction) when stimulated.
Both types of receptors are distributed among many different cells and are often
present on the same cells and may act in an antagonistic fashion. For instance,
stimulation of H1 receptors on neutrophils results in augmentation of neutrophil
chemotaxis, whereas H2 receptor stimulation results in its inhibition. The H2
receptor is especially abundant on parietal cells of the stomach mucosa and induces
the secretion of gastric acid as part of the normal physiology of the stomach. The
role of histamine receptors and hypersensitivity is discussed in Chapter 8.
Mast cell granules also contain chemotactic factors, two of which are neutrophil
chemotactic factor (NCF) and eosinophil chemotactic factor of anaphylaxis
(ECF-A). Chemotaxis is directional movement of cells along a chemical gradient
formed by a chemotactic factor. Neutrophils are the predominant cell needed to kill
bacteria in the early stages of inflammation. Eosinophils help regulate the
inflammatory response. Both cells are discussed in more detail later in this chapter.
Synthesis of mediators.
Activated mast cells initiate synthesis of other mediators of inflammation. These
include leukotrienes, prostaglandins, and platelet-activating factor, which are
produced from lipids (arachidonic acid) in the plasma membrane. Leukotrienes
(slow-reacting substances of anaphylaxis [SRS-A]) are sulfur-containing lipids
produced by lipoxygenase that initiate histamine-like effects: smooth muscle
contraction and increased vascular permeability. Leukotrienes appear to be
important in the later stages of the inflammatory response because they stimulate
slower and more prolonged inflammatory responses than does histamine.
Prostaglandins cause increased vascular permeability, neutrophil chemotaxis,
and pain by direct effects on nerves. They are long-chain, unsaturated fatty acids
produced by the action of the enzyme cyclooxygenase (COX) on arachidonic acid;
prostaglandins are classified into groups (E, D, A, F, and B) according to their
structure with numeral subscripts designating the number of double bonds.
Prostaglandins E1 and E2 cause increased vascular permeability and smooth muscle
contraction. COX exists in two different forms: COX-1 is found in most tissues and
COX-2 is associated with inflammation. Aspirin and other nonsteroidal anti-
inflammatory drugs inhibit both COX-1 and COX-2, but inhibition of COX-1 causes
complications, such as gastrointestinal toxicity. Selective COX-2 inhibitors are now
available.
Platelet-activating factor (PAF) is produced by removal of a fatty acid from the
plasma membrane phospholipids by phospholipase A2. Although mast cells are a
major source of PAF, this molecule also can be produced by neutrophils,
monocytes, endothelial cells, and platelets. The biologic activity of PAF is virtually
identical to that of leukotrienes, namely, causing endothelial cell retraction to
increase vascular permeability, leukocyte adhesion to endothelial cells, and platelet
activation.
Endothelium
The lining of blood vessels consists of a layer of endothelial cells that adhere to an
underlying matrix of connective tissue that contains a variety of proteins, including
collagen, fibronectin, and laminins. Endothelial cells regulate circulating
components of the inflammatory system and maintain normal blood flow by
preventing spontaneous activation of platelets and members of the clotting system.
Nitric oxide (NO) produced from arginine and prostacyclin (PGI2) from
arachidonic acid maintain blood flow and pressure and inhibit platelet activation.
PGI2 and NO are synergistic. NO is released continually to relax vascular smooth
muscle and suppress the effects of low levels of cytokines, thus maintaining
vascular tone. PGI2 production varies a great deal and is increased when additional
regulation is needed.
Damage to the endothelial cell lining of the vessel exposes the subendothelial
connective tissue matrix, which is prothrombogenic and initiates platelet activation
and formation of clots (the contact activation [intrinsic] clotting pathway).
Proinflammatory mediators (e.g., histamine, prostacyclin, and many others) affect
the endothelium, resulting in adherence of leukocytes to the vessel surface, invasion
of leukocytes into the tissue, and efflux of plasma from the vessel.
Platelets
Platelets are anucleate cytoplasmic fragments formed from megakaryocytes. They
circulate in the bloodstream until vascular injury occurs resulting in platelet
activation by many products of tissue destruction and inflammation, including
collagen, thrombin, and platelet-activating factor. Activated platelets (1) interact
with components of the coagulation cascade to stop bleeding; (2) degranulate,
releasing biochemical mediators such as serotonin, which has vascular effects
similar to those of histamine; and (3) synthesize thromboxane A2 (TXA2) from
prostaglandin H2. TXA2 is a potent vasoconstrictor and inducer of platelet
aggregation. Prolonged use of low-dose aspirin preferentially suppresses
production of TXA2 without interfering with the production of anti-inflammatory
PGI2 by the endothelium. Platelets also release growth factors that promote wound
healing. (Platelet function is described in detail in Chapter 20.)
Phagocytes
The primary role of most granulocytes (neutrophils, eosinophils, basophils) and
monocytes/macrophages is phagocytosis—the process by which a cell ingests and
disposes of damaged cells and foreign material, including microorganisms.
Neutrophils.
The neutrophil, or polymorphonuclear neutrophil (PMN), is a member of the
granulocytic series of white blood cells and is named for the characteristic staining
pattern of its granules as well as its multilobed nucleus. Neutrophils are the
predominant phagocytes in the early inflammatory site, arriving within 6 to 12
hours after the initial injury. Several inflammatory mediators (e.g., some bacterial
proteins, complement fragments C3a and C5a, and mast cell neutrophil chemotactic
factor) specifically and rapidly attract neutrophils from the circulation and activate
them.17
Because the neutrophil is a mature cell that is incapable of division and sensitive
to acidic environments, it is short lived at the inflammatory site and becomes a
component of the purulent exudate, or pus, which is removed from the body
through the epithelium or drained from the infected site via the lymphatic system.
(The lymphatic system is described in Chapter 23.) The primary roles of the
neutrophil are removal of debris and dead cells in sterile lesions, such as burns, and
destruction of bacteria in nonsterile lesions.
Eosinophils.
Another population of granulocytes is the eosinophil. Although eosinophils are
only mildly phagocytic, they have two specific functions: (1) serve as the body's
primary defense against parasites, and (2) help regulate vascular mediators released
from mast cells. The role of eosinophils in resistance to parasites occurs in
collaboration with specific antibodies produced by the acquired immune system
(discussed in Chapter 7).18
Regulation of mast cell–derived inflammatory mediators is critical to control
inflammation. The acute inflammatory response is needed only in a circumscribed
area and for a limited time. Therefore, control mechanisms are necessary to prevent
biochemical mediators from evoking more inflammation than necessary. Mast cell
eosinophil chemotactic factor–A (ECF-A) attracts eosinophils to the site of
inflammation. Eosinophil lysosomal granules contain enzymes that degrade
vasoactive molecules, thereby controlling the vascular effects of inflammation.
Histaminase degrades histamine, and arylsulfatase B degrades leukotrienes.
Basophils.
The basophil is the least prevalent granulocyte in the blood. It is very similar to
mast cells in the content of its granules and, in addition, is an important source of
the cytokine IL-4, which is a key regulator of the adaptive immune response.
Although often associated with allergies and asthma, its primary role is yet
unknown.
Monocytes and macrophages.
Monocytes are the largest normal blood cells (14 to 20 µm in diameter). Monocytes
are produced in the bone marrow, enter the circulation, and migrate to the
inflammatory site where they develop into macrophages. Monocytes also appear to
be the precursors of macrophages that are found in tissues (tissue macrophages)
including Kupffer cells in the liver, alveolar macrophages in the lungs, and
microglia in the brain. Macrophages are generally larger (20 to 40 µm) and are
more active as phagocytes than their monocytic precursors. Macrophages,
particularly those residing in the tissues, are often important cellular initiators of
the inflammatory response.
Monocyte-derived macrophages from the circulation may appear at the
inflammatory site as soon as 24 hours after the initial neutrophil infiltration, but
usually arrive 3 to 7 days later. Neutrophils and monocytes/macrophages differ
chiefly in the following ways:
1. Speed: Neutrophils arrive at the injury site first, whereas macrophages move
more sluggishly.
2. Active life span: Macrophages survive and divide in the acidic inflammatory site,
whereas neutrophils cannot.
3. Chemotactic factors: Neutrophils and macrophages are not attracted by the same
factors, such as macrophage chemotactic factor, which is released by neutrophils.
4. Enzymatic content of their lysosomes, or digestive vacuoles: Neutrophils have a
more active NADPH oxidase and produce more hydrogen peroxide; macrophage
phagolysosomes are more acidic, favoring the activity of acidic proteases and other
enzymes.
5. Role in the immune response: Macrophages, but not neutrophils, are involved in
activation of the adaptive immune system.
6. Role in wound repair: Macrophages are the primary cells that infiltrate tissue in
wounds, remove cells and cellular debris, promote angiogenesis, and produce
cytokines and growth factors that suppress further inflammation and initiate healing
by promoting epithelial cell division, activating fibroblasts, and promoting
synthesis of extracellular matrix and collagen.
The bactericidal activity of macrophages can increase markedly with the help of
inflammatory cytokines produced by cells of the acquired immune system (subsets
of T lymphocytes) or cells activated through Toll-like receptors (TLRs).
Macrophage activation results in two subpopulations of cells.19 M1 macrophages are
activated through TLRs by substances found in sites of inflammation and have
greater bacterial killing capacity. M2 macrophages are activated by lymphocyte-
produced cytokines and are primarily involved in healing and repair.20
Several bacteria are resistant to killing by granulocytes and can even survive
inside macrophages. Microorganisms, such as Mycobacterium tuberculosis
(tuberculosis), Mycobacterium leprae (leprosy), Salmonella typhi (typhoid fever),
Brucella abortus (brucellosis), and Listeria monocytogenes (listeriosis), can remain
dormant or multiply inside the phagolysosomes of macrophages.
Dendritic cells.
Dendritic cells provide one of the major links between the innate and acquired
immune responses. They are the primary phagocytic cells located in the peripheral
organs and skin, where molecules released from infectious agents are encountered,
recognized through PRRs, and internalized through phagocytosis. Dendritic cells
then migrate through the lymphatic vessels to lymphoid tissue, such as lymph nodes,
and interact with T lymphocytes to generate an acquired immune response.21
Through the production of a family of cytokines, they guide development of a
subset of T cells (helper cells) that coordinate the development of functional B and
T cells (discussed in Chapter 7).
Phagocytosis.
The two most important phagocytes are neutrophils and macrophages. Both cells
are circulating in the blood and must first leave the circulation and migrate to the
site of inflammation before initiating phagocytosis (Figure 6-9). Many products of
inflammation affect expression of surface molecules involved in cell-to-cell
adherence. Both leukocytes and endothelial cells begin expressing molecules
(selectins and integrins) that increase adhesion, or stickiness, causing the leukocytes
to adhere more avidly to the endothelial cells in the walls of the capillaries and
venules in a process called margination, or pavementing. Leukocyte-endothelial
interactions lead to diapedesis, or emigration of the cells through the inter-
endothelial junctions that have loosened in response to inflammatory mediators.22
FIGURE 6-9 Process of Phagocytosis. The process that results in phagocytosis is
characterized by three interrelated steps: adherence and diapedesis, tissue invasion by
chemotaxis, and phagocytosis. A, Adherence, margination, diapedesis, and chemotaxis. The
primary phagocyte in the blood is the neutrophil, which usually moves freely within the vessel
(1). At sites of inflammation, the neutrophil progressively develops increased adherence to the
endothelium, leading to accumulation along the vessel wall (margination or pavementing) (2). At
sites of endothelial cell retraction the neutrophil exits the blood by means of diapedesis (3).
Chemotaxis. In the tissues, the neutrophil detects chemotactic factor gradients through surface
receptors (1) and migrates towards higher concentrations of the factors (2). The high
concentration of chemotactic factors at the site of inflammation immobilizes the neutrophil (3).
B, Specific receptors for recognition and attachment. C, Phagocytosis. Opsonized
microorganisms bind to the surface of a phagocyte through specific receptors (1). The
microorganism is ingested into a phagocytic vacuole, or phagosome (2). Lysosomes fuse with
the phagosome, resulting in the formation of a phagolysosome (3). During this process the
microorganism is exposed to products of the lysosomes, including a variety of enzymes and
products of the hexose-monophosphate shunt (e.g., H2O2, O2
−). The microorganism is killed and
digested (4). Ab, Antibody; AbR, antibody receptor; C3b, complement component C3b; C3bR,
complement C3b receptor; PAMP, pathogen-associated molecular pattern; PRR, pattern
recognition receptor.
Once inside the tissue, leukocytes undergo a process of directed migration
(chemotaxis) by which they are attracted to the inflammatory site by chemotactic
factors.23 The primary chemotactic factors include many bacterial products,
neutrophil chemotactic factor produced by mast cells, the chemokine IL-8,
complement fragments C3a and C5a, and products of the clotting and kinin systems.
Red blood cells cannot repair themselves and are phagocytized by macrophages at
the end of their lifespan (Figure 6-10).
FIGURE 6-10 Phagocytosis of Red Blood Cell. This scanning electron micrographs shows the
progressive steps in phagocytosis. A, Red blood cells (R) attach to the surface of a macrophage
(M). B, Part of the macrophage (M) membrane starts to enclose the red cell (R). C, The red blood
cells are almost totally engulfed by the macrophage. (Modified from King DW et al: General pathology: principles and
dynamics, Philadelphia, 1983, Lea & Febiger.)
At the inflammatory site, the process of phagocytosis involves five steps: (1)
recognition and adherence of the phagocyte to its target, (2) engulfment (ingestion
or endocytosis), (3) formation of a phagosome, (4) fusion of the phagosome with
lysosomal granules within the phagocyte, and (5) destruction of the target.
Throughout the process, both the target and the digestive enzymes are isolated
within membrane-bound vesicles. Isolation protects the phagocyte itself from the
harmful effects of the target microorganisms, as well as its own enzymes.
Most phagocytes can trap and engulf bacteria using PRRs, although the process is
relatively slow. Opsonization greatly enhances adherence by acting as a glue to
tighten the affinity of adherence between the phagocyte and the target cell. The most
efficient opsonins are antibodies and C3b produced by the complement system.
Antibodies are made against antigens on the surface of bacteria and are highly
specific to that particular microorganism. Certain bacterial and fungal
polysaccharide coatings activate the alternative and lectin pathways of complement
activation, which deposits C3b on the bacterial surface and increases phagocytosis.
The surface of phagocytes contains a variety of specific receptors that will strongly
bind to opsonins. These include complement receptors that bind to C3b and Fc
receptors that bind to a site on antibody molecules.
Engulfment (endocytosis) is carried out by small pseudopods that extend from the
plasma membrane and surround the adherent microorganism, forming an
intracellular phagocytic vacuole, or phagosome (see Figures 6-9 and 6-10). After
the formation of the phagosome, lysosomes converge, fuse with the phagosome,
and discharge their contents, creating a phagolysosome. Destruction of the
bacterium takes place within the phagolysosome and is accomplished by both
oxygen-dependent and oxygen-independent mechanisms.
Oxygen-dependent killing mechanisms result from the production of toxic oxygen
species. Phagocytosis is accompanied by a burst of oxygen uptake by the phagocyte;
this is termed the respiratory burst and results from a shift in much of the cell's
glucose metabolism to the hexose-monophosphate shunt, which produces
nicotinamide adenine dinucleotide phosphate (NADPH). A membrane-associated
enzyme, NADPH oxidase, uses NADPH to generate superoxide (O2
−), hydrogen
peroxide (H2O2), and other reactive oxygen species that can be highly damaging to
bacteria. Hydrogen peroxide also can collaborate with the lysosomal enzyme
myeloperoxidase and halide anions (Cl− and Br−) to form acids that kill bacteria and
fungi.
Oxygen-independent mechanisms of microbial killing include (1) the acidic pH
(3.5 to 4.0) of the phagolysosome, (2) cationic proteins that bind to and damage
target cell membranes, (3) enzymatic attack of the microorganism's cell wall by
lysozyme and other enzymes, and (4) inhibition of bacterial growth by lactoferrin
binding of iron.
When a phagocyte dies at an inflammatory site, it frequently lyses (breaks open)
and releases its cytoplasmic contents into the tissue. For instance, contents of
neutrophil primary granules (lysozyme, hydrolases, neutral proteases) and
secondary granules (lysozyme, collagenase, gelatinase) can digest the connective
tissue matrix, causing much of the tissue destruction associated with inflammation.24
The destructive effects of many enzymes and reactive oxygen molecules released by
dying phagocytes are minimized by natural inhibitors found in the blood, such as
superoxide dismutase (breaks down superoxide), catalase (breaks down hydrogen
peroxide), and the antiproteinases α1-antitrypsin and α2-macroglobulin (both
produced by the liver). An inherited deficiency of α1-antitrypsin often leads to
chronic lung damage and emphysema as a result of inflammation. (The pulmonary
effects of α1-antitrypsin deficiency are described in Chapter 27.)
Natural Killer Cells and Lymphocytes
The main function of natural killer (NK) cells is recognition and elimination of
cells infected with viruses, although they also are somewhat effective at elimination
of other abnormal cells, specifically cancer cells.25 NK cells seem to be more
efficient in this role when they encounter an infected cell within the circulatory
system as opposed to within tissues. NK cells have inhibitory and activating
receptors that allow differentiation between infected or tumor cells and normal
cells. If the NK cell binds to a target cell through activating receptors, it produces
several cytokines and toxic molecules that can kill the target.26 NK cells and
lymphocytes, which are the principal cells of the adaptive immune response, will be
discussed in much more detail in Chapter 7.
Quick Check 6-4
1. What are pattern recognition receptors?
2. What are cytokines? How do cytokines promote inflammation?
3. What products do the mast cells release during inflammation, and what are their
effects?
4. What phagocytic cell types are involved in the acute inflammatory response?
What is the role of each?
5. What are the four steps in the process of phagocytosis?
Acute and Chronic Inflammation
Inflammation can be divided into phases of acute and chronic inflammation. The
acute inflammatory response is self-limiting—that is, it continues only until the
threat to the host is eliminated. This usually takes 8 to 10 days from onset to healing.
If the acute inflammatory response proves inadequate, a chronic inflammation may
develop and persist for weeks or months. If healing has not been initiated,
inflammation may progress to a granulomatous response that is designed to contain
the cause of tissue damage so it no longer poses any harm to the individual. The
characteristics of the early (i.e., acute) inflammatory response differ from those of
the later (i.e., chronic) response, and each phase involves different biochemical
mediators and cells that function together. Depending on the successful containment
of tissue damage and infection, the acute and chronic phases may lead to healing
without progression to the next phase.
Local Manifestations of Acute Inflammation
The cells and plasma protein systems of the inflammatory response interact to
produce all the characteristics of inflammation, whether local or systemic
(discussed in the next section), as well as determine the duration of inflammation,
either acute or chronic. All the local characteristics of acute inflammation (i.e.,
swelling, pain, heat, and redness [erythema]) result from vascular changes and the
subsequent leakage of circulating components into the tissue.
The exudate of inflammation results from increased vascular permeability and
varies in composition, depending on the stage of the inflammatory response and, to
some extent, the injurious stimulus. In early or mild inflammation, the exudate may
be watery (serous exudate) with very few plasma proteins or leukocytes, such as
the fluid in a blister. In more severe or advanced inflammation, the exudate may be
thick and clotted (fibrinous exudate), such as in the lungs of individuals with
pneumonia. If a large number of leukocytes accumulate, as in persistent bacterial
infections, the exudate consists of pus and is called a purulent (suppurative)
exudate. Purulent exudate is characteristic of walled-off lesions (cysts or
abscesses). If bleeding occurs, the exudate is filled with erythrocytes and is
described as a hemorrhagic exudate.
Systemic Manifestations of Acute Inflammation
The three primary systemic changes associated with the acute inflammatory
response are fever, leukocytosis (a transient increase in the levels of circulating
leukocytes), and increased levels of circulating plasma proteins.
Fever
Fever is partially induced by specific cytokines (e.g., IL-1, released from
neutrophils and macrophages). These are known as endogenous pyrogens to
differentiate them from pathogen-produced exogenous pyrogens. Pyrogens act
directly on the hypothalamus, the portion of the brain that controls the body's
thermostat. (Mechanisms of temperature regulation and fever are discussed in
Chapter 14.) A fever can be beneficial because some microorganisms (e.g., those
that cause syphilis or gonococcal urethritis) are highly sensitive to small increases
in body temperature. On the other hand, fever may have harmful side effects
because it may enhance the host's susceptibility to the effects of endotoxins
associated with gram-negative bacterial infections (bacterial toxins are described in
Chapter 8).
Leukocytosis
Leukocytosis is an increase in the number of circulating white blood cells (greater
than 11,000/ml3 in adults). During many infections, leukocytosis may be
accompanied by a left shift in the ratio of immature to mature neutrophils, so that
the more immature forms of neutrophils, such as band cells, metamyelocytes, and
occasionally myelocytes, are present in relatively greater than normal proportions.
(Chapter 20 contains a more complete discussion of the development and
maturation of blood cells.) Production of immature leukocytes increases primarily
from proliferation and release of granulocyte and monocyte precursors in the bone
marrow, which is stimulated by several products of inflammation.
Plasma Protein Synthesis
The synthesis of many plasma proteins, mostly products of the liver, is increased
during inflammation. These proteins, which can be either proinflammatory or anti-
inflammatory in nature, are referred to as acute-phase reactants (Table 6-4).
Acute-phase reactants reach maximal circulating levels within 10 to 40 hours after
the start of inflammation. IL-1 is indirectly responsible for the synthesis of acute-
phase reactants through the induction of IL-6, which directly stimulates liver cells to
synthesize most of these proteins.
TABLE 6-4
Circulating Levels of Acute-Phase Reactants During Inflammation
Function Increased Decreased
Coagulation components Fibrinogen None
Prothrombin
Factor VIII
Plasminogen
Protease inhibitors α1-Antitrypsin Inter-α1-antitrypsin
α1-Antichymotrypsin
Transport proteins Haptoglobin Transferrin
Hemopexin
Ceruloplasmin
Ferritin
Complement components C1s, C2, C3, C4, C5, C9, factor B, C1 inhibitor Properdin
Miscellaneous proteins α1-Acid glycoprotein Albumin
Fibronectin Prealbumin
Serum amyloid A (SAA) α1-Lipoprotein
C-reactive protein (CRP) β-Lipoprotein
Common laboratory tests for inflammation measure levels of acute-phase
reactants. For example, an increase in blood levels of acute-phase reactants,
primarily fibrinogen, is associated with an increased adhesion among erythrocytes
and a corresponding increase in the sedimentation rate. The erythrocyte
sedimentation rate is a measurement of the rate at which red blood cells sediment in
a tube over a prescribed time span (usually an hour). Although increased
erythrocyte sedimentation is a nonspecific reaction, it is considered a good
indicator of an acute inflammatory response.
Chronic Inflammation
Superficially, the difference between acute and chronic inflammation is duration;
chronic inflammation lasts 2 weeks or longer, regardless of cause. Chronic
inflammation is sometimes preceded by an unsuccessful acute inflammatory
response (Figure 6-11). For example, if bacterial contamination or foreign objects
(e.g., dirt, wood splinter, silica, and glass) persist in a wound, an acute response may
be prolonged beyond 2 weeks. Pus formation, suppuration (purulent discharge), and
incomplete wound healing may characterize this type of chronic inflammation.
FIGURE 6-11 The Chronic Inflammatory Response. Inflammation usually becomes chronic
because of the persistence of an infection, an antigen, or a foreign body in the wound. Chronic
inflammation is characterized by the persistence of many of the processes of acute
inflammation. In addition, large amounts of neutrophil degranulation and death, the activation of
lymphocytes, and the concurrent activation of fibroblasts result in the release of mediators that
induce the infiltration of more lymphocytes and monocytes/macrophages and the beginning of
wound healing and tissue repair. For more detailed information on each portion of the response,
see the figures referenced in this illustration.
Chronic inflammation can occur also as a distinct process without previous acute
inflammation. Some microorganisms (e.g., mycobacteria that cause tuberculosis)
have cell walls with a very high lipid and wax content, making them relatively
insensitive to breakdown by phagocytes. Other microorganisms (e.g., those that
cause leprosy, syphilis, and brucellosis) can survive within the macrophage and
avoid removal by the acute inflammatory response. Other microorganisms produce
toxins that damage tissue and cause persistent inflammation even after the organism
is killed. Finally, chemicals, particulate matter, or physical irritants (e.g., inhaled
dusts, wood splinters, and suture material) can cause a prolonged inflammatory
response.
Chronic inflammation is characterized by a dense infiltration of lymphocytes and
macrophages. If macrophages are unable to protect the host from tissue damage, the
body attempts to wall off and isolate the infected area, thus forming a granuloma
(Figure 6-12). For example, infections caused by some bacteria (listeriosis,
brucellosis), fungi (histoplasmosis, coccidioidomycosis), and parasites
(leishmaniasis, schistosomiasis, toxoplasmosis) can result in granuloma formation.
TNF-α primarily drives granuloma formation.27 Some macrophages differentiate
into large epithelioid cells, which specialize in taking up debris and other small
particles. Other macrophages fuse into multinucleated giant cells, which are active
phagocytes that can engulf very large particles—larger than those that can be
engulfed by a single macrophage. These two types of specialized cells form the
center of the granuloma, which is surrounded by a wall of lymphocytes. The
granuloma itself is often encapsulated by fibrous deposits of collagen and may
become cartilaginous or possibly calcified by deposits of calcium carbonate and
calcium phosphate.
FIGURE 6-12 Tuberculous Granuloma. A central area of amorphous caseous necrosis (C) is
surrounded by a zone of lymphocytes (L) and enlarged epithelioid cells (E). Activated
macrophages frequently fuse to form multinucleated cells (Langhans giant cells). In tuberculoid
granulomas the nuclei of the giant cells move to the cellular margins in a horseshoe-like
formation.
The classic granuloma associated with tuberculosis is characterized by a wall of
epithelioid cells surrounding a cheeselike proteinaceous center derived from dead
and decaying tissue (caseous necrosis) and mycobacteria.28 Decay of cells within the
granuloma results in the release of acids and the enzymatic contents of lysosomes
from dead phagocytes. In this inhospitable environment, the cellular debris is
broken down into its basic constituents, and a clear fluid may remain (liquefaction
necrosis). Eventually, this fluid diffuses out and leaves a hollow, thick-walled
structure that has replaced normal tissue and reduced the function of the lung.
Quick check 6-5
1. Describe how acute inflammation differs from chronic inflammation. What
characteristics do they share?
2. List the types of exudate produced in inflammation.
Wound Healing
The conclusion of inflammation is healing and repair. The most favorable outcome
is a return to normal structure and function if damage is minor, no complications
occur, and destroyed tissues are capable of regeneration (replacement of damaged
tissue with healthy tissue, such as occurs in the epithelia of the skin and intestines
and in some organs, such as the liver) (Figure 6-13). This restoration is called
resolution and may take up to 2 years, and local production of IL-10 appears to play
a critical role.29 Resolution may not be possible if extensive damage is present, the
tissue is not capable of regeneration, infection results in abscess or granuloma
formation, or fibrin persists in the lesion. In those cases, repair takes place instead
of resolution. Repair is the replacement of destroyed tissue with scar tissue. Scar
tissue is composed primarily of collagen that fills in the lesion and restores
strength but cannot carry out the physiologic functions of destroyed tissue, resulting
in loss of function.
FIGURE 6-13 Wound Healing by Primary and Secondary Intention and Phases of Wound
Healing. Phases of wound healing (coagulation, inflammation, proliferation, remodeling, and
maturation) and steps in wound healing by primary intention (left) and secondary intention
(right). Note large amounts of granulation tissue and wound contraction in healing by secondary
intention. (From Roberts JR, Custalow CB: Roberts and Hedges' clinical procedures in emergency medicine, ed 6, Philadelphia, 2013,
Saunders.)
Wound healing involves processes that (1) fill in, (2) seal, and (3) shrink the
wound. These characteristics of healing vary in importance and duration among
different types of wounds. A clean incision, such as a paper cut or a sutured surgical
wound, heals primarily through the process of collagen synthesis. Because this type
of wound has minimal tissue loss and close apposition of the wound edges, very
little sealing (epithelialization) and shrinkage (contraction) are required. Wounds
that heal under conditions of minimal tissue loss are said to heal by primary
intention (see Figure 6-13).
Other wounds do not heal as easily. Healing of an open wound, such as a stage IV
pressure ulcer (decubitus ulcer), requires a great deal of tissue replacement so that
epithelialization, scar formation, and contraction take longer and healing occurs
through secondary intention (see Figure 6-13). Healing by either primary or
secondary intention may occur at different rates for different types of tissue injury.
Epidermal wounds that heal by secondary intention and unsutured internal lesions
are not completely restored by healing. At best, repaired tissue regains 80% of its
original tensile strength. Only epithelial, hepatic (liver), and bone marrow cells are
capable of the complete mitotic regeneration of the normal tissue known as
compensatory hyperplasia. In fibrous connective tissue, such as joints and ligaments,
normal healing results in replacement of the original tissue with new tissue that does
not have exactly the same structure or function as that of the original. Some tissues
heal without replacement of cells. For example, damage resulting from myocardial
infarction heals with a scar composed of fibrous tissue rather than with cardiac
muscle.
Wound healing occurs in three overlapping phases: inflammation, proliferation
and new tissue formation, and remodeling and maturation.
Phase I: Inflammation
The early phase of wound healing, the transition from acute inflammation to
healing, begins almost immediately. The inflammatory phase includes coagulation
or hemostasis and the infiltration of cells that participate in wound healing,
including platelets, neutrophils, and macrophages (Figure 6-14). The fibrin mesh of
the blood clot acts as a scaffold for cells that participate in healing. Platelets
contribute to clot formation and, as they degranulate, release growth factors that
initiate proliferation of undamaged cells. Neutrophils clear the wound of debris and
bacteria and are later replaced by macrophages. Macrophages are essential to
wound healing because they clear debris, release wound healing mediators and
growth factors, recruit fibroblasts, and help promote formation of a new blood
supply (angiogenesis) during the proliferative phase of wound healing.
FIGURE 6-14 Time Course of Cells Infiltrating a Wound. Neutrophils and macrophages are the
predominant cells that infiltrate a wound during inflammation. Lymphocytes appear later and
peak at day 7. Fibroblasts are the predominant cells during the proliferative and remodeling
phases of the healing process. (Adapted from Townsend CM et al, editors: Sabiston textbook of surgery, ed 19, St Louis, 2012,
Elsevier.)
Phase II: Proliferation and New Tissue Formation
The proliferative phase begins 3 to 4 days after the injury and continues for as long
as 2 weeks. The wound is sealed and the fibrin clot is replaced by normal tissue or
scar tissue during this phase. The proliferative phase is characterized by
macrophage invasion of the dissolving clot and recruitment and proliferation of
fibroblasts (connective tissue cells), followed by fibroblast collagen synthesis,
epithelialization, contraction of the wound, and cellular differentiation.
Macrophages secrete a variety of biochemical mediators that promote healing,
including:
1. Transforming growth factor-beta (TGF-β) stimulates fibroblasts entering the
lesion to synthesize and secrete the collagen precursor procollagen.
2. Angiogenesis factors, such as vascular endothelial growth factor (VEGF) and
fibroblast growth factor-2 (FGF-2), stimulate vascular endothelial cells to form
capillary buds that grow into the lesion; decreased pH and decreased wound oxygen
tension also promote angiogenesis.30
3. Matrix metalloproteinases (MMPs) degrade and remodel extracellular matrix
proteins (e.g., collagen and fibrin) at the site of injury.31
Granulation tissue grows into the wound from surrounding healthy connective
tissue and consists of invasive cells, new lymphatic vessels, and new capillaries
derived from capillaries in the surrounding tissue, giving the granulation tissue a
red, granular appearance. During this process the healing wound must be protected.
Epithelialization is the process by which epithelial cells grow into the wound from
surrounding healthy tissue.32 Epithelial cells migrate under the clot or scab using
MMPs to unravel collagen. Migrating epithelial cells contact similar cells from all
sides of the wound and seal it. The epithelial cells remain active, undergoing
differentiation to give rise to the various epidermal layers (see Chapter 41).
Epithelialization of a skin wound can be hastened if the wound is kept moist,
preventing the fibrin clot from becoming a scab.
Fibroblasts are important cells during healing because they secrete collagen and
other connective tissue proteins. Fibroblasts are stimulated by macrophage-derived
TGF-β to proliferate, enter the lesion, and deposit connective tissue proteins in
débrided areas about 6 days after the fibroblasts have entered the lesion. Collagen is
the most abundant protein in the body.33 It contains high concentrations of the amino
acids glycine, proline, and lysine, many of which are enzymatically modified.
Modification of proline and lysine requires several cofactors that are absolutely
necessary for proper collagen polymerization and function. These include iron,
ascorbic acid (vitamin C), and molecular oxygen (O2); absence of any of these
results in impaired wound healing. As healing progresses, collagen molecules are
cross-linked by intermolecular covalent bonds to form collagen fibrils that are
further cross-linked to form collagen fibers. The complete process takes several
months.
In granulation tissue, TGF-β induces some fibroblasts to transition into
myofibroblasts, specialized cells responsible for wound contraction.34
Myofibroblasts have features of both smooth muscle cells and fibroblasts. They
appear microscopically similar to fibroblasts but differ in that their cytoplasm
contains bundles of parallel fibers similar to those found in smooth muscle cells.
Wound contraction occurs as extensions from the plasma membrane of
myofibroblasts establish connections between neighboring cells, contract their
fibers, and exert tension on the neighboring cells while anchoring themselves to the
wound bed. Wound contraction is necessary for closure of all wounds, especially
those that heal by secondary intention. Contraction is noticeable 6 to 12 days after
injury.
Phase III: Remodeling and Maturation
Tissue remodeling and maturation begins several weeks after injury and is normally
complete within 2 years. During this phase, there is continuation of cellular
differentiation, scar formation, and scar remodeling. The fibroblast is the major
cell of tissue remodeling with the deposition of collagen into an organized matrix.
Tissue regeneration and wound contraction continue in the remodeling and
maturation phase—a phase for recovering normal tissue structure that can persist
for years. For wounds that heal by scarring, scar tissue is remodeled and capillaries
disappear, leaving the scar avascular. Within 2 to 3 weeks after maturation has
begun, the scar tissue has gained about two thirds of its eventual maximal strength.
Dysfunctional Wound Healing
Dysfunctional wound healing and impaired epithelialization may occur during any
phase of the healing process. The cause of dysfunctional wound healing includes
ischemia, excessive bleeding, excessive fibrin deposition, a predisposing disorder
such as diabetes mellitus, obesity, wound infection, inadequate nutrients, numerous
drugs, and tobacco smoke.35
Oxygen-deprived (ischemic) tissue is susceptible to cellular death and infection,
which prolongs inflammation and delays healing. Ischemia reduces energy
production and impairs collagen synthesis and the tensile strength of regenerating
connective tissue.
Healing is prolonged if there is excessive bleeding. Large clots increase the
amount of space that granulation tissue must fill and serve as mechanical barriers to
oxygen diffusion. Accumulated blood is an excellent culture medium for bacteria
and promotes infection, thereby prolonging inflammation by increasing exudation
and pus formation. Decreased blood volume also inhibits inflammation because of
vessel constriction rather than the dilation required to deliver inflammatory cells,
nutrients, and oxygen to the site of injury.
Obesity delays wound healing because of impaired leukocyte function and
predisposition to infection, decreases in the number of growth factors, and
increases in the levels of proinflammatory cytokines. Additionally, there is
dysregulation in collagen synthesis and a decrease in angiogenesis.36
Excessive fibrin deposition is detrimental to healing. Fibrin released in response
to injury must eventually be reabsorbed to prevent organization into fibrous
adhesions. Adhesions formed in the pleural, pericardial, or abdominal cavities can
bind organs together by fibrous bands and distort or strangulate the affected organ.
Persons with diabetes are at risk for prolonged wound healing. Wounds are often
ischemic because of the potential for small-vessel diseases that impair the
microcirculation and alter (glycosylated) hemoglobin, which has an increased
affinity for oxygen and thus does not readily release oxygen in tissues.
Consequences of hyperglycemia also include suppression of macrophages and
increased risk for wound infection.
Wound infection is caused by the infiltration of pathogens. Pathogens damage
cells, stimulate the continued release of inflammatory mediators, consume nutrients,
and delay wound healing.
Optimal nutrition is important during all phases of healing because metabolic
needs increase. Leukocytes need glucose to produce adenosine 5′-triphosphate (5′-
ATP) necessary for chemotaxis, phagocytosis, intercellular killing, and initiation of
healing; therefore the wounds of persons with diabetes who receive insufficient
insulin heal poorly. Hypoproteinemia impairs fibroblast proliferation and collagen
synthesis. Prolonged lack of vitamins A and C results in poorly formed connective
tissue and greatly impaired healing because they are cofactors required for collagen
synthesis.37 Other nutrients, including iron, zinc, manganese, and copper, are also
required as cofactors for collagen synthesis. Malnutrition increases risk for wound
infection, delays healing, and reduces wound tensile strength.
Medications, including antineoplastic (anticancer) agents, nonsteroidal anti-
inflammatory drugs (NSAIDs), and steroids, delay wound healing. Antineoplastic
agents slow cell division and inhibit angiogenesis. Although NSAIDs inhibit
prostaglandin production and suppress acute inflammation and relieve pain, they
also can delay wound healing, particularly bone formation, and may contribute to
the formation of excessive scarring. Steroids prevent macrophages from migrating
to the site of injury and inhibit release of collagenase and plasminogen activator.
Steroids also inhibit fibroblast migration into the wound during the proliferative
phase and delay epithelialization. Toxic agents in tobacco smoke (i.e., nicotine,
carbon monoxide, and hydrogen cyanide) delay wound healing and increase the risk
for wound infection.
Dysfunctional collagen synthesis may involve excessive production of collagen,
leading to a hypertrophic scar or keloid.38 A hypertrophic scar is raised but
remains within the original boundaries of the wound and tends to regress over time
(Figure 6-15, A). A keloid is a raised scar that extends beyond the original
boundaries of the wound, invades surrounding tissue, and is likely to recur after
surgical removal (Figure 6-15, B). A familial tendency to keloid formation has been
observed, with a greater incidence in blacks than whites.
FIGURE 6-15 Hypertrophic Scar and Keloid Scar Formation. Hypertrophic scar (A) and keloid
scar (B) caused by excessive synthesis of collagen at suture sites. (A from Flint PW et al: Cummings
otolaryngology: head & neck surgery, ed 6, Philadelphia, 2015, Mosby; B from Damjanov I, Linder J: Anderson's pathology, ed 10, St Louis, 1996,
Mosby.)
Wound Disruption
A potential complication of wounds that are sutured closed is dehiscence, in which
the wound pulls apart at the suture line. Dehiscence generally occurs 5 to 12 days
after suturing, when collagen synthesis is at its peak. Approximately half of
dehiscence occurrences are associated with wound infection, but they also may be
the result of sutures breaking because of excessive strain. Obesity increases the risk
for dehiscence because adipose tissue is difficult to suture. Wound dehiscence
usually is heralded by increased serous drainage from the wound and a patient's
perception that “something gave way.” Prompt surgical attention is required.
Impaired Contraction
Wound contraction, although necessary for healing, may become excessive,
resulting in a deformity or contracture of scar tissue. Burns of the skin are
especially susceptible to contracture development, particularly at joints, resulting in
loss of movement around the joints. Internal contractures include duodenal
strictures caused by dysfunctional healing of a peptic ulcer; esophageal strictures
caused by chemical burns, such as lye ingestion; or abdominal adhesions caused by
surgery, infection, or radiation. Contracture may occur in cirrhosis of the liver,
constricting vascular flow and contributing to the development of portal
hypertension and esophageal varices. Proper positioning, range-of-motion
exercises, and surgery are among the physical means used to overcome excessive
skin contractures. Surgery is performed to release internal contractures.
Quick check 6-6
1. How does regeneration of tissue differ from repair of tissue?
2. What does it mean to heal by primary intention?
3. What is the role of fibroblasts in wound healing?
4. Describe various ways wound healing may be dysfunctional.
Pediatric Considerations
Age-Related Factors Affecting Innate Immunity in the Newborn
Child
• Newborn physiologic immunity acquired from mother through placenta and breast
milk.
• Newborns have transiently depressed inflammatory responses.
• Neutrophils are incapable of chemotaxis, lacking fluidity in the plasma membrane.
• Complement levels are diminished, especially components of the alternative
pathways (e.g., factor B), particularly in premature newborns.
• Monocyte/macrophage numbers are normal but chemotaxis of monocytes is
delayed.
• There is a tendency for infections associated with chemotactic defects, for
example, cutaneous abscesses caused by staphylococci and cutaneous candidiasis.
• There are diminished oxidative and bacterial responses in those stressed by in
utero infection or respiratory insufficiency.
• There is a tendency to develop severe overwhelming sepsis and meningitis when
infected by bacteria against which no maternal antibodies are present.
• The establishment of the gut microbiome is facilitated by breast milk.
• Cesarean delivered newborns have reduced gut microbial diversity.
Geriatric Considerations
Age-Related Factors Affecting Innate Immunity in the Elderly
• Normal numbers of cells of innate immunity but possible diminished function
(e.g., decreased phagocytic activity, decreased antibody production, and altered
cytokine synthesis)
• Increased incidence of chronic inflammation, possibly related to increased
production of proinflammatory mediators
• At risk for impaired healing and infection associated with chronic illness (e.g.,
diabetes mellitus, peripheral vascular disease, or cardiovascular disease) and
decreased phagocytosis.
• Use of medications interfering with healing (e.g., anti-inflammatory steroids)
• Loss of subcutaneous fat, diminishing layers of protection against injury
• Atrophied epidermis, including underlying capillaries, which decreases perfusion
and increases risk of hypoxia in wound bed
• Aging of the immune system, diminishing the effectiveness of vaccines
Did You Understand?
Innate Immunity
1. Neonates often have transiently depressed inflammatory function, particularly
neutrophil chemotaxis and alternative complement pathway activity.
2. Elderly persons are at risk for impaired wound healing, usually because of
chronic illnesses.
3. There are three layers of human defense: barriers; innate immunity, which
includes the inflammatory response; and adaptive (acquired) immunity.
4. Physical barriers are the first lines of defense that prevent damage to the
individual and prevent invasion by pathogens; these include the skin and mucous
membranes.
5. Antibacterial peptides (cathelicidins, defensins, collectins, and mannose-binding
lectin) in mucous secretions, perspiration, saliva, tears, and other secretions provide
a biochemical barrier against pathogenic microorganisms.
6. The skin and mucous membranes are colonized by commensal or mutualistic
microorganisms that provide protection by releasing chemicals that facilitate
immune responses, prevent colonization by pathogens, and facilitate digestion in the
gastrointestinal tract.
7. The second line of defense is the inflammatory response, a rapid and nonspecific
protective response to cellular injury from any cause. It can occur only in
vascularized tissue.
8. The macroscopic hallmarks of inflammation are redness, swelling, heat, pain,
and loss of function of the inflamed tissues.
9. The microscopic hallmarks of inflammation are vasodilation, increased capillary
permeability, and an accumulation of fluid and cells at the inflammatory site.
10. Inflammation is mediated by three key plasma protein systems: the complement
system, the clotting system, and the kinin system. The components of all three
systems are a series of inactive proteins that are activated sequentially.
11. The complement system can be activated by antigen-antibody reactions (through
the classical pathway) or by other products, especially bacterial polysaccharides
(through the lectin pathway or the alternative pathway), resulting in the production
of biologically active fragments that recruit phagocytes, activate mast cells, and
destroy pathogens.
12. The most biologically potent products of the complement system are C3b
(opsonin), C3a (anaphylatoxin), and C5a (anaphylatoxin, chemotactic factor).
13. The clotting system stops bleeding, localizes microorganisms, and provides a
meshwork for repair and healing.
14. Bradykinin is the most important product of the kinin system and causes
vascular permeability, smooth muscle contraction, and pain.
15. Control of inflammation regulates inflammatory cells and enzymes and
localizes the inflammatory response to the area of injury or infection.
16. Carboxypeptidase, histaminase, and C1 esterase inhibitor are inactivating
enzymes, and the fibrinolytic system and plasmin facilitate clot degradation after
bleeding is stopped.
17. Many different types of cells are involved in the inflammatory process including
mast cells, endothelial cells, platelets, phagocytes (neutrophils, eosinophils,
monocytes and macrophages, dendritic cells), natural killer (NK) cells, and
lymphocytes.
18. Most cells express plasma membrane pattern recognition receptors (PRRs) that
recognize molecules produced by infectious microorganisms (pathogen-associated
molecular patterns, or PAMPs), or products of cellular damage (damage-associated
molecular patterns, or DAMPs). Toll-like receptors (TLRs) and NOD-like receptors
are expressed on many inflammatory cells, recognize PAMPs and DAMPs, and
promote release of cytokines and inflammatory mediators that eliminate damaged
cells and protect against invasion by microbes.
19. The cells of the innate immune system secrete many biochemical mediators
(cytokines) that are responsible for activating other cells and regulating the
inflammatory response; these cytokines include chemokines, interleukins,
interferons, and other molecules.
20. Chemokines induce chemotaxis of leukocytes, fibroblasts, and other cells to
promote phagocytosis and wound healing.
21. Interleukins are produced primarily by lymphocytes and macrophages and
promote or inhibit inflammation by activating growth and differentiation of
leukocytes and lymphocytes.
22. The most important proinflammatory interleukins are interleukin-1 (IL-1),
interleukin-6 (IL-6), and tumor necrosis factor-alpha (TNF-α). Interleukins 6 and 10
down-regulate the inflammatory response.
23. Interferons are produced by cells that are infected by viruses. Once released
from infected cells, interferons can stimulate neighboring healthy cells to produce
substances that prevent viral infection.
24. The most important activator of the inflammatory response is the mast cell,
which is located in connective tissue near capillaries and initiates inflammation by
releasing biochemical mediators (histamine, chemotactic factors) from preformed
cytoplasmic granules and synthesizing other mediators (prostaglandins,
leukotrienes, and platelet-activating factor) in response to a stimulus. Basophils are
found in the blood and function similar to mast cells.
25. Histamine is the major vasoactive amine released from mast cells. It causes
dilation of capillaries and retraction of endothelial cells lining the capillaries, which
increases vascular permeability.
26. The endothelial cells lining the circulatory system (vascular endothelium)
normally regulate circulating components of the inflammatory system and maintain
normal blood flow by preventing spontaneous activation of platelets and members
of the clotting system.
27. During inflammation the endothelium expresses receptors that help leukocytes
leave the vessel and retract to allow fluid to pass into the tissues.
28. Platelets interact with the coagulation cascade to stop bleeding and release a
number of mediators that promote and control inflammation.
29. The polymorphonuclear neutrophil (PMN), the predominant phagocytic cell in
the early inflammatory response, exits the circulation by diapedesis through the
retracted endothelial cell junctions and moves to the inflammatory site by
chemotaxis.
30. Eosinophils release products that control the inflammatory response and are the
principal cell that kills parasitic organisms.
31. The macrophage, the predominant phagocytic cell in the late inflammatory
response, is highly phagocytic, is responsive to cytokines, and promotes wound
healing.
32. Dendritic cells connect the innate and acquired immune systems by collecting
antigens at the site of inflammation and transporting them to sites, such as the lymph
nodes, where immunocompetent B and T cells reside and are transformed into
functional cells.
33. Phagocytosis is a multistep cellular process for the elimination of pathogens and
foreign debris. The steps include recognition and attachment, engulfment, formation
of a phagosome and phagolysosome, and destruction of pathogens or foreign
debris. Phagocytic cells engulf microorganisms and enclose them in phagocytic
vacuoles (phagolysosomes), within which toxic products (especially metabolites of
oxygen) and degradative lysosomal enzymes kill and digest the microorganisms.
34. Opsonins, such as antibody and complement component C3b, coat
microorganisms and make them more susceptible to phagocytosis by binding them
more tightly to the phagocyte.
Acute and Chronic Inflammation
1. Acute inflammation is self-limiting and usually resolves within 8 to 10 days.
2. Local manifestations of inflammation are the result of the vascular changes
associated with the inflammatory process, including vasodilation and increased
capillary permeability. The symptoms include redness, heat, swelling, and pain.
3. The principal systemic effects of inflammation are fever and increases in levels
of circulating leukocytes (leukocytosis) and plasma proteins (acute-phase reactants
[i.e., IL-1 and IL-6]).
4. Chronic inflammation can be a continuation of acute inflammation that lasts 2
weeks or longer. It also can occur as a distinct process without much preceding
acute inflammation.
5. Chronic inflammation is characterized by a dense infiltration of lymphocytes and
macrophages. The body may wall off and isolate the infection to protect against
tissue damage by formation of a granuloma.
Wound Healing
1. Resolution (regeneration) is the return of tissue to nearly normal structure and
function. Repair is healing by scar tissue formation.
2. Damaged tissue proceeds to resolution (restoration of the original tissue structure
and function) if little tissue has been lost or if injured tissue is capable of
regeneration. This is called healing by primary intention.
3. Tissues that sustained extensive damage or those incapable of regeneration heal
by the process of repair resulting in the formation of a scar. This is called healing
by secondary intention.
4. Resolution and repair occur in two separate phases: the reconstructive phase in
which the wound begins to heal and the maturation phase in which the healed wound
is remodeled.
5. Dysfunctional wound healing can be related to ischemia, excessive bleeding,
excessive fibrin deposition, a predisposing disorder (such as diabetes mellitus),
wound infection, inadequate nutrients, numerous drugs, or altered collagen
synthesis.
6. Dehiscence is a disruption in which the wound pulls apart at the suture line.
7. A contracture is a deformity caused by the excessive shortening of collagen in
scar tissue.
Key Terms
Abscess, 149
Acute inflammation, 149
Acute-phase reactant, 149
Adaptive immunity, 134
Alternative pathway, 139
Anaphylatoxin, 138
Angiogenesis factor, 152
Antimicrobial peptide, 135
α1-Antitrypsin, 147
Basophil, 144
Blood clot, 139
Bradykinin, 141
C1 esterase inhibitor (C1 inh), 141
C1 inh deficiency, 141
Carboxypeptidase, 141
Cathelicidin, 135
Chemokine, 143
Chemotactic factor, 138
Chemotaxis, 145, 147
Chronic inflammation, 149
Classical pathway, 139
Clotting (coagulation) system, 139
Collagen, 153
Collectin, 135
Complement receptor, 142
Complement system, 140
Contact activation (intrinsic) pathway, 140
Contraction, 152
Contracture of scar tissue, 154
Cyst, 149
Cytokine, 143
Damage-associated molecular pattern (DAMP), 142
Defensin, 135
Degranulation, 144
Dehiscence, 154
Dendritic cell, 147
Diapedesis, 147
Endogenous pyrogen, 149
Endothelial cell, 146
Eosinophil, 146
Eosinophil chemotactic factor of anaphylaxis (ECF-A), 145
Epithelialization, 152
Epithelioid cell, 150
Exudate, 149
Fc receptor, 147
Fever, 149
Fibrinolytic system, 141
Fibrinous exudate, 149
Fibroblast, 153
Giant cell, 150
Granulation tissue, 153
Granuloma, 150
Hageman factor (factor XII), 140
Hemorrhagic exudate, 149
Hereditary angioedema, 141
Hexose-monophosphate shunt, 147
Histaminase, 141
Histamine, 145
Hypertrophic scar, 153
Inflammasomes, 142
Inflammation, 137
Inflammatory phase, 152
Inflammatory response, 134, 137
Innate immunity, 134
Interferon (IFN), 144
Interleukin (IL), 143
Interleukin-1 (IL-1), 144
Interleukin-6 (IL-6), 144
Interleukin-10 (IL-10), 144
Keloid, 154
Kinin system, 140
Lectin pathway, 139
Leukocytosis, 149
Leukotriene (slow-reacting substance of anaphylaxis [SRS-A]), 145
Lymphocyte, 141
Lysozyme, 135
Macrophage, 146
Mannose-binding lectin (MBL), 135
Margination (pavementing), 147
Mast cell, 144
Matrix metalloproteinase (MMP), 153
Monocyte, 146
Myofibroblast, 153
Natural killer (NK) cells, 147
Neutrophil (polymorphonuclear neutrophil [PMN]), 146
Neutrophil chemotactic factor (NCF), 145
Nitric oxide (NO), 146
NOD-like receptors (NLRs), 142
Normal flora, 135
Normal microbiome, 135
Opportunistic microorganism, 137
Opsonin, 138
Opsonization, 147
Pathogen-associated molecular pattern (PAMP), 142
Pattern recognition receptor (PRR), 142
Phagocyte, 138
Phagocytosis, 146
Phagolysosome, 147
Phagosome, 147
Plasma protein system, 138
Plasmin, 141
Plasminogen, 141
Platelet, 146
Platelet-activating factor (PAF), 145
Primary intention, 152
Proliferative phase, 152
Prostacyclin (PGI2), 146
Prostaglandin, 145
Purulent (suppurative) exudate, 149
Pyrogen, 149
Regeneration, 151
Repair, 152
Resolution, 151
Scar tissue, 152
Scavenger receptors, 142
Secondary intention, 152
Serous exudate, 149
T lymphocyte, 147
Tissue factor (extrinsic) pathway, 140
Tissue factor (TF; tissue thromboplastin), 140
Toll-like receptor (TLR), 142
Transforming growth factor, 144
Transforming growth factor-beta (TGF-β), 144
Tumor necrosis factor-alpha (TNF-α), 144
Wound contraction, 153
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7
Adaptive Immunity
Neal S. Rote
CHAPTER OUTLINE
Third Line of Defense: Adaptive Immunity, 158
Antigens and Immunogens, 160
Antibodies, 161
Classes of Immunoglobulins, 161
Antigen-Antibody Binding, 162
Function of Antibodies, 162
Immune Response: Collaboration of B Cells and T Cells, 166
Generation of Clonal Diversity, 166
Clonal Selection, 167
Cell-Mediated Immunity, 172
T-Lympohocyte Function, 172
PEDIATRIC CONSIDERATIONS: Age-Related Factors Affecting
Mechanisms of Self-Defense in the Newborn Child, 173
GERIATRIC CONSIDERATIONS: Age-Related Factors Affecting
Mechanisms of Self-Defense in the Elderly, 173
The third line of defense in the human body is adaptive (acquired) immunity, often
called the immune response or immunity, and consists of lymphocytes (Figure 7-1)
and serum proteins called antibodies. Once external barriers have been
compromised and inflammation (innate immunity, see Chapter 6) has been activated,
the adaptive immune response is called into action. Inflammation is the “first
responder” that contains the initial injury and slows the spread of infection, whereas
adaptive immunity slowly augments the initial defenses against infection and
provides long-term security against reinfection.
FIGURE 7-1 Lymphocytes. A scanning electron micrograph showing lymphocytes (yellow, like
cotton candy), red blood cells, and platelets. (Copyright Dennis Kunkel Microscopy, Inc.)
Third Line of Defense: Adaptive Immunity
Inflammation and adaptive immunity differ in several key ways. First, the
components of inflammation are activated immediately after tissue damage.
Adaptive immunity is inducible; the effectors of the immune response, lymphocytes
and antibodies, do not preexist but must be produced in response to infection. Thus,
adaptive immunity develops more slowly than inflammation. Second, the
inflammatory response is similar regardless of differences in the cause of tissue
damage or whether the inflammatory site is sterile or contaminated with infectious
microorganisms. The immune response is exquisitely specific. The lymphocytes
and antibodies induced in response to infection are extremely specific to the
infecting microbe. Third, the residual mediators of inflammation must be removed
quickly to limit damage to surrounding healthy tissue and allow healing. The
effectors of the immune response are long-lived and systemic, providing long-term
protection against specific infections. Finally, the inflammatory response to both
recurrent tissue damage and infection is identical. The immune response has
memory. If reinfected with the same microbe, protective lymphocytes and antibody
are produced immediately, thus providing permanent long-term protection against
infection.
Despite the differences, the innate and adaptive immune systems are highly
interactive and complementary. Many components of innate resistance are necessary
for the development of the adaptive immune response. Conversely, products of the
adaptive immune response activate components of innate resistance. Thus, both
systems are essential for complete protection against infectious disease.
The mechanisms underlying the immune response will be discussed in this
chapter. As with Chapter 6, a complete description of all the important components
and processes of an effective immune response would require far more space than
available. Therefore, this chapter will focus on the basic concepts and the most
important, or well-studied, mediators of the immune response.
The adaptive immune response has its own vocabulary (Figure 7-2). Antigens are
the molecular targets of antibodies and lymphocytes. Antigens are generally small
molecules, usually within proteins, carbohydrates, or lipids, found on the surface of
microbes or infected cells, although this definition will be expanded as we discuss
immunologic diseases in Chapter 8. In the fetus, well before being exposed to any
infectious microorganisms, lymphocytes have undergone extensive differentiation.
Some lymphoid stem cells enter the thymus and differentiate into T lymphocytes (T
cells, T indicates thymus derived), whereas others enter specific regions of the bone
marrow and differentiate into B lymphocytes (B cells, B indicates bone marrow
derived). Each type of cell develops origin-specific cell surface proteins that
identify them as T or B cells. Both B and T cells also develop cell surface antigen
receptors. The receptors are remarkable because an individual lymphocyte is
programmed to recognize only one specific antigen before having encountered that
antigen. It is estimated that before birth each individual has produced a population of
B and T lymphocytes capable of recognizing at least 108 different antigens. This
process is called generation of clonal diversity and refers to the process by which
the extensive diversity of antigen receptors on B and T cells is established (see
Figure 7-2).
FIGURE 7-2 Overview of the Immune Response. The immune response can be separated into
two phases: the generation of clonal diversity and clonal selection. During the generation of
clonal diversity, lymphoid stem cells from the bone marrow migrate to the central lymphoid
organs (the thymus or regions of the bone marrow), where they undergo a series of cellular
division and differentiation stages resulting in either immunocompetent T cells from the thymus
or immunocompetent B cells from the bone marrow. These cells are still naïve in that they have
never encountered foreign antigen. The immunocompetent cells enter the circulation and
migrate to the secondary lymphoid organs (e.g., spleen and lymph nodes), where they establish
residence in B- and T-cell–rich areas. The clonal selection phase is initiated by exposure to
foreign antigen. The antigen is usually processed by antigen-presenting cells (APCs) for
presentation to T-helper cells (Th cells). The intercellular cooperation among APCs, Th cells, and
immunocompetent T and B cells results in a second stage of cellular proliferation and
differentiation. Because antigen has “selected” those T and B cells with compatible antigen
receptors, only a small population of T and B cells undergo this process at one time. The result
is an active cellular immunity or humoral immunity, or both. Cellular immunity is mediated by a
population of effector T cells that can kill targets (T-cytotoxic cells) or regulate the immune
response (T-regulatory cells), as well as a population of memory cells (T-memory cells) that can
respond more quickly to a second challenge with the same antigen. Humoral immunity is
mediated by a population of soluble proteins (antibodies) produced by plasma cells and by a
population of memory B cells that can produce more antibody rapidly to a second challenge with
the same antigen.
Lymphocytes leave the primary lymphoid organs (bone marrow and thymus) as
immunocompetent, but naïve, B and T cells. The cells are immunocompetent in that
they have the capacity to respond to antigens, but they are naïve in that they have not
yet encountered antigen. These cells enter the blood and lymphatic vessels and
migrate to the secondary lymphoid organs (e.g., lymph nodes, spleen) of the
systemic immune system (Figure 7-3). Some take up residence in B cell and T cell
rich areas of those organs, and others reenter the circulation. Approximately 60% to
70% of circulating lymphocytes are immunocompetent T cells, and 10% to 20% are
immunocompetent B cells.
FIGURE 7-3 Lymphoid Tissues: Sites of B-Cell and T-Cell Differentiation. Immature lymphocytes
migrate through central (primary) lymphoid tissues: the bone marrow (central lymphoid tissue
for B lymphocytes) and the thymus (central lymphoid tissue for T lymphocytes). Mature
lymphocytes later reside in the T- and B-lymphocyte–rich areas of the peripheral (secondary)
lymphoid tissues.
A second process, clonal selection, is initiated when an infection occurs. This
process requires the cooperation among a variety of cells in the secondary
lymphoid organs; antigen needs to be processed by phagocytic cells, primarily
dendritic cells, which also express the processed antigen on their surfaces and
present the antigen to lymphocytes. Thus begins a symphony of cellular
interactions, referred to as clonal selection, involving several subsets of B and T
cells, intercellular adhesion through antigen receptors and specific intercellular
adhesion molecules, the production and response to multiple cytokines, and eventual
differentiation of immunocompetent B and T cells into highly specialized effector
cells. B cells develop into plasma cells that become factories for the production of
antibody. T cells develop into several subsets that can identify and kill a target cell
(T-cytotoxic cells, Tc cells), regulate the immune response by helping the clonal
selection process (T-helper cells, Th cells), or suppress inappropriate immune
responses (T-regulatory cells, Treg cells). Both B and T cells also differentiate into
very long-lived memory cells that exist for decades or, in some cases, for the life of
the individual. Memory cells are rapidly activated if a second infection occurs with
the same microbe.
Antibodies circulate in the blood and defend against extracellular microbes and
microbial toxins. This is referred to as the humoral immune response, or humoral
immunity. Effector T cells are found in the blood and tissues and defend against
intracellular pathogens (e.g., viruses) and cancer cells. This is referred to as the
cellular immune response, or cellular immunity (also cell-mediated immunity).
The preceding overview describes what is termed active immunity (active
acquired immunity), which develops in response to antigen. In certain clinical
situations, preformed antibody or lymphocytes may be administered to an
individual, termed passive immunity (passive acquired immunity). Examples
include individuals exposed to an infectious agent without having a preexisting
vaccine-induced immunity (e.g., hepatitis A virus or rabies virus) (Table 7-1).
Passive immunization with specific T cells has been used to treat several forms of
cancer. Whereas active acquired immunity is long lived, passive immunity is only
temporary because the donor's antibodies or T cells are eventually destroyed.
TABLE 7-1
Clinical Use of Antigen or Antibody
USE OF ANTIGEN OR ANTIBODY
Antigen
Source
Protection: Combat Active Disease Protection: Vaccination Diagnosis Therapy
Infectious
agents
Neutralize or destroy pathogenic
microorganisms (e.g., antibody response
against viral infections)
Induce safe and protective
immune response (e.g.,
recommended childhood
vaccines)
Measure circulating antigen
from infectious agent or
antibody (e.g., diagnosis of
hepatitis B infection)
Passive treatment with antibody to
treat or prevent infection (e.g.,
administration of antibody against
hepatitis A)
Cancers Prevent tumor growth or spread (e.g.,
immune surveillance to prevent early
cancers)
Prevent cancer growth or
spread (e.g., vaccination
with cancer antigens)
Measure circulating antigen
(e.g., circulating PSA for
diagnosis of prostate cancer)
Immunotherapy (e.g., treatment of
cancer with antibodies against
cancer antigens)
Environmental
substances
Prevent entrance into body (e.g., secretory
IgA limits systemic exposure to potential
allergens)
No clear example Measure circulating antigen or
antibody (e.g., diagnosis of
allergy by measuring
circulating IgE)
Immunotherapy (e.g.,
administration of antigen for
desensitization of individuals with
severe allergies)
Self-antigens Immune system tolerance to self-
antigens, which may be altered by an
infectious agent leading to autoimmune
disease (see Chapter 8)
Some cases of vaccination
alter tolerance to self-
antigens, leading to
autoimmune disease
Measure circulating antibody
against self-antigen for
diagnosis of autoimmune
disease (see Chapter 8)
Oral administration of self-
antigens to diminish production of
autoimmune disease–associated
autoantibodies
PSA, Prostate-specific antigen.
Antigens and Immunogens
We need to initially understand the molecules against which an immune response is
directed. Although the terms antigen and immunogen are commonly used as
synonyms, there are clinically important differences between the two. Antigen is
commonly used to describe a molecule that can bind with antibodies or antigen
receptors on B and T cells. A molecule that will induce an immune response is an
immunogen. Thus all immunogens are antigens but not all antigens are
immunogens. For instance, immunogenicity is frequently related to the size of the
antigen. In general, large molecules (those greater than 10,000 daltons), such as
proteins and polysaccharides, are most immunogenic. Many low-molecular-weight
molecules can function as haptens; they are too small to be immunogens by
themselves but become immunogenic after combining with larger molecules that
function as carriers for the hapten. Poison ivy contains an oily sap called urushiol
(molecular weight approximately 1500 daltons), which upon contact with the skin is
chemically altered, binds to large proteins in the skin, and becomes immunogenic,
resulting in a T-cell response and onset of a classic poison ivy rash. Similar
conditions will be discussed in Chapter 8.
Quick Check 7-1
1. Define acquired immunity.
2. Distinguish between innate and acquired immunity.
3. Distinguish between humoral and cell-mediated immunity.
4. What are the differences among antigens, immunogens, and haptens?
Antibodies
A basic understanding of antibodies and how they react with antigen provides a
foundation for more complex topics, such as the B-cell and T-cell antigen receptors,
the generation of clonal diversity, and intercellular collaborations during clonal
selection, which are discussed later in this chapter. The terms antibody and
immunoglobulin (Ig) are frequently used interchangeably. In general,
immunoglobulin is frequently used as a generic description of a general group of
antibodies, whereas antibody commonly denotes one particular set of
immunoglobulins known to have specificity for a particular antigen.
Classes of Immunoglobulins
There are five classes of immunoglobulins (IgG, IgA, IgM, IgE, and IgD), which
are characterized by differences in structure and function (Figure 7-4). Both IgG
and IgA have subclasses (Table 7-2).
FIGURE 7-4 Structures of Different Immunoglobulins. Secretory IgA, IgD, IgE, IgG, and IgM. The
black circles attached to each molecule represent carbohydrate residues.
TABLE 7-2
Properties of Immunoglobulins
Class Subclass Adult Serum Levels
(mg/dl)
Present in
Secretions
Complement
Activation
Opsonin Agglutinin Mast Cell
Activation
Placental
Transfer
IgG IgG1 800-900 + ++ ++ + − +++
IgG2 280-300 + + − + − +
IgG3 90-100 + +++ ++ + − +++
IgG4 50 − − − + + ++
IgM 120-150 + ++++ − ++++ − −
IgA IgA1 280-300 + − − + − −
IgA2 50 + − − + − −
sIgA 5 ++++ − − + − −
IgD 3 − − − − − −
IgE 0.03 + − − − +++ −
sIgA, Secretory immunoglobulin A; − indicates lack of activity; + to ++++ indicate relative activity or
concentration.
IgG is the most abundant class of immunoglobulins, constituting 80% to 85% of
the immunoglobulins in the blood and accounting for most of the protective activity
against infections. During pregnancy maternal IgG is transported across the
placenta and protects the newborn child during the first 6 months of life.
IgA is found in the blood and in bodily secretions as secretory IgA (subclass
IgA2). Secretory IgA is a dimer consisting of two IgA2 molecules held together
through a J chain and secretory piece. The secretory piece is attached to dimeric
IgA during transportation through mucosal epithelial cells to protect against
degradation by enzymes also found in secretions.
IgM is the largest immunoglobulin and usually exists as a pentamer (a molecule
consisting of five identical smaller molecules) that is stabilized by a J chain. It is the
first antibody produced during the initial, or primary, response to antigens. IgM is
usually synthesized early in neonatal life, but may be increased as a response to
infection in utero.
IgD is found in low concentrations in the blood. Its primary function is as an
antigen receptor on the surface of early B cells.
IgE is normally at low concentrations in the circulation. It has very specialized
functions as a mediator of many common allergic responses (see Chapter 8) and in
the defense against parasitic infections.1
Molecular Structure
There are three parts to an antibody molecule (Figure 7-5). Two identical fragments
have the ability to bind antigen and are termed antigen-binding fragments (Fab).
The third fragment is termed the crystalline fragment (Fc). The Fab portions
contain the recognition sites (receptors) for antigens and confer the molecule’s
specificity toward a particular antigen. The Fc portion is responsible for most of the
biologic functions of antibodies.
FIGURE 7-5 Antigen-Antibody Binding. CH, Constant region heavy chain; VL, Variable region light
chain; VH, Variable region heavy chain; CL, Constant region light chain; Fab, Fragment antigen
binding; Fc, Crystalline fragment; CDR's, Complementary determining regions; FR's, Framework
regions; Red lines are disulfide linkages.
An immunoglobulin molecule consists of four polypeptide chains: two identical
light (L) chains and two identical heavy (H) chains. The class of antibody is
determined by different amino acid sequences in the heavy chains. The light and
heavy chains are held together by noncovalent bonds and covalent disulfide
linkages. A set of disulfide bridges between the heavy chains occurs in the hinge
region and, in some instances, lends a degree of flexibility at that site.
Each L and H chain is further subdivided structurally into constant (C) and
variable (V) regions. The constant regions have relatively stable amino acid
sequences within a particular immunoglobulin class.Conversely, among different
antibodies, the sequences of the variable regions have a large number of amino acid
differences and these are called complementary determining regions (CDR). They
determine the specificity of an antibody for a particular antigen. The regions
between the CDR’s are called framework regions (FR) and they have more stable
amino acid sequences (see Figure 7-5).
Antigen-Antibody Binding
Because antigens are relative small, a large molecule (e.g., protein, polysaccharide,
nucleic acid) usually contains multiple and diverse antigens. The precise area of the
antigen that is recognized by an antibody is called its antigenic determinant, or
epitope. The matching portion on the antibody is sometimes referred to as the
antigen-binding site, or paratope. The antigen fits into the antigen binding site of
the antibody with the specificity of a key into a lock and is held there by noncovalent
chemical interactions.
Function of Antibodies
The chief function of antibodies is to protect against infection. The mechanism can
be either direct—through the action of antibody alone or indirect—requiring
activation of other components of the innate immune response (Figure 7-6).
Directly, antibodies can affect infectious agents or their toxic products by
neutralization (inactivating or blocking the binding of antigens to receptors),
agglutination (clumping insoluble particles that are in suspension), or
precipitation (making a soluble antigen into an insoluble precipitate). For instance,
many pathogens initiate infection by attaching to specific receptors on cells. Viruses
that cause the common cold or the influenza virus must attach to specific receptors
on respiratory tract epithelial cells. Some bacteria, such as Neisseria gonorrhoeae
that causes gonorrhea, must attach to specific sites on urogenital epithelial cells.
Antibodies may protect the host by covering sites on the microorganism that are
needed for attachment, thereby preventing infection. Many viral infections can be
prevented by vaccination with inactivated or attenuated (weakened) viruses designed
to induce neutralizing antibody production at the site of the entrance of the virus
into the body. Vaccination against influenza using an inhaled vaccine particularly
induces protective IgA in the respiratory tract.
FIGURE 7-6 Direct and Indirect Functions of Antibody. Protective activities of antibodies can be
direct (through the action of antibody alone) or indirect (requiring activation of other
components of the innate immune response, usually through the Fc region). Direct means
include neutralization of viruses or bacterial toxins before they bind to receptors on the surface
of the host's cells. Indirect means include activation of the classical complement pathway
through C1, resulting in formation of the membrane-attack complex (MAC), or increased
phagocytosis of bacteria opsonized with antibody and complement components bound to
appropriate surface receptors (FcR and C3bR).
Some bacteria secrete toxins that harm individuals. For instance, specific bacterial
toxins cause the symptoms of tetanus or diphtheria. Most toxins are proteins that
bind to surface molecules on cells and damage those cells. Protective antibodies
produced against the toxin (referred to as antitoxins) can bind to the toxins, prevent
their interaction with host cells, and neutralize their biologic effects (see Chapter 8).
Indirectly, through the Fc portion, antibodies activate components of innate
resistance, including complement and phagocytes (Figure 7-7). Through the
classical pathway, complement component C1 will be activated by binding
simultaneously to the Fc regions of two adjacent antibodies bound to a microbe,
resulting in activation of the entire cascade. Phagocytic cells express receptors that
bind the Fc portion of antibody; thus antibody is an opsonin that facilitates
phagocytosis of bacteria.3 IgM is the best complement-activating antibody, and IgG
is the best opsonin. Some antibodies are more protective than others. It is now a
common procedure to clone the “best” antibodies (monoclonal antibodies) for use
in diagnostic tests and for therapy (Box 7-1).
Box 7-1
Monoclonal Antibodies
Most humoral immune responses are polyclonal—that is, a mixture of antibodies
produced from multiple B lymphocytes. Most antigenic molecules have multiple
antigenic determinants, each of which induces a different group of antibodies.
Thus, a polyclonal response is a mixture of antibody classes, specificities, and
function, some of which are more protective than others.
Monoclonal antibody is produced in the laboratory from one B cell that has been
cloned; thus the entire antibody is of the same class, specificity, and function. The
advantages of monoclonal antibodies are that (1) a single antibody of known
antigenic specificity is generated rather than a mixture of different antibodies; (2)
monoclonal antibodies have a single, constant binding affinity; (3) monoclonal
antibodies can be diluted to a constant titer (concentration in fluid) because the
actual antibody concentration is known; and (4) the antibody can be easily purified.
Thus, a highly concentrated antibody with optimal function has been used to
develop extremely specific and sensitive laboratory tests (e.g., home and laboratory
pregnancy tests) and therapies (e.g., for certain infectious diseases or several
experimental therapies for cancer).
IgE
IgE is a special class of antibody that protects the individual from infection with
large parasitic worms (helminths).4 However, when IgE is produced against
relatively innocuous environmental antigens, it is also the primary cause of
common allergies (e.g., hay fever, dust allergies, bee stings). The role of IgE in
allergies is discussed in Chapter 8.
Large multicellular parasites usually invade mucosal tissues. Many antigens from
the parasites induce IgE, as well as other antibody classes. IgG, IgM, and IgA bind to
the surface of parasites, activate complement, generate chemotactic factors for
neutrophils and macrophages, and serve as opsonins for those phagocytic cells.
This response, however, does not greatly damage parasites. The only inflammatory
cell that can adequately damage a parasite is the eosinophil because of the special
contents of its granules, including major basic protein, eosinophil cationic protein,
eosinophil peroxidase, and eosinophil neurotoxin, each of which can damage
infectious worms. Thus, IgE is designed to specifically initiate an inflammatory
reaction that preferentially attracts eosinophils to the site of parasitic infection.
Mast cells in the tissues have Fc receptors that specifically and with high affinity
bind IgE. IgE antibodies against antigens of the parasite are rapidly bound to the
mast cell surface. Soluble parasite molecules with multiple antigenic determinants
diffuse to neighboring mast cells and simultaneously bind to multiple IgE
molecules. This reaction initiates a cascade of effects that can ultimately kill the
parasite. The steps of the cascade are presented in Figure 7-7.
FIGURE 7-7 IgE Function. (1) Soluble antigens from a parasitic infection cause production of IgE
antibody by B cells. (2) Secreted IgE binds to IgE-specific receptors on the mast cell. (3)
Additional soluble parasite antigen cross-links the IgE on the mast cell surface, (4) leading to
mast cell degranulation and release of many proinflammatory products, including eosinophil
chemotactic factor of anaphylaxis (ECF-A). (5) ECF-A attracts eosinophils from the circulation.
(6) The eosinophil attaches to the surface of the parasite and releases potent lysosomal
enzymes that damage microorganisms.
Secretory Immune System
Immunocompetent lymphocytes migrate among secondary lymphoid organs and
tissue as part of the systemic immune system. Another, partially independent,
immune system protects the external surfaces of the body through lacrimal and
salivary glands and a network of lymphoid tissues residing in the breasts, bronchi,
intestines, and genitourinary tract. This system is called the secretory (mucosal)
immune system (Figure 7-8). Plasma cells in those sites secrete antibodies in bodily
secretions such as tears, sweat, saliva, mucus, and breast milk to prevent pathogenic
microorganism from infecting the body's surfaces and possibly penetrating to cause
systemic disease.5 Alternatively, the microorganisms may reside in the membranes
without causing disease, be shed, and cause infection for other individuals. Thus, an
individual may become a carrier for a particular infectious organism. For instance,
in the 1950s two vaccines were developed to prevent infection with poliovirus,
which enters through the gastrointestinal tract. The Sabin vaccine was administered
orally as an attenuated (i.e., inactivated so as to render relatively harmless) live
virus. This route caused a transient, limited infection and induced effective systemic
and secretory immunity that prevented both the disease and the establishment of a
carrier state. The Salk vaccine, on the other hand, consisted of killed viruses
administered by injection in the skin. It induced adequate systemic protection but did
not generally prevent an intestinal carrier state. Thus, recipients of the Salk vaccine
were protected from disease but could still shed the virus and infect others.
FIGURE 7-8 Secretory Immune System. A, Lymphocytes from the mucosal-associated
lymphoid tissues circulate throughout the body in a pattern separate from other lymphocytes.
For example, lymphocytes from the gut-associated lymphoid tissue circulate through the
regional lymph nodes, the thoracic duct, and the blood and return to other mucosal-associated
lymphoid tissues rather than to lymphoid tissue of the systemic immune system. B, Lymphoid
tissue associated with mucous membranes is called mucosal-associated lymphoid tissue
(MALT).
IgA is the dominant secretory immunoglobulin, although IgM and IgG also are
present in secretions. The primary role of IgA is to prevent the attachment and
invasion of pathogens through mucosal membranes, such as those of the
gastrointestinal, pulmonary, and genitourinary tracts. Dimeric IgA antibodies
containing the J chain are produced by plasma cells of the mucosa. Mucosal
epithelium expresses a cell surface immunoglobulin receptor that binds and
internalizes IgA. The IgA, along with the epithelial receptor (secretory piece), is
secreted as secretory IgA (sIgA).
The lymphoid tissues of the secretory immune system are connected; thus many
foreign antigens in a mother's gastrointestinal tract (e.g., polio virus) induce
secretion of specific antibodies into the breast milk. Colostral antibodies (i.e., those
found in the colostrum of breast milk) may protect the nursing newborn against
infectious disease agents that enter through the gastrointestinal tract. Although
colostral antibodies provide the newborn with passive immunity against
gastrointestinal infections, they do not provide systemic immunity because transport
across the newborn's gut into the bloodstream is discontinued after the first 24 hours
of life. Maternal antibodies that pass across the placenta into the fetus before birth
provide passive systemic immunity.
Immune Response: Collaboration of B Cells
and T Cells
Generation of Clonal Diversity
The immune response occurs in two phases: generation of clonal diversity and
clonal selection (Table 7-3 and see Figure 7-2). Clonal diversity is the production
of a large population of B cells and T cells before birth that have the capacity to
recognize almost any foreign antigen found in the environment. This process
mostly occurs in specialized lymphoid organs (the primary [central] lymphoid
organs): the bone marrow for B cells and the thymus for T cells.6 The result is the
differentiation of lymphoid stem cells into B and T lymphocytes with the ability to
react against almost any antigen that will be encountered throughout life. It is
estimated that B and T cells can collectively recognize more than 108 different
antigenic determinants. Lymphocytes are released from these organs into the
circulation as immunocompetent cells that have the capacity to react with antigens
and migrate to the circulation and other (secondary) lymphoid organs in the body.
TABLE 7-3
Generation of Clonal Diversity vs. Clonal Selection
Generation of Clonal Diversity Clonal Selection
Purpose? To produce large numbers of T and B lymphocytes with
maximum diversity of antigen receptors
Select, expand, and differentiate clones of T and B cells against specific
antigen
When does it
occur?
Primarily in fetus Primarily after birth and throughout life
Where does it
occur?
Central lymphoid organs: thymus for T cells, bone marrow
for B cells
Peripheral lymphoid organs, including lymph nodes, spleen, and other
lymphoid tissues
Is foreign antigen
involved?
No Yes, antigen determines which clones of cells will be selected
What hormones or
cytokines are
involved?
Thymic hormones, IL-7, others Many cytokines produced by Th cells and APCs
Final product? Immunocompetent T and B cells that can react with antigen,
but have not seen antigen, and migrate to secondary
lymphoid organs
Plasma cells that produce antibody, effector T cells that help (Th cells),
kill targets (Tc cells), or regulate immune responses (Treg cells); memory
B and T cells
APCs, Antigen-presenting cells; Tc, T-cytotoxic cells; Th, T-helper cells; Treg cells, T-regulatory cells.
Development of B Lymphocytes
Lymphocytes destined to become B cells circulate through the specialized regions
of the bone marrow, where they are exposed to hormones and cytokines that induce
proliferation and differentiation into B cells (see Figure 7-2). Lymphoid stem cells
in the bone marrow interact with stromal cells through a variety of intercellular
adhesion molecules. As the stem cell begins to mature, it progressively develops a
variety of necessary surface markers important for the further differentiation and
proliferation of the B cell.7 The next stage in development is formation of the B-cell
receptor (BCR).
The B-cell receptor (BCR) is a complex of antibody bound to the cell surface
and other molecules involved in intracellular signaling (Figure 7-9). Its role is to
recognize an antigen and communicate that information to the cell's nucleus. The
BCRs in immunocompetent cells are membrane-associated IgM (mIgM) and IgD
(mIgD) immunoglobulins that have identical specificities for antigen. The mIgM is
a monomer rather than the pentamer primarily found in the blood.
FIGURE 7-9 B-cell Antigen Receptor and T-cell Antigen Receptor. A, The antigen receptor on the
surface of B cells (BCR complex) is a monomeric (single) antibody with a structure similar to
that of circulating antibody, with an additional transmembrane region (TM) that anchors the
molecule to the cell surface. The active BCR complex contains molecules (Igα and Igβ) that are
responsible for intracellular signaling after the receptor has bound antigen. B, The T-cell
receptor (TCR) consists of an α- and a β-chain joined by a disulfide bond. Each chain consists of
a constant region (Cα and Cβ) and a variable region (Vα and Vβ). Each variable region contains
CDRs and FRs in a structure similar to that of antibody. The active TCR is associated with
several molecules that are responsible for intracellular signaling after antigen binding. These
include the CD3, which is a complex of γ (gamma), ξ (epsilon), and δ (delta) subunits and a
complex of two ζ (zeta) molecules. The ζ molecules are attached to a cytoplasmic protein
kinase (ZAP70) that is critical to intracellular signaling.
As described previously, the variable regions of antibodies, as well as the BCR,
contain CDR areas. The diversity of these CDRs is responsible for the variety of
antigens that can be recognized by immunocompetent B cells.8 The enormous
repertoire of specificities is made possible by rearrangement of existing DNA
during B-cell development in the primary lymphoid organs, a process known as
somatic recombination. Multiple loci in the DNA that encode for the variable
regions of immunoglobulins are recombined to generate receptors that collectively
can recognize and bind to any possible antigen.8 To create the variable region of a
light chain, different regions are rearranged using enzymes encoded by
recombination activating genes (RAG-1, RAG-2). The DNA is cut and spliced
(repaired) so that after this manipulation, the progeny of a single lymphocyte will
synthesize immunoglobulins with identical variable regions. Those variable
regions, however, are cut and spliced differently from those of another lymphocyte,
making each cell unique and therefore able to react with different antigens. The
gene for the H chain undergoes similar rearrangement.
Somatic rearrangement of the variable regions will frequently result in a BCR
that recognizes the individual's own antigens, which may result in inadvertent attack
on “self” antigens expressed on various tissue and organs causing autoimmune
disease or hypersensitivites. Many of these “autoreactive” B cells are eliminated in
the bone marrow. It is estimated that more than 90% of developing B cells are
induced to undergo apoptosis. This process is referred to as central tolerance, so
that resultant immunocompetent B cells are against foreign antigens and “tolerant”
to self-antigens. The process of peripheral tolerance is discussed on p. 173.
B-cell differentiation also is characterized by the development of a variety of
important surface molecules that are markers for B cells. These include CD21 (a
complement receptor) and CD40 (adhesion molecule required for later interactions
with T cells).
Development of T Lymphocytes
The process of T-cell proliferation and differentiation is similar to that for B cells
(see Figure 7-2). The primary lymphoid organ for T-cell development is the
thymus.9 Lymphoid stem cells journey through the thymus, where, under influence
of thymic hormones and the cytokine IL-7, they are driven to undergo cell division
and simultaneously produce receptors (T-cell receptors [TCRs]) against the
diversity of antigens the individual will encounter throughout life. They exit the
thymus through the blood vessels and lymphatics as mature (immunocompetent) T
cells with antigen-specific receptors on the cell surface and establish residence in
secondary lymphoid organs.
Production of the TCR proceeds in a manner very similar to that described
earlier for B cells. The most common TCR resembles an antibody Fab region and
consists of two protein chains, α- and β-chains, each of which has a variable region
and a constant region (see Figure 7-9). The variable regions also undergo somatic
recombination. As with the BCR, a set of intracellular signaling molecules co-
assemble in the membrane with the TCR. The complex of these signaling molecules
is called CD3.10 Thus, all immunocompetent T cells can be identified by the
presence of CD3 on the surface.
Differentiation of T cells in the thymus also results in expression in a variety of
other important surface molecules. Initially, proteins called CD4 and CD8 are
concurrently expressed on the developing cells. CD4 cells develop into T-helper
cells (Th cells), whereas CD8 cells become T-cytotoxic cells (Tc cells).
Approximately 60% of immunocompetent T cells in the circulation express CD4
and 40% express CD8.
Central tolerance also occurs in the thymus where more than 95% of developing
T cells are deleted. Like B-cells, T-cells can also become autoreactive.
Quick Check 7-2
1. What are the major functions of antibodies?
2. What is the difference between the secretory and systemic immune systems?
3. What are the different types of T cells, and what function does each have?
Clonal Selection
Antigens initiate the second phase of the immune response, clonal selection. Clonal
selection is the processing of antigen for a specific immune response. This process
involves a complex interaction among cells in the secondary lymphoid organs (see
Figure 7-2). To initiate an effective immune response, most antigens must be
processed because they cannot react directly with most cells of the immune system
and must be shown or presented to the immune cells in a specific manner. This is the
job of antigen-processing (antigen-presenting) cells (usually dendritic cells,
macrophages, or similar cells), generally referred to as APCs. The interaction
among APCs, subpopulations of T cells that facilitate immune responses (T-helper
[Th] cells), and immunocompetent B or T cells results in differentiation of B cells
into active antibody-producing cells (plasma cells) and T cells into effector cells,
such as T-cytotoxic cells. Both lines also develop into memory cells that respond
even faster when that antigen enters the body again. Thus, activation of the immune
system produces a long-lasting protection against specific antigens (see Figure 7-2).
Defects in any aspect of cellular collaboration will lead to defects in cell-mediated
immunity, humoral immunity, or both and, depending on the particular defect,
potentially the individual's death from infection (see Chapter 8).
Primary and Secondary Immune Responses
The immune response to antigen has classically been divided into two phases—the
primary and secondary responses—that are most easily demonstrated by measuring
concentrations of circulating antibodies over time (Figure 7-10). After a single
initial exposure to most antigens, there is a latent period, or lag phase, during which
clonal selection occurs. After approximately 5 to 7 days, IgM antibody is detected in
the circulation. This is the primary immune response, characterized typically by
initial production of IgM followed by production of IgG against the same antigen.
The quantity of IgG may be about equal to or less than the amount of IgM. The
amount of antibody in a serum sample is frequently referred to as the titer; a higher
titer indicates more antibodies. If no further exposure to the antigen occurs, the
circulating antibody is catabolized (broken down) and measurable quantities fall.
The individual's immune system, however, has been primed.
FIGURE 7-10 Primary and Secondary Immune Responses. The initial administration of antigen
induces a primary response during which IgM is initially produced, followed by IgG. Another
administration of the antigen induces the secondary response in which IgM is transiently
produced and larger amounts of IgG are produced over a longer period of time.
A second challenge by the same antigen results in the secondary immune
response, which is characterized by the more rapid production of a larger amount
of antibody than the primary response. The rapidity of the secondary immune
response is the result of memory cells that require less further differentiation. IgM
may be transiently produced in the secondary response, but IgG production is
increased considerably, making it the predominant antibody class. Natural infection
(e.g., rubella) may result in measurable levels of protective IgG for the life of the
individual. Some vaccines (e.g., polio) also may produce extremely long-lived
protection, although most vaccines require boosters at specified intervals.
Antigen Processing and Presentation
For most antigens, the first step in clonal selection is processing and presentation by
APCs. Antigens are usually expressed on large molecules found on microbes, which
undergo phagocytosis and destruction by dendritic cells and macrophages. These
are referred to as exogenous antigens. Other antigens, endogenous antigens,
originate within a cell that has been infected by a virus or has become cancerous.
Processing results in the release of small antigenic determinants, which are
presented on the surface of APCs by specialized molecules, molecules of the major
histocompatibility complex (MHC). MHC molecules in humans also are called
human leukocyte antigens (HLA) (discussed in more detail in Chapter 8) and are
related to their role in transplantation. Major histocompatibility complex (MHC)
molecules are glycoproteins found on the surface of all human cells except red
blood cells. They are divided into two general classes, class I and class II, based on
their molecular structure, distribution among cell populations, and function in
antigen presentation. MHC class I molecules are composed of a large alpha (α)
chain along with a smaller chain called β2-microglobulin. MHC class II molecules
are composed of α- and β-chains that differ from the ones used for MHC class I.
The α- and β-chains of the MHC molecules are encoded from different genetic loci
located as a large complex of genes on human chromosome 6 (Figure 7-11). MHC
genes are probably the most polymorphic of any human genes; therefore, no two
individuals, except identical twins, will have a complete set of identical MHC
molecules.
FIGURE 7-11 Antigen-Presenting Molecules.
MHC class I molecules present endogenous antigens, which are primarily
recognized by T-cytotoxic (Tc) cells. Because MHC class I molecules are expressed
on all cells, except red blood cells, any change in that cell caused by viral infection
or malignancy may result in foreign antigens being presented. MHC class II
molecules present exogenous antigens (Figure 7-12). Antigen presented by MHC
class II molecules is preferentially recognized by T-helper (Th) cells. Thus, antigen
presentation to Tc cells is MHC class I restricted and presentation to Th cells is
MHC class II restricted. MHC class II molecules are co-expressed with MHC class I
molecules on a limited number of cells that have APC function, including
macrophages, dendritic cells, and B lymphocytes.
FIGURE 7-12 Antigen Processing. Antigen processing and presentation are required for
initiation of most immune responses. Foreign antigen may be either endogenous (cytoplasmic
protein) or exogenous (e.g., bacterium). Endogenous antigenic peptides are transported into the
endoplasmic reticulum (ER) (1), where the MHC molecules are being assembled. In the ER,
antigenic peptides bind to the α-chains of the MHC class I molecule (2), and the complex is
transported to the cell surface (3). The α- and β-chains of the MHC class II molecules are also
being assembled in the endoplasmic reticulum (4), but the antigen-binding site is blocked by a
small molecule (invariant chain) to prevent interactions with endogenous antigenic peptides.
The MHC class II–invariant chain complex is transported to phagolysosomes (5), where
exogenous antigenic fragments have been produced as a result of phagocytosis (6). In the
phagolysosomes, the invariant chain is digested and replaced by exogenous antigenic peptides
(7), after which the MHC class II–antigen complex is inserted into the cell membrane (8).
Thus, the term antigen processing relates to the process by which large
exogenous and endogenous antigens are cut up by enzymes into small antigenic
fragments that are linked with the appropriate MHC molecules and inserted into the
membrane of the APC.11 Lipid antigens are frequently presented by a molecule
unrelated to the MHC, CD1, which is not discussed here.
Cellular Interactions in the Immune Response
The second step in clonal selection is a finally tuned set of intercellular
collaborations that result in the production of effector cells (plasma cells, Th cells,
Tc cells) and memory cells.12 Each collaboration requires three complementary
intracellular signaling events: antigen-specific recognition through the TCR
complex, activation of intercellular adhesion molecules, and the response to
specific groups of cytokines. Without each signaling event, a protective immune
response will not be produced.
T-helper lymphocytes.
Regardless of whether an antigen primarily induces a cellular or humoral immune
response, APCs usually must present antigens to T-helper cells (Th cells). The APC
presents antigen held by the polymorphic regions (α1 and β1) of the α- and β-chains
of MHC class II molecules.13 The antigen also binds to the TCR on the Th cell (see
Figure 7-9). The strength of the intercellular antigen binding is increased by CD4 on
the Th cell, which binds to a nonpolymorphic region of the β2 region of the MHC
class II molecule. The cytoplasmic portions of CD3 and CD4 interact to activate
intracellular signaling pathways. A second co-stimulatory signal results from the
interaction of a variety of adhesion molecules; the most critical being B7 on the
APC and CD28 on the Th cell.
The third signal occurs through Th-cell cytokine receptors. In the early stages of
Th-cell differentiation, IL-1 secreted by the APC provides this signal through the IL-
1 receptor on the Th cell (Figure 7-13). The initial differentiation response by the
Th cell includes the production of the cytokine IL-2 and up-regulation of IL-2
receptors. IL-2 is secreted and acts in an autocrine (self-stimulating) fashion to
induce further maturation and proliferation of the Th cell. Without IL-2 production,
the Th cell cannot efficiently mature into a functional helper cell.
FIGURE 7-13 Development of T-Cell Subsets. The most important step in clonal selection is the
production of populations of T-helper (Th) cells (Th1, Th2, and Th17) and T-regulatory (Treg) cells
that are necessary for the development of cellular and humoral immune responses. In this
model, APCs (1) (probably multiple populations) may influence whether a precursor Th cell (Thp
cell) (2) will differentiate into a Th1, Th2, Th17, or Treg cell (3). Differentiation of the Thp cell is
initiated by three signaling events. The antigen signal is produced by the interaction of the T-cell
receptor (TCR) and CD4 with antigen presented by MHC class II molecules. A set of co-
stimulatory signals is produced from interactions between adhesion molecules (not shown). A
third signal is produced by the interactions of cytokines (particularly interleukin-1 [IL-1]) with
appropriate cytokine receptors (IL-1R) on the Thp cell. The Thp cell up-regulates IL-2 production
and expression of the IL-2 receptor (IL-2R), which acts in an autocrine fashion to accelerate Thp
cell differentiation and proliferation. Commitment to a particular phenotype results from the
relative concentrations of other cytokines. IL-12 and IFN-γ produced by some populations of
APCs favor differentiation into the Th1 cell phenotype; IL-4, which is produced by a variety of
cells, favors differentiation into the Th2 cell phenotype; IL-6 and TGF-β (T-cell growth factor)
facilitate differentiation into Th17 cells; IL-2 and TGF-β induce differentiation into Treg cells. The
Th1 cell is characterized by the production of cytokines that assist in the differentiation of T-
cytotoxic (Tc) cells, leading to cellular immunity, whereas the Th2 cell produces cytokines that
favor B-cell differentiation and humoral immunity. Th1 and Th2 cells affect each other through
the production of inhibitory cytokines: IFN-γ will inhibit development of Th2 cells, and IL-4 will
inhibit the development of Th1 cells. Th17 cells produce cytokines that affect phagocytes and
increase inflammation. Treg cells produce immunosuppressive cytokines that prevent the
immune response from being excessive. APC, Antigen-presenting cell; IFN, interferon; MHC,
major histocompatibility complex; TGF, transforming growth factor.
At this point and depending on the predominant cytokines in the immediate
environment, Th cells undergo differentiation into one of several subsets: Th1, Th2,
Th17, or Treg cells.14 These subsets have different functions: Th1 cells
preferentially provide help in developing Tc cells (cell-mediated immunity), Th2
cells provide more help for developing B cells (humoral immunity), Th17 cells are
lymphokine-secreting cells that activate macrophages, and Treg cells limit the
immune response (these will discussed later in this chapter).15 The Th subsets differ
considerably in the spectrum of cytokines they produce. Additionally, Th1 and Th2
cells may suppress each other so that the immune response may favor either
antibody formation, with suppression of a cell-mediated response, or the opposite.
For example, antigens derived from viral or bacterial pathogens and those derived
from cancer cells seem to induce a greater number of Th1 cells relative to Th2
cells, whereas antigens derived from multicellular parasites and allergens may
result in production of more Th2 cells. Many antigens (e.g., tetanus vaccine),
however, will produce excellent humoral and cell-mediated responses
simultaneously. Th cells are necessary for development of most humoral and
cellular immune responses; therefore the virus that causes acquired immune
deficiency syndrome (AIDS) results in life-threatening infections because it
specifically infects and destroys Th cells (see Chapter 8).
Superantigens.
Several pathogenic microorganisms, particularly viruses and bacteria, manipulate
the normal interaction between APCs and Th cells to the detriment of the individual
and the benefit of the microbe. A group of microbial molecules are called
superantigens (SAGs). SAGs bind to the portion of the TCR outside of its normal
antigen-specific binding site, as well as to MHC class II molecules outside of their
antigen-presentation sites (Figure 7-14). Some SAGs also react with CD28 on the Th
cells and provide a co-stimulatory signal. Thus, SAGs are not processed by an APC
to be presented to an immune cell. This binding, which is independent of antigen
recognition, provides a signal for Th-cell activation, proliferation, and cytokine
production. The normal antigen-specific recognition between Th cells and APCs
results in activation of relatively few cells—only those cells with specific TCRs
against that antigen. SAGs activate a large population of Th cells, regardless of
antigen specificity, and induce excessive production of cytokines, including IL-2,
interferon gamma (IFN-γ), and tumor necrosis factor-alpha (TNF-α). The
overproduction of inflammatory cytokines results in symptoms of a systemic
inflammatory reaction, including fever, low blood pressure, and, potentially, fatal
shock. Some examples of SAGs are the bacterial toxins produced by Staphylococcus
aureus and Streptococcus pyogenes (SAGs that cause toxic shock syndrome and
food poisoning).16
FIGURE 7-14 Superantigens. The T-cell receptor (TCR) and major histocompatibility complex
(MHC) class II molecule are normally held together by processed antigen. Superantigens, such
as some bacterial exotoxins, bind directly to the variable region of the TCR β-chain and the MHC
class II molecule. Each superantigen activates sets of Vβ chains independently of the antigen
specificity of the TCR.
T-cytotoxic lymphocytes.
The differentiation of immunocompetent T cells into effector T-cytotoxic cells (Tc
cells) requires similar intercellular communications as described for Th cells, with
some very important differences. Rather than interacting with an APC, the
immunocompetent Tc cell recognizes antigen presented by MHC class I molecules
on the surface of a virus-infected cell or cancerous cell (Figure 7-15). The Tc cell
expresses CD8, rather than CD4. CD8 binds to the MHC class I molecule and, as
with Th cell differentiation, the proximity of the CD3 and CD8 cytoplasmic portions
activates intercellular signaling pathways. Cytokine signals, especially IL-2, are
produced by Th1 cells and activate cytokine receptors on the Tc cells.
FIGURE 7-15 Tc-Cell Clonal Selection. The immunocompetent Tc cell can react with antigen but
cannot yet kill target cells. During clonal selection, this cell reacts with antigen presented by
MHC class I molecules on the surface of a virally infected or cancerous abnormal cell. (1) The
antigen–MHC class I complex is recognized simultaneously by the T-cell receptor (TCR), which
binds to antigen, and CD8, which binds to the MHC class I molecule. (2) A separate signal is
provided by cytokines, particularly IL-2 from Th1 cells. (3) In response to these signals, the Tc
cell develops into an effector Tc cell with the ability to kill abnormal cells.
B-cell clonal selection.
A further sequence of cellular interactions is required to produce an effective
antibody response. The immunocompetent B cell is also an APC and expresses
surface mIgM and mIgD B-cell receptors (BCRs) (Figure 7-16). Unlike the T-cell
receptor that can only see processed and presented antigens, the BCR can react with
soluble antigens that have not been processed. B cells also express surface CD21,
which is a receptor for opsonins produced by complement activation. Antigen
binding through the BCR and CD21 activates the B cell, resulting in internalization,
processing, and presentation of antigen fragments by MHC class II molecules.17 The
antigen presented on the B-cell surface is recognized by a Th2 cell through the TCR
and CD4. The intercellular bridges created through antigen and other intercellular
adhesion molecules induce the Th2 cell to secrete cytokines (particularly IL-4) that
initiate B-cell proliferation and maturation into plasma cells.18
FIGURE 7-16 B-Cell Clonal Selection. Immunocompetent B cells undergo proliferation and
differentiation into antibody-secreting plasma cells. Multiple signals are necessary (1). The B
cell itself can directly bind soluble antigen through the B-cell receptor (BCR) and act as an
antigen-processing cell. Antigen is internalized, processed (2), and presented (3) to the TCR on
a Th2 cell by MHC class II molecules (4). A cytokine signal is provided by the Th2 cell cytokines
(e.g., IL-4) that react with the B cell (5). The B cell differentiates into plasma cells that secrete
antibody (6).
A major component of B-cell maturation is class switch, the process that results
in the change in antibody production from one class to another (e.g., IgM to IgG
during the primary immune response). Before exposure to antigens and Th2 cells,
the B cell produces IgM and IgD, which are used as cell membrane receptors.
During the clonal selection process, a B cell proliferates and develops into
antibody-secreting plasma cells, and each B cell has the option of becoming a
secretor of IgM or changing the class of antibody to a secreted form of IgG, IgA, or
IgE. Class switch occurs by another round of somatic recombination with the
variable region of the antibody heavy chain being combined with a different
constant region of the heavy chain. Because the variable region is conserved and the
light chain remains unchanged, the antigenic specificity of the antibody also
remains unchanged. The particular constant region chosen by each cell during class
switch appears to be, at least partially, under the control of specific Th2 cytokines.
For instance, IL-4 and IL-13 appear to preferentially stimulate switch to IgE
secretion, and transforming growth factor-beta (TGF-β) and IL-5 appear to play
major roles in class switch to IgA secretion. Thus, during clonal selection, a B cell
may produce a population of plasma cells that are capable of producing many
different classes of antibodies against the same antigen.
Although most antigens require B cells to interact with Th cells, a few antigens
can bypass the need for cellular interactions and can directly stimulate B-cell
maturation and proliferation. These are called T-cell–independent antigens (Figure
7-17). They are mostly bacterial products that are large and are likely to have
repeating identical antigenic determinants that bind and cross-link several BCRs.
The accumulated intracellular signal is adequate to induce differentiation into a
plasma cell but is not adequate to induce a change in the class of antibody that will
be produced. Therefore, T-cell–independent antigens usually induce relatively pure
IgM primary and secondary immune responses.
FIGURE 7-17 Activation of a B Cell by a T-Cell–Independent Antigen. Molecules containing
repeating identical antigenic determinants may interact simultaneously with several receptors
on the surface of the B cell and induce the proliferation and production of immunoglobulins.
Because Th2 cells do not participate, class switch does not occur and the resultant antibody
response is IgM.
Memory cells.
During the clonal selection process, both B cells and T cells differentiate and
proliferate into an extremely large population of long-lived memory cells.19
Memory cells remain inactive until subsequent exposure to the same antigen. Upon
reexposure, these memory cells do not require much further differentiation and will
therefore rapidly become new plasma cells or effector T cells without the cellular
interactions described previously.
Cell-Mediated Immunity
The rather straightforward function of antibodies has been discussed earlier in this
chapter. The function of effector T cells is more complex and utilizes the principles
of intercellular recognition necessary for clonal selection.
T-Lymphocyte Function
The clonal selection process produces several subsets of effector T cells. Th cells
and T memory cells have already been discussed. Other effector T cells include T-
cytotoxic (Tc) cells that attack and destroy cells expressing antigens from
intracellular (endogenous) origins, T-regulatory cells (Treg) that limit (suppress)
the immune response, and T-lymphokine producing cells that secrete cytokines that
activate other cells.
T-Cytotoxic Lymphocytes
T-cytotoxic (Tc) cells are responsible for the cell-mediated destruction of tumor
cells or cells infected with viruses. In a fashion similar to intercellular recognition
during the clonal selection process, the Tc cell must directly adhere to the target cell
through antigen presented by MHC class I molecules and CD8 (Figure 7-18).
Because of the broad cellular distribution of MHC class I molecules, Tc cells can
recognize antigens on the surface of almost any type of cell that has been infected
by a virus or has become cancerous. Unlike clonal selection, the roles of co-
stimulatory signals through adhesion molecules and cytokines are of less
importance here. Attachment to a target cell activates multiple killing mechanisms
through which the Tc cell induces the target cell to undergo apoptosis.
FIGURE 7-18 Cellular Killing Mechanisms. Several cells have the capacity to kill abnormal (e.g.,
virally infected, cancerous) target cells. (1) T-cytotoxic (Tc) cells recognize endogenous antigen
presented by MHC class I molecules. The Tc cell mobilizes multiple killing mechanisms that
induce apoptosis of the target cell. (2) Natural killer (NK) cells identify and kill target cells
through receptors that recognize abnormal surface changes. NK cells specifically kill targets
that do not express surface MHC class I molecules. (3) Several cells, including macrophages
and NK cells, can kill by antibody-dependent cellular cytotoxicity (ADCC). IgG antibodies bind to
foreign antigen on the target cell, and cells involved in ADCC bind IgG through Fc receptors (FcR)
and initiate killing. The insert is a scanning electron microscopic view of Tc cells (L) attacking a
much larger tumor cell (Tu). (Insert from Abbas A, Lichtman A: Cellular and molecular immunology, ed 5, Philadelphia, 2003,
Saunders.)
Various other cells kill targets in a fashion similar to Tc lymphocytes. Prominent
among these cells are natural killer cells. Natural killer (NK) cells are a special
group of lymphoid cells that are similar to T cells but lack antigen-specific
receptors. Instead, they express a variety of cell surface activation receptors (similar
to pattern recognition receptors, see Chapter 6) that identify protein changes on the
surface of cells infected with viruses or that have become cancerous. After
attachment, the NK cell kills its target in a manner similar to that of Tc cells. NK
cells also have receptors for MHC class I. However, NK cells lack CD8; therefore
binding to MHC class I molecules results in inactivation of the NK cell. Thus, NK
cells complement the effects of Tc cells. In some instances, a virus-infected or
cancerous cell will “protect” itself by down-regulating MHC class I molecule
expression. Without surface MHC class I molecules a cell becomes resistant to Tc-
cell recognition and killing. NK cells primarily kill target cells that have suppressed
the expression of MHC class I.
NK cells, as well as some macrophages, can specifically kill targets through use
of antibodies. NK cells express Fc receptors for IgG. If antigens on the infected or
cancerous cell bind IgG, the NK cell can attach through Fc receptors and activate its
normal killing mechanisms. This is referred to as antibody-dependent cellular
cytotoxicity (ADCC).
Lymphokine-Secreting T Cells
Two subsets of Th cells amplify inflammation. Th1 cells, in addition to assisting Tc-
cell clonal selection, secrete cytokines that activate M1 macrophages to increase
phagocytic and microbial killing functions (described in Chapter 6). The most
important cytokine for macrophage activation is interferon-γ (IFN-γ). Th2 cells, in
addition to assisting B-cell clonal selection, secrete cytokines (e.g., IL-4, IL-13) that
activate M2 macrophages for healing and repair of damaged tissue (described in
Chapter 6). Th17 cells secrete a set of cytokines (e.g., IL-17, IL-22, chemokines) that
recruit phagocytic cells to a site of inflammation.20 Th17-cell cytokines also may
activate cells, particularly epithelial cells, to produce antimicrobial proteins in
defense against certain bacterial and fungal pathogens.
T-Regulatory Lymphocytes
T-regulatory (Treg) cells are a diverse group of T cells that control the immune
response, usually suppressing the response and maintaining tolerance against self-
antigens.21 This process occurs in the secondary lymphoid organs and other tissues,
known as peripheral tolerance, in contrast to the process of central tolerance
described earlier. This population of Treg cells that differentiate from the Th-cell
population expresses CD4 and binds to antigens presented by MHC class II
molecules. Unlike other Th cells, however, Treg cells express consistently high
levels of CD25 (the IL-2 receptor). Differentiation from the Th precursor cell is
controlled, primarily by TGF-β and IL-2. Treg cells produce very high levels of
immunosuppressive cytokines TGF-β and IL-10, which generally decrease Th1 and
Th2 activity by suppressing antigen recognition and Th-cell proliferation.
Quick Check 7-3
1. What are antigen-presenting cells?
2. Define BCR and TCR.
3. What is the role of T-helper cells?
4. Why are cytokines important to the immune response?
5. What is the difference between central tolerance and peripheral tolerance?
Age-related mechanisms of self-defense in the newborn child and in the elderly
are listed in the Pediatric Considerations and Geriatric Considerations boxes.
Pediatric Considerations
Age-Related Factors Affecting Mechanisms of Self-Defense in the
Newborn Child
Normal human newborns are immunologically immature; they have deficient
antibody production, phagocytic activity, and complement activity, especially
components of alternative pathways (e.g., factor B).
The newborn cannot produce all classes of antibody; IgM is produced by the
newborn (develops in the last trimester) to in utero infections (e.g.,
cytomegalovirus, rubella virus, and Toxoplasma gondii); only limited amounts of
IgA are produced in the newborn; IgG production begins after birth and rises
steadily throughout the first year of life.
Maternal antibodies provide protection within the newborn's circulation (see figure
below).
Deficits in specific maternal transplacental antibody may lead to a tendency to
develop severe, overwhelming sepsis and meningitis in the newborn.
Antibody Levels in Umbilical Cord Blood and in Neonatal Circulation. Early in gestation, maternal
IgG begins active transport across the placenta and enters the fetal circulation. At birth, the
fetal circulation may contain nearly adult levels of IgG, which is almost exclusively from the
maternal source. The fetal immune system has the capacity to produce IgM and small amounts
of IgA before birth (not shown). After delivery, maternal IgG is rapidly destroyed and neonatal IgG
production increases.
Geriatric Considerations
Age-Related Factors Affecting Mechanisms of Self-Defense in the
Elderly
Immune function decreases with age; diminished T-cell function and reduced
antibody responses to antigenic challenge occur with age.
The thymus reaches maximum size at sexual maturity and then undergoes involution
until it is a vestigial remnant by middle age; by 45 to 50 years of age, the thymus
is only 15% of its maximum size.
With age there is a decrease in thymic hormone production and the organ's ability to
mediate T-cell differentiation.
Did You Understand?
Third Line of Defense: Adaptive Immunity
1. Adaptive immunity is a state of protection, primarily against infectious agents,
that differs from inflammation by being slower to develop, being more specific,
and having memory that makes it much longer lived.
2. The adaptive immune response is most often initiated by cells of the innate
system. These cells process and present portions of invading pathogens (i.e.,
antigens) to lymphocytes in peripheral lymphoid tissue.
3. The adaptive immune response is mediated by two different types of lymphocytes
—B lymphocytes and T lymphocytes. Each has distinct functions. B cells are
responsible for humoral immunity that is mediated by circulating antibodies
(immunoglobulins), whereas T cells are responsible for cell-mediated immunity, in
which they kill targets directly or stimulate the activity of other leukocytes.
4. Adaptive immunity can be either active or passive depending on whether immune
response components originated in the host or came from a donor.
Antigens and Immunogens
1. Antigens are molecules that bind and react with components of the immune
response, such as antibodies and receptors on B and T cells. Most antigens can
induce an immune response, and these antigens are called immunogens.
2. All immunogens are antigens but not all antigens are immunogens.
3. Some pathogens are successful because they mimic “self” antigens but avoid
inducing an immune response.
4. Large molecules, such as proteins, polysaccharides, and nucleic acids, are most
immunogenic. Thus molecular size is an important factor for antigen
immunogenicity.
5. Haptens are antigens too small to be immunogens by themselves but become
immunogenic after combining with larger molecules.
6. The antigenic determinant, or epitope, is the precise chemical structure with
which an antibody or B-cell/T-cell receptor reacts.
7. Self-antigens are antigens on an individual's own cells. The individual's immune
system does not normally recognize self-antigens as immunogenic, a condition
known as tolerance.
8. The response to antigen can be divided into two phases: the primary and
secondary responses. The primary response of humoral immunity is usually
dominated by IgM, with lesser amounts of IgG. The secondary immune response
has a more rapid production of a larger amount of antibodies, predominantly IgG.
Antibodies
1. The humoral immune response consists of molecules (antibodies) produced by B
cells. B cells are lymphocytes.
2. Antibodies are plasma glycoproteins that can be classified by chemical structure
and biologic activity as IgG, IgM, IgA, IgE, or IgD.
3. A typical antibody molecule is constructed of two identical heavy chains and two
identical light chains (either κ or λ) and has two Fab portions that bind antigen and
an Fc portion that interacts with complement or receptors on cells.
4. The protective effects of antibodies may be direct through the action of antibody
alone or indirect requiring activation of other components of the innate immune
response.
5. IgE is a special class of antibody produced against environmental antigens that
are the primary cause of common allergies. It also protects the individual from
infection by large parasitic worms (helminthes).
6. The secretory immune system protects the external surfaces of the body through
secretion of antibodies in bodily secretions, such as tears, sweat, saliva, mucus, and
breast milk. IgA is the dominant secretory immunoglobulin.
Immune Response: Collaboration of B Cells and T
Cells
1. The generation of clonal diversity results in production of B and T lymphocytes
with receptors against millions of antigens that possibly will be encountered in an
individual's lifetime occurs in the fetus in the primary lymphoid organs: the thymus
for T cells and portions of the bone marrow for B cells..
2. The generation of clonal diversity is the differentiation of lymphoid stem cells
into B and T lymphocytes. Lymphoid stem cells interact with stromal cells through a
variety of adhesion factors. As the stem cell matures it develops a variety of surface
markers or receptors, one of the earliest is IL-7 receptor. IL-7, produced by stromal
cells is critical for driving differentiation and proliferation of the B cell.
3. The next stage in development is formation of the B-cell receptor (BCR). The
role of the BCR is to recognize antigen and communicate that information to the
cell's nucleus.
4. The variable regions of antibodies, as well as the BCR, contain CDR areas. The
diversity of these CDRs is responsible for the variety of antigens recognized by
immunocompetent B cells. The enormous repertoire of antibody specificities is
made possible by rearrangement of existing DNA during B-cell development in the
primary lymphoid organs, a process called somatic recombination.
5. Somatic rearrangement of the antibody variable regions will frequently result in a
BCR that recognizes the individual's own antigens, which may result in attack on
“self” antigens expressed on various tissue and organs. Many of these
“autoreactive” B cells are eliminated in the bone marrow. Most of the developing B
cells undergo apoptosis. This entire process is referred to as central tolerance.
6. The process of T-cell proliferation and differentiation is similar to that for B
cells. The primary lymphoid organ for T-cell development is the thymus. Lymphoid
stem cells travel through the thymus, where thymic hormones and the cytokine IL-7
promote lymphoid stem cell division and the production of receptors. They exit the
thymus as mature immunocompetent T cells with antigen-specific receptors on the
cell surface.
7. T cell receptor, or TCR, proceeds in a manner similar to BCR. Initially proteins
called CD4 and CD8 are expressed on the developing cells. Eventually CD4 cells
develop into T-helper cells (Th cells) and CD 8 cells become T-cytotoxic cells.
Other mature T cells include T-regulatory cells (Treg) and memory cells.
8. The generation of clonal diversity concludes when immunocompetent T and B
cells migrate from the primary lymphoid organs into the circulation and secondary
lymphoid organs to await antigen.
9. The induction of an immune response, or clonal selection, begins when antigen
enters the individual's body.
10. Most antigens must first interact with antigen-presenting cells (APCs) (e.g.,
macrophages). Dendritic cells present in the skin, mucosa, and lymphoid tissues
also present antigen.
11. Antigen is processed in the APCs and presented on the cell surface by molecules
of the MHC. The particular MHC molecule (class I or class II) that presents antigen
determines which cell will respond to that antigen. Th cells require that the antigen
be presented in a complex with MHC class II molecules. Tc cells require that the
antigen be presented by MHC class I molecules.
12. The T cell sees the presented antigen through the T-cell receptor (TCR) and
accessory molecules: CD4 or CD8. CD4 is found on Th cells and reacts specifically
with MHC class II. CD8 is found on Tc cells and reacts specifically with MHC class
I.
13. Th cells consist of Th1 cells, which help Tc cells respond to antigen; Th2 cells,
which help B cells develop into plasma cells; and Th17 cells, which help activate
macrophages.
14. Tc cells bind to and kill cellular targets such as cells infected with viruses or
cancer cells.
15. The natural killer (NK) cell has some characteristics of the Tc cells and is
important for killing target cells in which viral infection or malignancy has resulted
in the loss of cellular MHC molecules.
Pediatric Considerations: Age-Related Factors
Affecting Mechanisms of Self-Defense in the
Newborn Child
1. Neonates often have transiently depressed inflammatory function, particularly
neutrophil chemotaxis and alternative complement pathway activity.
2. The T-cell–independent immune response is adequate in the fetus and neonate, but
the T-cell–dependent immune response develops slowly during the first 6 months of
life.
3. Maternal IgG antibodies are transported across the placenta into the fetal blood
and protect the neonate for the first 6 months, after which they are replaced by the
child's own antibodies.
Geriatric Considerations: Age-Related Factors
Affecting Mechanisms of Self-Defense in the
Elderly
1. Elderly persons are at risk for impaired wound healing, usually because of
chronic illnesses.
2. T-cell function and antibody production are somewhat deficient in elderly
persons. Elderly individuals also tend to have increased levels of circulating
autoantibodies (antibodies against self-antigens).
Key Terms
Active immunity (active acquired immunity), 159
Adaptive (acquired) immunity, 158
Agglutination, 162
Antibody, 161
Antibody-dependent cellular cytotoxicity (ADCC), 172
Antigen, 158, 160
Antigen-binding fragment (Fab), 161
Antigen-binding site (paratope), 162
Antigen processing, 168
Antigen-processing (antigen-presenting) cell (APC), 167
Antigenic determinant (epitope), 162
B-cell receptor (BCR), 166
B lymphocyte (B cell), 158
CD3, 167
CD4, 167
CD8, 167
CDR, 167
Cellular immunity, 159
Central tolerance, 167
Class switch, 170
Cloncal diversity, 159, 166
Clonal selection, 159, 167
Complementary-determining region (CDR), 162
Crystalline fragment (Fc), 161
Dendritic cell, 167
Hapten, 160
Human leukocyte antigens (HLA), 168
Humoral immunity, 159
Immune response, 158
Immunity, 158
Immunocompetent, 159
Immunogen, 160
Immunoglobulin (Ig), 161
Lymphocyte, 158
Lymphoid stem cell, 166
Major histocompatibility complex (MHC), 168
Memory cell, 159
Natural killer (NK) cell, 172
Neutralization, 162
Passive immunity (passive acquired immunity), 159
Peripheral tolerance, 173
Plasma cell, 159
Precipitation, 162
Primary immune response, 167
Primary (central) lymphoid organ, 166
Secondary immune response, 167
Secondary lymphoid organ, 159
Secretory (mucosal) immune system, 164
Secretory immunoglobulin, 164
Somatic recombination, 167
Superantigen (SAG), 169
Systemic immune system, 164
T-cell receptor (TCR), 167
T-cytotoxic (Tc) cell, 159
T-helper (Th) cell, 159
T lymphocyte (T cell), 158
T-regulatory (Treg) cell, 159, 169, 172
Th1 cell, 168
Th2 cell, 169
Th17 cell, 169
Titer, 167
References
1. Wu LC, Zarrin AA. The production and regulation of IgE by the immune
system. Nat Rev Immunol. 2014;14(4):247–259.
2. Sela-Culang I, et al. The structural basis of antibody-antigen recognition.
Front Immunol. 2013;4:302.
3. Guilliams M, et al. The function of Fcγ receptors in dendritic cells and
macrophages. Nat Rev Immunol. 2014;14(2):94–108.
4. Fitzsimmons CM, et al. Helminth allergens, parasite-specific IgE, and its
protective role in human immunity. Front Immunol. 2014;5:61.
5. Rescigno M. Mucosal immunology and bacterial handling in the intestine.
Best Pract Res Clin Gastroenterol. 2013;27(1):17–24.
6. Miyazaki K, et al. The establishment of B versus T cell identity. Trends
Immunol. 2014;35(5):205–210.
7. Clark MR, et al. Orchestrating B cell lymphopoiesis through interplay of
IL-7 receptor and pre-B cell receptor signalling. Nat Rev Immunol.
2014;14(2):69–89.
8. Shih H-Y, Krangel MS. Chromatin architecture, CCCTC-binding factor, and
V(D)J recombination: managing long-distance relationships at antigen
receptor loci. J Immunol. 2013;190(10):4915–4921.
9. Boehm T, Swann JB. Thymus involution and regeneration: two sides of the
same coin? Nat Rev Immunol. 2013;13(11):831–838.
10. Brownlie RJ, Zamoyska R. T cell receptor signalling networks: branching,
diversified and bounded. Nat Rev Immunol. 2013;13(4):257–269.
11. Blum JS, et al. Pathways of antigen processing. Annu Rev Immunol.
2013;31(2013):443–473.
12. Batista FD, Dustin ML. Cell:cell interactions in the immune system. Immunol
Rev. 2013;251(1):7–12.
13. Fooksman DR. Organizing MHC class II presentation. Front Immunol.
2014;5:158.
14. Yamane H, Paul WE. Early signaling events that underlie fate decisions of
naïve CD4+ T cells toward distinct T-helper cell subsets. Immunol Rev.
2013;252(1):12–23.
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8
Infection and Defects in Mechanisms of
Defense
Neal S. Rote
CHAPTER OUTLINE
Infection, 176
Microorganisms and Humans: A Dynamic
Relationship, 176
Countermeasures Against Infectious
Microorganisms, 187
Deficiencies in Immunity, 189
Initial Clinical Presentation, 189
Primary (Congenital) Immune Deficiencies, 190
Secondary (Acquired) Immune Deficiencies, 192
Evaluation and Care of Those with Immune
Deficiency, 193
Replacement Therapies for Immune Deficiencies,
193
Acquired Immunodeficiency Syndrome (AIDS),
194
Hypersensitivity: Allergy, Autoimmunity, and Alloimmunity, 199
Mechanisms of Hypersensitivity, 199
Antigenic Targets of Hypersensitivity Reactions,
206
The defensive system protecting the body from infection is a finely tuned network,
but it is not perfect. Sometimes infectious agents can inhibit or escape defense
mechanisms or the system may break down, leading to inadequate protection or
inappropriate activation. An inadequate response (commonly called an immune
deficiency) may range from relatively mild defects to life-threatening severity.
Inappropriate responses (hypersensitivity reactions) may be (1) exaggerated against
noninfectious environmental substances (allergy); (2) misdirected against the body's
own cells (autoimmunity); or (3) directed against beneficial foreign tissues, such as
transfusions or transplants (alloimmunity). Several of these inappropriate responses
can be serious or life-threatening. This chapter provides an overview of conditions
under which our protective systems have failed.
Infection
Modern health care has shown great progress in preventing and treating infectious
diseases. In the United States, heart disease and malignancies greatly surpass
infectious disease as major causes of death. However, endemic diseases, such as
chronic hepatitis, human immunodeficiency virus (HIV), other sexually transmitted
infections, and foodborne infections, remain major challenges.1 Most deaths related
to infections occur in individuals whose protective systems are compromised
(children, elderly, and those with chronic disease). Influenza/pneumonia (eighth
leading cause of death) and sepsis (eleventh leading cause) accounted for more than
89,000 deaths (3.5% of the total number of deaths) in 2011.2 Other infections
resulted in an additional 27,000 deaths.
Infectious disease remains a significant threat to life in many parts of the world,
including India, Africa, and Southeast Asia.3 The advent of sanitary living
conditions, clean water, uncontaminated food, vaccinations, and antimicrobial
medications has improved the health of many; but inefficient healthcare systems,
endemic poverty, political unrest, and other factors have slowed progress in some
regions. As a result of these initiatives, smallpox has been eradicated from the globe
(the last reported case was in 1975 in Somalia). Worldwide, polio has declined by
more than 99% and eradicated from the Western hemisphere. Measles was decreased
by 78% and was nearly eliminated in the Western hemisphere. Although vaccines
and antimicrobials have diminished the frequency of some infectious diseases, the
emergence of new diseases, such as West Nile virus, severe acute respiratory
syndrome (SARS), Middle East respiratory syndrome coronavirus (MERS-CoV),
and Hantavirus, and the uncontrolled spread of diseases, such as Ebola virus
infection, into new regions of Africa, as well as the continued development of many
multiple drug–resistant microorganisms, are examples of the current intense
challenges in the struggle to prevent and control infectious disease. Some tropical
diseases are emerging for the first time in the United States, possibly a result of
global warming.
Microorganisms and Humans: A Dynamic
Relationship
The increase in antibiotic resistance, in particular, places more importance on
maintenance of an intact inflammatory and immune system. Individuals with
immune deficiencies become easily infected with opportunistic microorganisms—
those that normally would not cause disease but seize the opportunity provided by
the person's decreased immune or inflammatory responses.
Unlike opportunistic infections, true pathogens have devised means to circumvent
the normal controls provided by the innate and adaptive system. Several factors
influence the capacity of a pathogen to cause disease.
• Communicability: Ability to spread from one individual to others (e.g., measles
and pertussis spread very easily; human immunodeficiency virus [HIV] is of lower
communicability)
• Infectivity: Ability of the pathogen to invade and multiply in the host (e.g., herpes
simplex virus can survive for long periods in a latent stage)
• Virulence: Capacity of a pathogen to cause severe disease (e.g., measles virus is of
low virulence; rabies and Ebola viruses are highly virulent)
• Pathogenicity: Ability of an agent to produce disease—success depends on
communicability, infectivity, extent of tissue damage, and virulence (e.g., HIV can
kill T lymphocytes)
• Portal of entry: Route by which a pathogenic microorganism infects the host
(e.g., direct contact, inhalation, ingestion, or bites of an animal or insect)
• Toxigenicity: Ability to produce soluble toxins or endotoxins, factors that greatly
influence the pathogen's degree of virulence
Infectivity is facilitated by the ability of pathogens to attach to cell surfaces,
release enzymes that dissolve protective barriers, multiply rapidly, escape the action
of phagocytes, or resist the effect of low pH. After penetrating protective barriers
(invasion), pathogens then multiply and spread through the lymph and blood to
tissues and organs, where they continue multiplying and cause disease. In humans
the route of entrance of many pathogenic microorganisms also becomes the site of
shedding of new infectious agents to other individuals, completing a cycle of
infection.
Infectious disease can be caused by microorganisms that range in size from 20
nanometers (nm) (poliovirus) to 10 meters (m) (tapeworm). Classes of pathogenic
microorganisms and their characteristics are summarized in Table 8-1. Some
mechanisms of tissue damage caused by microorganisms are summarized in Table
8-2. The multiple layers of defense against infection are described in Chapters 6 and
7. Table 8-3 contains examples of microorganisms that defeat our protective
systems.
TABLE 8-1
Classes of Microorganisms Infectious to Humans
Class Size Site of Reproduction Example
Virus 20-300 nm Intracellular Poliomyelitis
Chlamydiae 200-1000 nm Intracellular Urethritis
Rickettsiae 300-1200 nm Intracellular Rocky Mountain spotted fever
Mycoplasma 125-350 nm Extracellular Atypical pneumonia
Bacteria 0.8-15 mcg Skin Staphylococcal wound infection
Mucous membranes Cholera
Extracellular Streptococcal pneumonia
Intracellular Tuberculosis
Fungi 2-200 mcg Skin Tinea pedis (athlete's foot)
Mucous membranes Candidiasis (e.g., thrush)
Extracellular Sporotrichosis
Intracellular Histoplasmosis
Protozoa 1-50 mm Mucosal Giardiasis
Extracellular Sleeping sickness
Helminths 3 mm to 10 m Intracellular Trichinosis
Extracellular Filariasis
TABLE 8-2
Examples of Microorganisms That Cause Tissue Damage
PATHOGENS THAT DIRECTLY CAUSE TISSUE DAMAGE
Produce Exotoxin
Streptococcus pyogenes Tonsillitis, scarlet fever
Staphylococcus aureus Boils, toxic shock syndrome, food poisoning
Corynebacterium diphtheria Diphtheria
Clostridium tetani Tetanus
Vibrio cholerae Cholera
Produce Endotoxin
Escherichia coli Gram-negative sepsis
Haemophilus influenzae Meningitis, pneumonia
Salmonella typhi Typhoid
Shigella Bacillary dysentery
Pseudomonas aeruginosa Wound infection
Yersinia pestis Plague
Cause Direct Damage with Invasion
Variola Smallpox
Varicella-zoster Chickenpox, shingles
Hepatitis B virus Hepatitis
Poliovirus Poliomyelitis
Measles virus Measles, subacute sclerosing panencephalitis
Influenza virus Influenza
Herpes simplex virus Cold sores
PATHOGENS THAT INDIRECTLY CAUSE TISSUE DAMAGE
Produce Immune Complexes
Hepatitis B virus Kidney disease
S. pyogenes Glomerulonephritis
Treponema pallidum Kidney damage in secondary syphilis
Most acute infections Transient renal deposits
Cause Cell-Mediated Immunity
Mycobacterium tuberculosis Tuberculosis
Mycobacterium leprae Tuberculoid leprosy
Lymphocytic choriomeningitis virus Aseptic meningitis
Borrelia burgdorferi Lyme arthritis
Herpes simplex virus Herpes stromal keratitis
Data modified from Janeway CA et al: Immunobiology: the system in health and disease, ed 5, New York,
2001, Garland.
TABLE 8-3
Examples of Mechanisms Used by Pathogens to Resist the Immune
System
Mechanisms Effect on Immunity Example of Specific
Microorganisms
Destroy or Block Component of Immune System
Produce toxins Kills phagocyte or interferes with chemotaxis
Prevents phagocytosis by inhibiting fusion between phagosome and lysosomal granules
Staphylococcus
Streptococcus
Mycobacterium tuberculosis
Produce antioxidants (e.g.,
catalase, superoxide
dismutase)
Produce protease to digest
IgA
Prevents killing by O2-dependent mechanisms
Promotes bacterial attachment
Mycobacterium sp.
Salmonella typhi
Neisseria gonorrhoeae (urinary
tract infection), Haemophilus
influenzae, and Streptococcus
pneumoniae (pneumonia)
Produce surface molecules
that mimic Fc receptors and
bind antibody
Prevents activation of complement system
Prevents antibody functioning as opsonin
Staphylococcus
Herpes simplex virus
Mimic Self-Antigens
Produce surface antigens
(e.g., M protein, red blood
cell antigens) that are
similar to self-antigens
Pathogen resembles individual's own tissue; in some individuals, antibodies can be
formed against self-antigen, leading to hypersensitivity disease (e.g., antibody to M
protein also reacts with cardiac tissue, causing rheumatic heart disease; antibody to red
blood cell antigens can cause anemia)
Group A Streptococcus (M protein)
Mycoplasma pneumoniae (red cell
antigens)
Change Antigenic Profile
Undergo mutation of
antigens or activate genes
that change surface
molecules
Immune response delayed because of failure to recognize new antigen Influenza
HIV
Some parasites
Bacterial Disease
Bacteria are prokaryocytes (lacking a discrete nucleus) and are relatively small.
They can be aerobic or anaerobic and motile or immotile. Spherical bacteria are
called cocci, rodlike forms are called bacilli, and spiral forms are termed
spirochetes. Gram stain differentiates the microorganisms as gram-positive or
gram-negative bacteria. Examples of human diseases caused by specific bacteria are
listed in Table 8-4. The general structure of bacteria is reviewed in Figure 8-1.
TABLE 8-4
Examples of Common Bacterial Infections
Microorganism Gram Stain Respiratory
Pathway
Intracellular or
Extracellular
Respiratory Tract Infections
Upper Respiratory Tract Infections
Corynebacterium diphtheriae (diphtheria) Gram + Facultative anaerobic Extracellular
Haemophilus influenzae Gram − Facultative anaerobic Extracellular
Streptococcus pyogenes (group A) Gram + Facultative anaerobic Extracellular
Otitis Media
Haemophilus influenzae Gram − Facultative anaerobic Extracellular
Streptococcus pneumoniae Gram + Facultative anaerobic Extracellular
Lower Respiratory Tract Infections
Bacillus anthracis (pulmonary anthrax) Gram + Facultative anaerobic Extracellular
Bordetella pertussis (whooping cough) Gram − Aerobic Extracellular
Chlamydia pneumonia Not stainable Aerobic Obligate intracellular
Escherichia coli Gram − Facultative anaerobic Extracellular
Haemophilus influenzae Gram − Facultative anaerobic Extracellular
Legionella pneumophila Gram − Aerobic Facultative intracellular
Mycobacterium tuberculosis Gram +
(weakly)
Aerobic Extracellular
Mycoplasma pneumoniae Not stainable Aerobic Extracellular
Neisseria meningitidis (develops into meningitis) Gram − Aerobic Extracellular
Pseudomonas aeruginosa Gram − Aerobic Extracellular
Streptococcus agalactiae (group B; develops to meningitis) Gram + Facultative anaerobic Extracellular
Streptococcus pneumoniae Gram + Facultative anaerobic Extracellular
Yersinia pestis (plague) Gram − Facultative anaerobic Extracellular
Gastrointestinal Infections
Inflammatory Gastrointestinal Infections
Bacillus anthracis (gastrointestinal anthrax) Gram + Facultative anaerobic Extracellular
Clostridium difficile Gram + Anaerobic Extracellular
Escherichia coli O157:H7 Gram − Facultative anaerobic Extracellular
Vibrio cholerae Gram − Facultative anaerobic Extracellular
Invasive Gastrointestinal Infections
Brucella abortus (brucellosis, undulant fever, leading to sepsis, heart
infection)
Gram − Aerobic Intracellular
Helicobacter pylori (gastritis and peptic ulcers) Gram − Microaerophilic Extracellular
Listeria monocytogenes (leading to sepsis and meningitis) Gram + Aerobic Intracellular
Salmonella typhi (typhoid fever) Gram − Anaerobic Extracellular
Shigella sonnei Gram − Facultative anaerobic Extracellular
Food Poisoning
Bacillus cereus Gram + Facultative anaerobic Extracellular
Clostridium botulinum Gram + Anaerobic Extracellular
Clostridium perfringens Gram + Anaerobic Extracellular
Staphylococcus aureus Gram + Facultative anaerobic Extracellular
Sexually Transmitted Infections
Chlamydia trachomatis (pelvic inflammatory disease) Not stainable Aerobic Intracellular
Neisseria gonorrhoeae (urethritis) Gram − Aerobic Facultative intracellular
Treponema pallidum (spirochete; syphilis) Gram − Aerobic Extracellular
Skin and Wound Infections
Bacillus anthracis (cutaneous anthrax) Gram + Facultative anaerobic Extracellular
Borrelia burgdorferi (Lyme disease; spirochete) Gram − Aerobic Extracellular
Clostridium tetani (tetanus) Gram + Anaerobic Extracellular
Clostridium perfringens (gas gangrene) Gram + Anaerobic Extracellular
Mycobaterium leprae (leprosy) Gram +
(weakly)
Aerobic Extracellular
Pseudomonas aeruginosa Gram − Aerobic Extracellular
Rickettsia prowazekii (rickettsia; typhus) Gram − Aerobic Obligate intracellular
Staphylococcus aureus Gram + Facultative anaerobic Extracellular
Streptococcus pyogenes (group A) Gram + Facultative anaerobic Extracellular
Eye Infections
Chlamydia trachomatis (conjunctivitis) Not stainable Aerobic Obligate intracellular
Haemophilus aegyptius (pink eye) Gram − Facultative anaerobic Extracellular
Zoonotic Infections
Bacillus anthracis (anthrax) Gram + Facultative anaerobic Extracellular
Brucella abortus (brucellosis, also called undulant fever) Gram − Aerobic Intracellular
Borrelia burgdorferi (spirochete; Lyme disease) Gram − Aerobic Extracellular
Listeria monocytogenes Gram + Aerobic Intracellular
Rickettsia rickettsii (rickettsia; Rocky Mountain spotted fever) Gram − Aerobic Obligate intracellular
Rickettsia prowazekii (rickettsia; typhus) Gram − Aerobic Obligate intracellular
Yersinia pestis (plague) Gram − Facultative anaerobic Extracellular
Nosocomial Infections
Enterococcus faecalis Gram + Facultative anaerobic Extracellular
Enterococcus faecium Gram + Facultative anaerobic Extracellular
Escherichia coli (cystitis) Gram − Facultative anaerobic Extracellular
Pseudomonas aeruginosa Gram − Obligate anaerobic Extracellular
Staphylococcus aureus Gram + Facultative anaerobic Extracellular
Staphylococcus epidermidis Gram + Facultative anaerobic Extracellular
FIGURE 8-1 General Structure of Bacteria. A, The structure of the bacterial cell wall determines
its staining characteristics with Gram stain. A gram-positive bacterium has a thick layer of
peptidoglycan (left). A gram-negative bacterium has a thick peptidoglycan layer and an outer
membrane (right). B, Example of a gram-positive (darkly stained microorganisms, arrow) group
A Streptococcus. This microorganism consists of cocci that frequently form chains. C, Example
of a gram-negative (pink microorganisms, arrow) Neisseria meningitides in cerebrospinal fluid.
Neisseria form complexes of two cocci (diplococci). (A from Murray PR et al: Medical microbiology, ed 7, Philadelphia,
2013, Saunders; B, C from Murray PR et al: Medical microbiology, ed 4, St Louis, 2002, Mosby.)
Bacterial survival and growth depend on the effectiveness of the body's defense
mechanisms and on the bacterium's ability to resist these defenses. A vast amount of
information has been published about bacterial pathogenesis. The main aspects of
how bacteria cause disease may be illustrated in how one particular microorganism,
Staphylococcus aureus, has adapted to become a life-threatening pathogen.
Staphylococcus aureus has become a major cause of hospital-acquired
(nosocomial) infections and is now spreading throughout the community. This
microorganism is a common commensal inhabitant of normal skin and nasal
passages (estimates depict that from 30% to 80% of individuals may be nasal
carriers) and can be transmitted by direct skin-to-skin contact or by contact with
shared items or surfaces that have become contaminated by another person (e.g.,
towels, used bandages).4
Although a relatively benign commensal microorganism under normal
conditions, S. aureus is well equipped to act as a life-threatening pathogen when the
opportunity arises; thus it is an opportunistic microorganism. Skin infections may
occur at sites of trauma, such as cuts and abrasions, and at areas of the body covered
by hair (e.g., back of neck, groin, buttock, armpit, beard area of men). Most
infections are relatively mild and localized, appearing as red and swollen pustules
on the skin, containing pus or other drainage. They can develop into abscesses,
boils, carbuncles, cellulitis, or furunculosis. Invasive disease may originate from
wound infections (e.g., trauma, surgical wounds, indwelling medical devices,
prosthetic joints) and lead to fatal septicemia and abscesses in internal organs (e.g.,
lungs, kidney, bones, skeletal muscle, meninges, or heart) (Figure 8-2).
FIGURE 8-2 Staphylococcus aureus Infections. Different strains of S. aureus (gram-positive
cocci in sputum from an individual with pneumonia [center photograph]) cause a variety of
infections. The particular infection may depend on the toxin produced: exfoliative toxin (scalded
skin syndrome), enterotoxins A-G (food poisoning), or toxic shock syndrome toxin-1 (TSST-
1). (Toxic shock syndrome, carbuncle, impetigo, and wound infection photos from Cohen J, Powderly W G: Infectious diseases, ed 3, St Louis,
2010, Mosby; folliculitis photo from Goldman L, Ausiello D: Cecil medicine, ed 24, Philadelphia, 2012, Saunders; center photo and photos of food
poisoning and endocarditis from Kumar V et al: Robbins and Cotran pathologic basis of disease, ed 8, Philadelphia, 2010, Saunders; furuncle
photo from Long S et al: Principles and practice of pediatric infectious diseases, ed 4, Philadelphia, 2012, Saunders; scalded skin syndrome and
pneumonia photos from Mandell G et al: Principles and practice of infectious diseases, ed 7, Philadelphia, 2010, Churchill Livingstone.)
Microscopically, staphylococci are gram-positive cocci that generally grow in
grapelike clusters. However, this microorganism possesses a myriad of potential
virulence factors that determine the severity, location, and clinical features of
infection. It should be noted that individual strains of this opportunistic pathogen
utilize only some of the entire array of virulence factors.
Microorganisms frequently exist as part of complex multicellular masses called
biofilms. Biofilms consist of mixed species of microorganisms, including bacteria,
fungi, and viruses. Growth of bacteria in biofilms offers survival advantage by
protection from the host's responses and exposure to antibiotics. These structures
are associated with otitis media; urinary tract infections secondary to indwelling
catheters; foot ulcers in diabetic persons; infected burn wounds; vaginitis;
osteomyelitis; pneumonia secondary to cystic fibrosis; and diseases of the oral
cavity related to dental plaque, such as dental caries and periodontitis. S. aureus
biofilms are associated with persistent nasopharyngeal colonization and
colonization of implanted devices.5
A variety of surface proteins mediate adherence among microorganisms in
biofilms and to connective tissue (laminin, fibrin, fibronectin) and endothelium.
Attachment to collagen occurs in strains causing osteomyelitis and septic arthritis.
The capsular polysaccharide mediates attachment to prosthetic devices and also
protects against phagocytosis. One surface protein, protein A, binds IgG by the Fc
portion so that the Fab regions are facing outward.
Thus, the bacteria appear coated with a self-protein, and, with the Fc bound
directly to protein A, the IgG cannot activate complement or act as an opsonin.6 A
coagulase that induces fibrin clotting on the bacterial surface also masks bacterial
antigens under a surface of self-proteins. Staphylococcal protein A and also a
protein called staphylococcal binder of immunoglobulin are secreted and bind and
neutralize IgG. Staphylococcus produces proteins that inhibit complement activity,
including activation of C3 and C5, preventing production of C3b, C3a, and C5a.7
Some strains of S. aureus are programmed to avoid innate immunity. They can
produce inhibitors of antimicrobial peptides and avoid recognition by Toll-like
receptors.8 Even when engulfed by a phagocyte, S. aureus may resist intracellular
oxidative killing by inactivating hydrogen peroxide and other reactive oxygen
species. They also resist lysozyme by changing the chemistry of the cell wall.9
Many bacteria use toxins as virulence factors, including exotoxins and
endotoxins. Exotoxins are secreted molecules and are immunogenic eliciting
production of antibodies known as antitoxins (important for vaccine development,
see page 187). The most poisonous yet discovered is botulinum neurotoxin
produced by Clostridium botulinum; less than 1 ng/kg is toxic to humans. Strains of
S. aureus are capable of producing a wide array of secreted toxic molecules or
exotoxins. These include those that damage the cell membrane (α-toxin, which
forms pores in membranes; hemolysin, which destroys erythrocytes; β-toxin, which
is a sphingomyelinase; δ-toxin, a detergent-like toxin; and leukocidin, which lyses
phagocytes). Other toxins include coagulase, which causes blood clots;
staphylokinase, which breaks down clots; exfoliative toxins, which cause separation
of the epidermis resulting in scalded skin syndrome; lipase, which degrades lipids
on the skin surface and facilitates abscess formation; enterotoxins, which cause food
poisoning; and superantigens (discussed in Chapter 7).10 Each infectious strain of S.
aureus may produce a few of these toxins so that strains differ in their capacities to
cause particular diseases; thus, different strains may cause purulent dermal
infections, food poisoning, or toxic shock syndrome.
Antibiotic resistance has become a major problem with S. aureus. For several
decades pathogenic strains have commonly produced β-lactamase, an enzyme that
destroys penicillin. More recently, staphylococci have developed resistance to
broad-spectrum antibiotics, including methicillin-like antibiotics (methicillin-
resistant Staphylococcus aureus [MRSA]), which were widely used to treat
penicillin-resistant microorganisms.
It is clear that S. aureus succeeds as an opportunistic pathogen because of a wide
array of virulence factors that neutralize important components of the innate and
adaptive immune systems, destroy tissue, and resist much of our repertoire of
antibiotics. The major remaining option is the development of an effective vaccine,
a task that is sometimes difficult.11 As mentioned in the beginning of this section, S.
aureus is only one of many bacteria that have developed similar characteristics.
Gram-negative microbes produce an endotoxin (lipopolysaccharide [LPS]) that
is a structural portion of the cell wall and is released during growth, lysis, or
destruction of the bacteria or during treatment with antibiotics. Therefore,
antibiotics cannot prevent the toxic effects of the endotoxin. Bacteria that produce
endotoxins are called pyrogenic bacteria because they activate the inflammatory
process and produce fever. The innermost part of the lipopolysaccharide, lipid A,
consists of polysaccharide and fatty acids and is responsible for the substance's
toxic effects.
Bacteremia occurs when bacteria are present in the blood. Gram-negative sepsis
(sepsis or septicemia) occurs when bacteria are growing in the blood and release
large amounts of endotoxin, which can cause endotoxic shock with up to 50%
mortality.12 Released endotoxin, as well as other bacterial products, reacts with
pattern recognition receptors (PRRs) and induces the overproduction of
proinflammatory cytokines, particularly tumor necrosis factor-alpha (TNF-α),
interleukin-1 (IL-1), and interleukin-6 (IL-6), which may secondarily be
immunosuppressive.13 Endotoxin also is a potent activator of the complement and
clotting systems, leading to a degree of capillary permeability sufficient to permit
escape of large volumes of plasma into surrounding tissue, contributing to
hypotension and, in severe cases, cardiovascular shock (see Chapter 24). Activation
of the coagulation cascade leads to the syndrome of disseminated (or diffuse)
intravascular coagulation (see Chapter 21).
Viral Disease
Viral diseases are the most common afflictions of humans and range from the
common cold, caused by many viruses, and the “cold sore” of herpes simplex virus
to cancers and acquired immunodeficiency syndrome (AIDS). Examples of human
diseases caused by specific viruses are listed in Table 8-5. Viruses are very simple
microorganisms consisting of nucleic acid protected from the environment by a
layer or layers of proteins (capsid). The viral genome can be double-stranded DNA
(dsDNA), single-stranded DNA (ssDNA), double-stranded RNA (dsRNA), or single-
stranded RNA (ssRNA). A select group of viruses (e.g., human immunodeficiency
virus [HIV], herpesviruses, influenza virus) bud from the surface of an infected cell,
retaining a portion of the cell's plasma membrane (envelope) as added protection.
Viral replication depends totally on their ability to infect a permissive host cell—a
cell that cannot resist viral invasion and replication. Thus, viruses are obligatory
intracellular microbes. Transmission is usually from one infected individual to an
uninfected individual by aerosols of respiratory tract fluids, contact with infected
blood, sexual contact, or transmission from an animal reservoir (zoonotic
infection) usually through a vector, such as mosquitoes.14
TABLE 8-5
Examples of Human Diseases Caused by Specific Viruses
Baltimore
Classification
Family Virus Envelope Main Route of Transmission Disease
dsDNA Adenoviruses Adenovirus No Droplet contact Acute febrile pharyngitis
Herpesviruses Herpes simplex type 1
(HSV-1)
Yes Direct contact with saliva or
lesions
Lesions in mouth, pharynx,
conjunctivitis
Herpes simplex type 2
(HSV-2)
Yes Sexually, contact with lesions
during birth
Sores on labia, meningitis in children
Herpes simplex type 8
(HSV-8)
Yes Sexually?, body fluids Kaposi sarcoma
Epstein-Barr virus (EBV) Yes Saliva Mononucleosis, Burkitt lymphoma
Cytomegalovirus (CMV) Yes Body fluids, mother's milk,
transplacental
Mononucleosis, congenital infection
Varicella-zoster virus
(VZV)
Yes Droplet contact Chickenpox, shingles
ssDNA Papovaviruses Papillomavirus No Direct contact Warts, cervical carcinoma
dsRNA Reoviruses Rotavirus No Fecal-oral Severe diarrhea
ssRNA+ Picornaviruses Coxsackievirus No Fecal-oral, droplet contact Nonspecific febrile illness,
conjunctivitis, meningitis
Hepatitis A virus No Fecal-oral Acute hepatitis
Poliovirus No Fecal-oral Poliomyelitis
Rhinovirus No Droplet contact Common cold
Flaviviruses Hepatitis C virus Yes Blood, sexually Acute or chronic hepatitis,
hepatocellular carcinoma
Yellow fever virus Yes Mosquito vector Yellow fever
Dengue virus Yes Mosquito vector Dengue fever
West Nile virus Yes Mosquito vector Meningitis, encephalitis
Togaviruses Rubella virus Yes Droplet contact, transplacental Acute or congenital rubella
Coronaviruses SARS Yes Droplets in aerosol or direct
contact
Severe respiratory tract disease
Caliciviruses Norovirus No Fecal-oral Gastroenteritis
ssRNA− Orthomyxoviruses Influenza virus Yes Droplet contact Influenza
Paramyxoviruses Measles virus Yes Droplet contact Measles
Mumps virus Yes Droplet contact Mumps
Parainfluenza virus Yes Droplet contact Croup, pneumonia, common cold
Respiratory syncytial
virus (RSV)
Yes Droplet contact, hand-to-mouth Pneumonia, influenza-like syndrome
Rhabdoviruses Rabies virus Yes Animal bite, droplet contact Rabies
Bunyaviruses Hantavirus Yes Aerosolized animal fecal
material
Viral hemorrhagic fever
Filoviruses Ebola virus Yes Direct contact with body fluids Viral hemorrhagic fever
Marburg virus Yes Direct contact with body fluids Viral hemorrhagic fever
Arenavirus Lassa virus Yes Aerosolized animal fecal
material
Viral hemorrhagic fever
ssRNA+ with RT Retroviruses HIV Yes Sexually, blood products AIDS
dsDNA with RT Hepadnaviruses Hepatitis B virus Yes All body fluids Acute or chronic hepatitis,
hepatocellular carcinoma
To understand the basic concepts of viral pathogenicity, it may be best to look
closely at a single virus. Influenza is a ssRNA virus with a segmented genome (eight
pieces of ssRNA). It is transmitted through aerosols or body fluids and is highly
infectious. Symptoms begin 1 to 4 days after infection and may include chills, fever,
sore throat, muscle aches, severe headaches, coughing, weakness, generalized
discomfort, nausea, and vomiting and may lead to pneumonia. It can be fatal,
particularly in young children and older adults.15 The normal rate of infectivity is
about 5% to 15%, with a mortality of about 0.1%, and in most cases recovery occurs
in 1 to 2 weeks. Yearly seasonal influenza outbreaks result in about 250,000 to
500,000 deaths worldwide.
The life cycle of every virus is completely intracellular and involves several
steps, the first being attachment to a receptor on the target cell (Figure 8-3). The
influenza virion expresses two surface proteins that are essential to virulence. The
hemagglutinin (HA) protein is a glycoprotein that is necessary for entrance into
cells by binding to glycan receptors on the surface of respiratory tract epithelium.
The viral surface neuraminidase (NA) is an enzyme that is necessary for release of
new virions from infected cells by cleaving cellular sialic acids (a common
component of mammalian cell membranes). The specificity of this virus-receptor
interaction (tropism) dictates the range of host cells that a particular virus will
infect and, therefore, the clinical symptoms that reflect the alteration of the function
of the infected cells. Other viruses also use specific receptors; for example, HIV
attaches to CD4 on T-helper cells, Epstein-Barr virus (EBV, a cause of
mononucleosis and Burkitt lymphoma) attaches to complement receptor 2 (CR2) on
B lymphocytes, and Rhinovirus (a group of viruses that cause the common cold)
attaches to intracellular adhesion molecule-1 (ICAM-1) on respiratory tract
epithelium.
FIGURE 8-3 Stages of Viral Infection of a Host Cell. The virion (1) becomes attached to the cell's
plasma membrane by absorption; (2) releases enzymes that weaken the membrane and allow it
to penetrate the cell; (3) uncoats itself; (4) replicates; and (5) matures and escapes from the cell
by budding from the plasma membrane. The infection then can spread to other host cells.
Attachment is followed by penetration (entrance into the cell by endocytosis or
membrane fusion), uncoating (release of viral nucleic acid from the viral capsid by
viral or host enzymes), replication (synthesis of mRNA and viral proteins),
assembly (formation of new virions), and release (exit from the cell by lysis or
budding). The influenza virus enters the respiratory tract epithelial cells by
endocytosis. Low pH leads to intermembrane fusion between the endosome and
viral envelop and uncoating.16 The viral ssRNA is transported to the nucleus where
transcription and replication occur using the viral RNA-dependent RNA
polymerase.17 Viral proteins assemble in the cytoplasm to form the matrix around
the viral genome, and the virion buds from the cell surface. Infected cells usually
die as a direct effect of the virus. The severity of clinical symptoms is usually
secondary to the level of cytokines produced by the infected cells or in response to
death of the cells.
The effects of virus on the infected cell vary greatly. Some viruses, such as
herpesviruses, will initiate a latency phase during which the host cell is transformed
(i.e., herpes simplex viruses 1 and 2 establish latency in neurons). During this phase,
the viral DNA may be integrated into the DNA of the host cell and become a
permanent passenger in that cell and its progeny. In response to stimuli, such as
stress, hormonal changes, or disease, the virus may exit latency and enter a
productive cycle. Herpesviruses 1 and 2 are released from the neurons and infect
skin epithelium, where lesions in the skin are a result of the immune response
against the infected epithelium.
Cytopathic effects caused by other viruses include the following:
1. Cessation of DNA, RNA, and protein synthesis (e.g., herpesvirus)
2. Disruption of lysosomal membranes, resulting in release of digestive lysosomal
enzymes that can kill the cell (e.g., herpesvirus)
3. Fusion of host cells, producing multinucleated giant cells (e.g., respiratory
syncytial virus)
4. Alteration of the antigenic properties, or identity, of the infected cell, causing the
individual's immune system to attack the cell as if it were foreign (e.g., hepatitis B
virus)
5. Transformation of host cells into cancerous cells, resulting in uninhibited and
unregulated growth (e.g., human papillomavirus)
6. Promotion of secondary bacterial infection in tissues damaged by viruses
The principal method by which influenza virus eludes the immune system is by
changing viral surface antigens, a process known as antigenic variation.
Antibodies against the HA and NA antigens are responsible for protection against
influenza infection. Infections are seasonal and protection gained from the previous
year's infection does not totally protect against influenza in the following year
because the HA and NA antigens undergo yearly change. Usually antigenic variation
is relatively minor (antigenic drift) and results from mutations. Individuals
frequently have partial protection resulting from the previous year's infection,
which lessens the clinical effects of the disease. Two groups of influenza virus,
influenza A and influenza B, infect humans and the yearly vaccine against influenza
is a trivalent mixture of inactivated proteins from two influenza A subtypes and one
influenza B subtype. Influenza B almost exclusively infects humans and mutates at a
much lower rate than influenza A. Influenza A has antigenically distinct subtypes
based on HA (17 forms) and NA (10 forms) antigens. Currently, subtypes H1N1,
H1N2, and H3N2 are the primary causes of influenza worldwide.
Influenza A periodically undergoes major antigenic changes (antigenic shifts)
(Figure 8-4). Influenza A can infect birds and mammals and shifts occur in animals
coinfected by a human and an avian strain of influenza. The genome is segmented
and the segments can undergo recombination, during which the human virus obtains
a new HA or NA antigen. Without a shift occurring, clinical influenza is usually
considered epidemic (the number of new infections exceeds the number usually
observed at other times of the year). When major antigenic changes occur, previous
protection may not exist, resulting in a major pandemic (an epidemic that spreads
over a large area, such as a continent or worldwide) and much more severe disease.
FIGURE 8-4 Antigenic Shifts in Influenza Virus. One theory proposes that antigenic shifts occur
when a human influenza virus (blue) and an avian influenza virus (red) coinfect a species that is
permissive for both. The eight ssRNA strands are co-expressed in the same infected cell,
resulting in mixing of the strands so that a hybrid virus can be produced. The hybrid virus
indicated here contains all the genetic information of the original virus that infected humans, but
contains a new hemagglutinin (HA)-containing stand from the avian virus. This virus expresses
a new HA antigen and will be less susceptible to residual immunity that normally provides
partial protection against yearly influenza infections.
A major worry regards zoonotic influenza during which a lethal influenza virus
that infects birds or other animals suddenly develops the capacity to infect humans.14
These infections are monitored closely by agencies, such as the Centers for Disease
Control and Prevention (CDC) in Atlanta. The CDC is currently monitoring human
cases of several zoonotic influenza outbreaks, including swine influenza virus
(H1N1), a pathogenic H5N1 avian influenza virus, and a new strain of avian
influenza (H7N9) that recently appeared.
Viral pathogens bypass many defense mechanisms by hiding within cells and
away from normal inflammatory or immune responses. Some viruses spread from
cell-to-cell through the bloodstream (e.g., influenza, rubella) and are highly
sensitive to neutralizing antibodies that block viral spread and eventually cure the
infection; therefore the disease is described as self-limiting. Other viruses (e.g.,
measles, herpes) are inaccessible to antibodies after initial infection because they
remain inside infected cells, spreading by direct cell-to-cell contact. Most viruses
have developed additional defense mechanisms. For instance, influenza virus
produces NS1 protein (viral non-structural protein-1) that blocks the antiviral
effects of type I interferon.
Fungal Disease
Fungi are relatively large eukaryotic microorganisms with thick walls that have two
basic structures: single-celled yeasts (spheres) or multicellular molds (filaments or
hyphae) (Figure 8-5). Some fungi can exist in either form and are called dimorphic
fungi. The cell walls of fungi are rigid and multilayered and composed of
polysaccharides different from the peptidoglycans of bacteria. The lack of
peptidoglycans allows fungi to resist the action of bacterial cell wall inhibitors such
as penicillin and cephalosporin. Molds are aerobic, and yeasts are facultative
anaerobes, which adapt to, but do not require, anaerobic conditions. They usually
reproduce by simple division or budding.
FIGURE 8-5 Morphology of Fungi. (A) Fungi may be either mold or yeast forms, or dimorphic. (B)
Photograph showing Candida albicans with both the mycelial and the yeast forms. (C) Oral
infection with C. albicans (candidiasis, i.e., thrush). (D) Gram stain of sputum showing that
clinical isolates of C. albicans present as chains of elongated budding yeasts (× 1000). (A, B from
Goering R et al: Mims' medical microbiology, ed 5, London, 2013, Saunders. C from McPherson R, Pincus M: Henry's clinical diagnosis and
management by laboratory methods, ed 22, Philadelphia, 2012, Saunders; D courtesy Dr. Stephen Raffanti.)
Diseases caused by fungi are called mycoses. Mycoses can be superficial, deep,
or opportunistic. Superficial mycoses occur on or near skin or mucous membranes
and usually produce mild and superficial disease. Fungi that invade the skin, hair, or
nails are known as dermatophytes. The diseases they produce are called tineas
(ringworm), for example, tinea capitis (scalp), tinea pedis (feet), and tinea cruris
(groin). Chapter 41 discusses the various skin disorders caused by fungi.
Pathologic fungi cause disease by adapting to the host environment. Fungi that
colonize the skin can digest keratin. Other fungi can grow with wide temperature
variations in lower oxygen environments. Still other fungi have the capacity to
suppress host immune defenses. Phagocytes and T lymphocytes are important in
controlling fungi. Low white blood cell counts promote fungal infection and
infection control is particularly important for individuals who are
immunosuppressed. Common pathologic fungi are summarized in Table 8-6.
TABLE 8-6
Common Pathogenic Fungi
Primary Site of Infection Fungus Disease (Primary) Symptoms
Superficial (no tissue invasion, little
inflammation)
Malassezia furfur Tinea versicolor, seborrheic
dermatitis, dandruff
Red rash on body
Cutaneous (no tissue invasion,
inflammatory response)
Dermatophytes
Trichophyton
mentagrophytes
Trichophyton rubrum
Microsporum canis
Tinea pedis (athlete's foot)
Tinea cruris (jock itch)
Tinea corporis (ringworm)
Scaling, fissures, pruritus
Rash, pruritus
Lesion, raised border, scaling
Candida albicans Cutaneous candidiasis Lesions in most areas of skin, mucous membranes,
thrush, vaginal infection
Subcutaneous (tissue invasion) Sporothrix schenckii Sporotrichosis Ulcers or abscesses on skin and other organ systems
Systemic (dimorphic; causes disease in
healthy individuals)
Stachybotrys chartarum, or
“black mold”
Black mold disease Rash, headaches, nausea, pain
Coccidioides immitis Coccidioidomycosis Valley fever, flulike symptoms
Histoplasma capsulatum Histoplasmosis Lung, flulike symptoms, disseminates to multiple
organs, eye
Blastomyces dermatitidis Blastomycosis Flulike symptoms, chest pains
Systemic (opportunistic) Aspergillus fumigatus,
Aspergillus flavus
Aspergillosis Invasive to lungs and other organs
Pneumocystis jiroveci Pneumocystis pneumonia (PCP) Pneumonia
Cryptococcus neoformans Cryptococcosis Pneumonia-like illness, skin lesions, disseminates to
brain, meningitis
Candidia albicans Systemic candidiasis Sepsis, endocarditis, meningitis
AIDS, Acquired immunodeficiency syndrome; DNA, deoxyribonucleic acid; ds, double-stranded; HIV,
human immunodeficiency virus; RNA, ribonucleic acid; RT, reverse transcriptase; SARS, severe acute
respiratory syndrome; ss, single-stranded.
Candida albicans is the most common cause of fungal infections in humans. It is
an opportunistic yeast that is a commensal inhabitant in the normal microbiome of
many healthy individuals, residing in the skin, gastrointestinal tract, mouth (30% to
55% of healthy individuals), and vagina (20% of healthy women). Candida albicans
is normally under the control of local defense mechanisms, including members of
the bacterial microbiome that produce antifungal agents. In healthy individuals
antibiotic therapy can diminish the microbiome (e.g., diminished levels of
Lactobacillus in the gastrointestinal or vaginal microbiome). Candida overgrowth
may occur, resulting in localized infection such as vaginitis or oropharyngeal
infection (thrush).
In immunocompromised individuals, particularly those with diminished levels of
neutrophils (neutropenia), disseminated infection may occur. Candida is the most
common fungal infection in people with cancer (particularly acute leukemia and
other hematologic cancers), transplantation (bone marrow and solid organ), and
HIV/AIDS. Invasive candidiasis also may be secondary to indwelling catheters,
intravenous lines, or peritoneal dialysis, which provides direct entrance into the
bloodstream.
Disseminated candidiasis may involve deep infections of several internal organs,
including abscesses in the kidney, brain, liver, and heart, and is characterized by
persistent or recurrent fever, gram-negative shock-like symptoms (hypotension,
tachycardia), and disseminated intravascular coagulation (DIC). The death rates of
septic or disseminated candidiasis are in the range of 30% to 40%.
Parasitic Disease
Parasitic microorganisms establish a relationship in which the parasite benefits at
the expense of the other species. Parasites range from a unicellular protozoan to
large worms. Parasitic worms (helminths) include intestinal and tissue nematodes
(e.g., hookworm, roundworm), flukes (e.g., liver fluke, lung fluke), and tapeworms.
A protozoan is a eukaryotic, unicellular microorganism with a nucleus and
cytoplasm. Pathogenic protozoa include malaria (Plasmodium), amoebae (e.g.,
Entamoeba histolytica, which causes amoebic dysentery), and flagellates (e.g.,
Giardia lamblia, which causes diarrhea; Trypanosoma, which causes sleeping
sickness). Although less common in the United States, parasites and protozoa are
common causes of infections worldwide, with a significant effect on the mortality
and morbidity of individuals in developing countries. Important parasites of humans
are listed in Table 8-7.
TABLE 8-7
Examples of Parasites That Are Important in Humans
Category Subgroup Species Disease Organs Affected/Symptoms
Protozoa Ameboid Entamoeba histolytica Amebiasis Dysentary, liver abscess
Flagellate Giardia lamblia Giardiasis* Diarrhea
Trichomonas vaginalis Trichomoniasis Inflammation of reproductive organs
Trypanosoma cruzi, T. brucei Chagas disease: African sleeping
sickness
Generalized, blood and lymph nodes, progressing to
cardiac and CNS
Ciliate Balantidium coli Balantidiasis Small intestines, invasion of colon, diarrhea
Sporozoa
(nonmotile)
Cryptosporidium parvum, C.
hominis
Cryptosporidiosis* Intestine, diarrhea
Plasmodium spp. Malaria Blood, liver
Toxoplasma gondii Toxoplasmosis* Intestine, eyes, blood, heart, liver
Helminths Flukes (trematodes) Fasciola hepatica Fasciolosis Liver destruction
Schistosoma mansoni Schistosomiasis Blood, diarrhea, bladder, generalized symptoms
Tapeworms
(cestodes)
Taenia solium Pork tapeworm Encysts in muscle, brain, liver
Roundworms
(nematodes)
Ascaris lumbricoides Ascariasis Intestinal obstruction, bile duct obstruction
Necator americanus
(hookworm)
Hookworm disease Intestinal parasite
Trichinella spiralis Trichinosis* Intestine, diarrhea, muscle, CNS, death
Wuchereria bancrofti Filariasis, elephantiasis Lymphatics
Enterobius vermicularis
(pinworm)
Pinworm infection Intestines
Onchocerca volvulus Onchocerciasis Blindness, dermatitis
*Most common in the United States.
Malaria is one of the most common infections worldwide. In 2012, the World
Health Organization (WHO) estimated that there were 207 million cases of malaria
with an estimated 627,000 deaths; 90% were in Africa where 82% of the deaths were
children younger than age 5 years.18 Malaria is caused by Plasmodium falciparum, a
protozoan (unicellular) parasite.
Many protozoan parasites are transmitted through vectors or ingested. Vectors
include the tsetse fly (Trypanosoma cruzi, which causes Chagas disease in South
America; Trypanosoma brucei, which causes sleeping sickness in Africa) and sand
fleas (leishmaniasis). Water and food can be contaminated with protozoal parasites
(e.g., E. histolytica, G. lamblia). Transmission of Plasmodium is through the bite of
an infected female Anopheles mosquito, where the parasite grows in the salivary
gland.
The initial attachment to cells depends on the presence of the microorganism in
the bloodstream or gastrointestinal tract. Microorganisms in the bloodstream have
surface proteins that allow them to attach to various receptors to infect
macrophages, red blood cells, or organ cells such as the liver. For example,
multiplication of Plasmodium occurs in erythrocytes and results in the release of
additional parasites that infect other erythrocytes. Periodic (48 to 72 hours) lysis of
the erythrocytes results in anemia and induction of cytokines (e.g., TNF-α, IFN-γ,
IL-1) that provoke fever, chills, sweating, headache, muscle pains, and vomiting,
Severe symptoms include anemia, pulmonary edema, and other complications
causing death. Neurologic complications may result from infected red blood cells
adhering to endothelium in capillaries of the brain.
Countermeasures Against Infectious
Microorganisms
The body's innate and adaptive responses against microorganisms are numerous
and involve an interaction between the immune and inflammatory systems.
Pathogenic microorganisms, however, have developed means of circumventing the
individual's protective defenses. Therefore prophylactic or interventive procedures
have been developed either to prevent the pathogen from initiating disease
(vaccines, public health measures) or to destroy the pathogen once the disease
process has started (antimicrobials). Most vaccine development has focused on
preventing the most severe and common infections (Table 8-8). With the initial
success of antibiotic therapy, there was no perceived need for vaccination against
many common and non–life-threatening infections. The increasing problem of
antibiotic-resistant pathogens, however, has forced a reappraisal of that strategy, and
a greater emphasis now is being placed on the development of new vaccines.
TABLE 8-8
Reduction in Vaccine-Preventable Diseases in the United States as of
2009
Disease Baseline 20th Century Annual Cases* 2011* Cases % Reduction
Diphtheria 175,885 0 100
Measles 503,282 212 99.9
Mumps 152,209 370 99.4
Pertussis 147,271 15,216 90.8
Smallpox 48,164 0 100
Polio 16,316 0 100
Rubella 47,745 4 99.9
Tetanus 1,314 9 99.9
Haemophilus influenzae type b, invasive 20,000 1,170 94.2
*Average number of reported cases over multiple years before initiation of vaccine.
From Centers for Disease Control and Prevention: *2012 data from provisional cases of selected notifiable
diseases, MMWR Morb Mortal Wkly Rep 60(51):1762-1765, 2011. Available at:
http://www.cdc.gov/vaccines/pubs/pinkbook/downloads/appendices/G/cases-deaths.pdf.
Infection Control Measures
Although effective means of safeguarding populations from exposure to infectious
disease are well-known, lack of implementation or breakdowns in application of
these initiatives has led to the reemergence of some infectious diseases, particularly
in less developed countries. The following are some examples of environmental
infection control measures:
1. Sanitary disposal of sewage, garbage, and animal waste
2. Provision of water treatment and prevention of water contamination
3. Maintenance of sanitation practices for the transport, preparation, and serving of
food
4. Control of insect vectors by draining standing water and implementation of
mosquito eradication programs
5. Support of research to develop safe agents for insecticide-resistant insect vectors
Antimicrobials
Since initiation of the widespread use of penicillin during World War II, antibiotics
have significantly prevented the spread of infections. Antibiotics are natural
products of fungi, bacteria, and related microorganisms that affect the growth of
http://www.cdc.gov/vaccines/pubs/pinkbook/downloads/appendices/G/cases-deaths.pdf
other microorganisms. Some antibacterial antibiotics are bactericidal (kill the
microorganism), whereas others are bacteriostatic (inhibit growth until the
microorganism is destroyed by the individual's own protective mechanisms). The
mechanisms of action of most antibiotics are (1) inhibition of the function or
production of the cell wall/membrane, (2) prevention of protein synthesis, (3)
blockage of DNA replication, or (4) interference with folic acid metabolism (Table
8-9). Because viruses use the enzymes of the host's cells, there has been far less
success in developing antiviral antibiotics.
TABLE 8-9
Chemicals or Antimicrobials Identified That Prevent Growth of or Destroy
Microorganisms
Mechanism of Action Agents
Inhibits synthesis of cell wall Penicillins, cephalosporins, monobactams, carbapenems, vancomycin, bacitracin, cycloserine,
fosfomycin
Cell membrane inhibitors Amphotericin, ketoconazole, polymycin
Damages cytoplasmic membrane Polymyxins, polyene antifungals, imidazoles
Alters metabolism of nucleic acid Quinolones, rifampin, nitrofurans, nitroimidazoles
Inhibits protein synthesis Aminoglycosides, tetracyclines, chloramphenicol, macrolides, clindamycin, spectinomycin
Inhibits folic acid synthesis (needed for protein
synthesis)
Sulfonamides, trimethoprim
Alters energy metabolism Trimethoprim, dapsone, isoniazid
Adapted from Brenner GM, Stevens CW: Pharmacology, ed 4, Philadelphia, 2013, Saunders.
Immediately after antibiotics became widely used, antibiotic-resistant
microorganisms were observed. By 1944 an adequate supply of penicillin allowed
its widespread use to treat infections. In 1946 a hospital in Britain reported that 14%
of all Staphylococcus aureus infections were penicillin resistant, producing β-
lactamase, an enzyme that destroys penicillin. The same hospital reported an
increase to 59% by 1950 and to greater than 89% in the 1990s.
More than 2 million individuals develop antibiotic-resistant infections yearly,
resulting in more than 23,000 deaths. Antibiotic resistance to a single antibiotic has
rapidly progressed to multiple-antibiotic resistance. The CDC released a lengthy
document on Antibiotic Resistance Threats in the United States, 2013, in which 18
pathogens were sorted into “Urgent Threats,” “Serious Threats,” and “Concerning
Threats.”19 The most urgent threats are Clostridium difficile (C. difficile),
carbapenem (an “antibiotic of last resort” against penicillin-resistant organisms)
resistant Enterobacteriaceae species (i.e., Klebsiella and E. coli), and drug-resistant
Neisseria gonorrhoeae (N. gonorrhoeae).
Many other infections considered routine and easily treatable are now resistant to
almost all currently available antibiotics, including methicillin-resistant
Staphylococcus aureus [MRSA] and Streptococcus pneumoniae, which causes
pneumonia, meningitis, and acute otitis media (middle ear infection), which were
once routinely susceptible to penicillin. Additionally, there are major increases in
resistant Salmonella typhi (typhoid fever), Shigella (bloody diarrhea), Acinetobacter
(pneumonia), Campylobacter (bloody diarrhea), Enterococcus (sepsis, wound
infection, urinary tract infection), Pseudomonas aeruginosa (burn infection, sepsis),
and Mycobacterium tuberculosis (tuberculosis).20 Antibiotic-resistant fungi (e.g.,
fluconazole-resistant Candida albicans) have evolved and malarial parasites have
recently developed broad drug resistance, including to chloroquine—the previous
mainstay of the preventive and therapeutic arsenal of antimalarial drugs.
Antibiotic resistance is usually a result of genetic mutations that can be
transmitted directly to neighboring microorganisms by plasmid exchange or
incorporation of free DNA. Some microorganisms can inactivate antibiotics,
penicillin resistance being the classic example. Other forms of resistance result
from modification of the target molecule. Azidothymidine (AZT) is a family of
antivirals that suppresses the enzymatic activity of reverse transcriptase, a viral-
specific enzyme responsible for the replication of viral RNA and production of a
DNA copy. HIV frequently mutates and produces an AZT-resistant reverse
transcriptase. Multidrug transporters in the microorganism's membrane mediate a
third mechanism of resistance. These transporters affect the rate of intracellular
accumulation of the antimicrobial by preventing entrance or, more commonly, by
increasing active efflux of the antibiotic. Antibiotic-resistant strains of M.
tuberculosis are protected from aminoglycosides and tetracycline by a multidrug
pump that increases efflux.
Why have multiple antibiotic–resistant microorganisms appeared? Lack of
compliance in completing the therapeutic regimen with antibiotics allows the
selective resurgence of microorganisms that are more relatively resistant to the
antibiotic. Overuse of antibiotics can lead to the destruction of the normal
microbiome, allowing the selective overgrowth of antibiotic-resistant strains or
pathogens that had previously been controlled. There also is concern that overuse of
antibiotics to promote growth in cattle results in ingestion of antibiotic-containing
meat.21
Active Immunization
Recovery from an infection generally results in the strongest resistance to a future
infection with the same microbe. Vaccines are biologic preparations of antigens that
when administered stimulate production of protective antibodies or cellular
immunity against a specific pathogen without causing potentially life-threatening
disease. The purpose of vaccination is to induce long-lasting protective immune
responses under safe conditions. The primary immune response from vaccination is
generally short lived; therefore booster injections are used to push the immune
response through multiple secondary responses that result in large numbers of
memory cells and sustained protective levels of antibody or T cells, or both.
Mass vaccination programs have been tremendously successful and have led to
major changes in the health of the world's population.22 In the early 1950s an
estimated 50 million cases of smallpox occurred each year, with about 15 million
deaths. The World Health Organization (WHO) conducted an aggressive
immunization campaign from 1967 to 1977 that resulted in the global eradication of
smallpox by 1979. Many vaccines are used in the United States and the Centers for
Disease Control and Prevention (CDC) provides updated vaccine schedules at their
website: www.cdc.gov/vaccines/recs/schedules/default.htm.
Development of a successful vaccine is costly and depends on several factors.
These include identification of the protective immune response and the appropriate
antigen to induce that response. For instance, individuals with ongoing HIV
infection produce a great deal of antibody against several HIV antigens. But, for
development of a successful vaccine, we must first understand which antibody, if
any, will protect against an initial infection.
Once a good candidate antigen is identified, it must be developed into an
effective, cost-efficient, stable, and safe vaccine. Most vaccines against viral
infection (measles, mumps, rubella, varicella [chickenpox]) contain live viruses that
are weakened (attenuated virus) so they continue to express appropriate antigens
but establish only a limited and easily controlled infection. Limited replication of
the virus appears to afford better long-term protection than using viral antigen.
Current exceptions are the hepatitis B vaccine, which uses a recombinant viral
protein, and the hepatitis A vaccine, which is an inactivated (killed) virus and
normally should not cause an infection.
Even attenuated viruses can establish life-threatening infections in individuals
whose immune systems are deficient or suppressed. The risk of infection by the
vaccine strain of virus is extremely small, but it may affect the choice of
recommended vaccines. For instance, the Sabin polio vaccine was an attenuated
virus that was administered orally. It provided systemic protection and induced a
secretory immune response to prevent growth of the poliovirus in the intestinal
tract. Being a live virus, the vaccine could cause polio in some children who had
unsuspected immune deficiencies (about 1 case in 2.4 million doses). The Salk
vaccine was a completely inactivated virus administered by injection. It induced
protective systemic immunity but did not provide adequate secretory immunity.
Therefore even if the individual was protected from systemic infection by
poliovirus, the virus could establish a limited infection in the individual's intestinal
http://www.cdc.gov/vaccines/recs/schedules/default.htm
mucosa, be shed, and infect others. When polio was epidemic, the oral vaccine was
preferred. However, the live attenuated vaccine itself caused about eight cases of
paralytic polio per year in the United States in individuals with inadequate immune
systems. As a result, the current recommendation of the CDC is vaccination with the
killed virus.
Some common bacterial vaccines are killed microorganisms or extracts of
bacterial antigens. The vaccine against pneumococcal pneumonia consists of a
mixture of capsular polysaccharides from 23 strains of Streptococcus pneumoniae.
Of the more than 90 known strains of this microorganism, these 23 cause the most
severe illnesses. However, the capsular vaccine is not very immunogenic in young
children. A conjugated vaccine is available that contains capsular polysaccharides
from 13 strains conjugated to carrier proteins in order to increase immunogenicity.
A similar vaccine is available for Haemophilus influenzae type b (Hib).
Some bacterial pathogens are not invasive, but colonize mucosal membranes or
wounds and release potent exotoxins that act locally or systemically. Vaccination
against systemic exotoxins (e.g., diphtheria, tetanus, pertussis) has been achieved
using toxoids—purified exotoxins that have been chemically detoxified without loss
of immunogenicity. Pertussis (whooping cough) vaccine has been changed from a
killed whole-cell vaccine to cellular extract (acellular) vaccine that contains the
pertussis toxoid and additional bacterial antigens. This change has dramatically
reduced adverse side effects (fever, local inflammatory reactions, and others) of
vaccination.
With so many recommended vaccines, there has been an effort to combine
vaccines in order to minimize the number of required injections. One of the first
licensed vaccine mixtures was DPT, which now usually contains diphtheria (D) and
tetanus (T) toxoids and acellular pertussis vaccine (aP). More recent mixtures
include DTaP with inactivated poliovirus, either with Hib conjugate to tetanus toxoid
or with hepatitis B antigen.
Common problems confronting vaccination programs include access to the
programs in less developed countries or lack of compliance of the susceptible
population even when vaccination programs are available. A certain percentage of
the population will be genetically unresponsive or less responsive to a particular
vaccine and therefore will not produce a protective immune response. As many as
10% of the population may not respond adequately to the recommended series of
injections. With most vaccines, the percentage of unresponsive individuals is low,
and they will benefit from successful immunization of the rest of the population.
Depending on the microorganism, a certain percentage of the population (usually
about 85%) should be immunized in order to achieve protection of the total
population. This is referred to as herd immunity. If this level of immunization is not
achieved, outbreaks of infection can occur. More recently resistance to
immunization with measles has increased, and in early 2008 the number of measles
cases in the United States increased by about fourfold. In several European countries
antivaccine groups have disrupted immunization programs. As a result the incidence
of pertussis (whooping cough) increased by 10 to 100 times in those countries
compared with neighboring countries that maintained a high incidence of
immunization. Immunizations should be complete before children start school.
The reluctance to vaccinate has generally been based on potential vaccine
dangers.23 As with any medicine, complications can arise. In the case of vaccines,
these include pain and redness at the injection site, fever, allergic reactions to
vaccine ingredients, infection associated with attenuated viruses in immune-deficient
individuals, and others. More severe dangers do exist, although they are extremely
rare. More commonly the reluctance to vaccination is based on inadequate
information.24 A common fear related to the presence of the preservative thimerosal
in vaccines. Thimerosal is a mercury-containing compound that had been used as a
preservative since the 1930s. Although no cases of mercury toxicity have been
reported secondary to vaccination, thimerosal was removed from all vaccines in
2001, with the exception of some inactivated influenza vaccines. In 2003 groups in
northern Nigeria claimed that the oral vaccine was unsafe and were tainted with
antifertility drugs (estradiol), HIV, and cancer-causing agents.25 The reasoning
appeared to be secondary to mounting distrust of Western nations because of
conflicts in the Middle East. The effect was suspension of polio immunization for
almost 1 year in two Nigerian states and reduction of immunization in three other
states. The incidence of polio rose dramatically, and more than 27,000 cases of
paralysis resulted. The goal of the WHO is to eradicate polio worldwide by 2022.
As of November 2014, the total global number of wild polio (naturally occurring)
cases was 291; the highest number of cases were in Pakistan (246).26
Passive Immunotherapy
Passive immunotherapy is a form of countermeasure against pathogens in which
preformed antibodies are given to the individual. Passive immunotherapy with
human immunoglobulin has been approved for several infections, including
hepatitis A and hepatitis B. Treatment of potential rabies infection after a bite
combines passive and active immunization. The rabies virus proliferates very
slowly.27 Individuals who have been bitten receive a onetime injection with human
rabies immunoglobulin, or, more recently, with monoclonal antibody to slow
further viral proliferation, followed by multiple injections with a killed viral
vaccine to induce greater protective immunity. More specific therapy with
monoclonal antibodies is being evaluated for other infectious diseases. A
monoclonal antibody against respiratory syncytial virus has been approved for
therapy, and recently an experimental monoclonal antibody preparation seems to
have neutralized the Ebola virus.
In the past, vaccines and therapeutic antibodies were developed only for the most
deadly pathogens. With the increase in antibiotic-resistant microorganisms, the
development and widespread use of new vaccines and antibodies against these
microorganisms must be considered.28
Quick Check 8-1
1. How do antigenic changes in viral pathogens promote disease?
2. What are three mechanisms pathogens use to block the immune system?
3. What is the difference between an endotoxin and an exotoxin?
4. How do bacteria develop antibiotic resistance?
Deficiencies in Immunity
An immune deficiency is the failure of the immune or inflammatory response to
function normally, resulting in increased susceptibility to infections. Primary
(congenital) immune deficiency is caused by a genetic defect, whereas secondary
(acquired) immune deficiency is caused by another condition, such as cancer,
infection, or normal physiologic changes, such as aging. Acquired forms of
immune deficiency are far more common than the congenital forms.
Initial Clinical Presentation
The clinical hallmark of immune deficiency is a tendency to develop unusual or
recurrent, severe infections. The most severe primary immune deficiencies develop
in young children, 2 years old and younger. Preschool and school-age children
normally may have 6 to 12 infections per year, and adults may have 2 to 4 infections
per year. Most of these are not severe and are limited to viral infections of the upper
respiratory tract, recurrent streptococcal pharyngitis, or mild otitis media (middle
ear infections).
Potential immune deficiencies should be considered if the individual has
experienced severe, documented bouts of pneumonia, otitis media, sinusitis (sinus
infection), bronchitis, septicemia (blood infection), or meningitis or infections with
rare opportunistic microorganisms (e.g., Pneumocystis carinii).29 Infections are
generally recurrent with only short intervals of relative health, and multiple
simultaneous infections are common. Individuals with immune deficiencies often
have eight or more purulent ear infections, two or more serious sinus infections,
and two or more pneumonias, recurrent abscesses, or persistent fungal infections
(particularly thrush) within a year. Invasive fungal infections are rare in healthy
individuals and strongly indicate a defective immune system. Recurrent internal
infections, such as meningitis, osteomyelitis, or sepsis, are common. Prolonged
antibiotic use is commonly ineffective by oral or injected routes and may
necessitate intravenous administration. Children frequently present with failure to
thrive because of chronic diarrhea and other chronic symptoms. A familial history
of immune deficiency may be found in some types of primary deficiency.
Routine care of individuals with immune deficiencies must be tempered with the
knowledge that the immune system may be totally ineffective. It is unsafe to
administer conventional immunizing agents or blood products to many of these
individuals because of the risk of causing an uncontrolled infection. Infection is a
particular problem when attenuated vaccines that contain live but weakened
microorganisms are used (e.g., live polio vaccine; vaccines against measles,
mumps, and rubella).
The type of recurrent infections may indicate the type of immune defect.
Deficiencies in T-cell immune responses are associated with recurrent infections
caused by certain viruses (e.g., varicella herpes, cytomegalovirus), fungi, and yeasts
(e.g., Candida, Histoplasma), or atypical microorganisms (e.g., P. carinii). B-cell
deficiencies and phagocyte deficiencies, however, are suggested if the individual
has documented, recurrent infections with microorganisms that require
opsonization (e.g., encapsulated bacteria) or with viruses against which humoral
immunity is normally effective (e.g., rubella). Some complement deficiencies
resemble defects in antibody or phagocyte function, but others are associated with
disseminated infections with bacteria of the genus Neisseria (Neisseria meningitides
and Neisseria gonorrhoeae).
Primary (Congenital) Immune Deficiencies
Most primary immune deficiencies are the result of single gene defects (Table 8-10).
Generally, the mutations are sporadic and not inherited: a family history exists in
only about 25% of individuals. The sporadic mutations occur before birth, but the
onset of symptoms may be early or later, depending on the particular syndrome. In
some instances, symptoms of immune deficiency appear within the first 2 years of
life. Other immune deficiencies are progressive, with the onset of symptoms
appearing in the second or third decade of life.
TABLE 8-10
Examples of Primary Immune Deficiencies
Classification Example Immune Deficiency Outcome
Combined Immune Deficiencies: Without Nonimmune Defects
Defective development of both B
and T cells
Severe combined
immunodeficiencies
(SCIDs)
X-linked SCID
Lack of both T and B cells, little or no
antibody production or cellular immunity
Defective interleukin receptors needed for
lymphocyte maturation
Recurrent, life-threatening infections with variety of
microorganisms
Recurrent, life-threatening infections with variety of
microorganisms
Defects in cooperation among B
cells, T cells, and antigen-
presenting cells
Bare lymphocyte
syndrome
No antigen presentation because of lack of
MHC class I or MHC class II molecules on
cell surface
Recurrent, life-threatening infections with variety of
microorganisms
Combined Immune Deficiencies: With Nonimmune Defects
Defect in actin cytoskeleton Wiskott-Aldrich
syndrome (WAS)
Decreased IgM antibody Recurrent infections with encapsulated bacteria;
thrombocytopenia; eczema
Defective development of T cells
in central lymphoid organ
(thymus)
DiGeorge syndrome Lack of T cells Recurrent, life-threatening fungal and viral
infections; defective parathyroid gland; abnormal
facial development
Predominantly Antibody Deficiencies
Defect in class-switch to IgA Selective IgA
deficiency
Diminished or absent IgA Asymptomatic or recurrent mild sinus, pulmonary,
and gastrointestinal infections
Defect in development of B cells
in the bone marrow
Bruton
agammaglobulinemia
Few B cells Recurrent bacterial infections
Phagocytic Defects
Defects in production of
neutrophils
Severe congenital
neutropenia
Lack of neutrophils Recurrent, life-threatening bacterial infections
Defects in bacterial killing Chronic
granulomatous
disease
Lack of production of oxygen products (e.g.,
hydrogen peroxide)
Recurrent infections with bacteria that are sensitive
to killing by oxygen-dependent mechanisms
Defects in Innate Immunity
Defect in development of cellular
immunity against specific antigen
Chronic
mucocutaneous
candidiasis
Lack of T-cell response to Candida Recurrent and disseminated infections with fungus
Candida albicans
Complement Deficiencies
Defective production of C3 C3 deficiency Little or no C3 produced Recurrent, life-threatening bacterial infections
Defective production of
component of membrane attack
complex
C6, C7, C8, or C9
deficiency
Little or no C6, C7, C8, or C9 produced Recurrent disseminated infections with Neisseria
gonorrhoeae or N. meningitides
Defective production of
component of lectin pathway
Mannose-binding
lectin (MBL)
deficiency
Little or no activation of lectin pathway Recurrent infections with bacteria and yeast with
mannose-containing capsules
Individually, primary immune deficiencies are rare. For instance, only 30 to 50
new cases of severe combined immunodeficiency (SCID) are diagnosed in the
United States yearly. However, more than 250 different deficiencies have been
identified, and the number is growing rapidly.30 Together, primary immune
deficiencies are more common than cystic fibrosis, hemophilia, childhood
leukemia, or many other well-known diseases. Many are subtle with minor
deficiencies, but several result from major defects and lead to recurrent life-
threatening infections. The distribution between genders is about even, although
some specific diseases have a male or female predominance. The three most
commonly diagnosed deficiencies are common variable immune deficiency (34%
of individuals with primary immune deficiencies), selective immunoglobulin A
(IgA) deficiency (24%), and IgG subclass deficiency (17%).
Primary immune deficiencies have recently been reclassified into nine groups,
based on the principal component of the immune or inflammatory systems that is
defective.31 The major groups include combined with or without nonimmune defects
(both B and T lymphocytes are deficient, although this group contains some
diseases previously classified as T-cell defects), predominantly antibody
deficiencies, immune dysregulation (defects in control of lymphocyte proliferation,
T-regulatory cells defects), phagocytic defects (inadequate numbers or function),
defects in innate immunity, and complement defects. To provide a better
understanding of the diversity and severity of primary immune deficiencies, a few
select examples will be discussed.
Combined Deficiencies
Combined deficiencies include the most life-threatening disorders and result from
defects that directly affect the development of both T and B lymphocytes. However,
the severity depends upon the degree to which B and T cells are affected.32 The most
severe disorders are called severe combined immunodeficiencies (SCIDs). Most
individuals with SCIDs have few detectable lymphocytes in the circulation and
secondary lymphoid organs (spleen, lymph nodes). The thymus usually is
underdeveloped because of the absence of T cells. Immunoglobulin levels,
especially IgM and IgA, are absent or greatly reduced. Several forms of SCID are
caused by autosomal recessive enzymatic defects that result in the accumulation of
toxic metabolites, and rapidly dividing cells, such as lymphocytes, are especially
sensitive. For instance, deficiency of adenosine deaminase (ADA deficiency) results
in the accumulation of toxic purines. X-linked SCID results from a common defect
in most of the important interleukin (IL) receptors needed for lymphocyte
maturation (e.g., IL-2, IL-4, IL-7, and others).
Even if nearly adequate numbers of B and T cells are produced, their cooperation
may be defective. The bare lymphocyte syndrome is an immune deficiency
characterized by an inability of lymphocytes and macrophages to produce major
histocompatibility complex (MHC) class I or class II molecules. Without MHC
molecules, antigen presentation and intercellular cooperation cannot occur
effectively. Children with this deficiency develop serious, life-threatening infections
and usually die before the age of 5 years.
Some combined immune deficiencies result in depressed development of a small
portion of the immune system. For instance, an individual can be unable to produce
a certain class of antibody, as in Wiskott-Aldrich syndrome (WAS, an X-linked
recessive disorder), where IgM antibody production is greatly depressed. Antibody
responses against antigens that elicit primarily an IgM response, such as
polysaccharide antigens from bacterial cell walls (e.g., P. aeruginosa, S.
pneumoniae, Haemophilus influenzae, and other microorganisms with
polysaccharide outer capsules), are deficient.
Many combined immune deficiencies also are associated with other characteristic
defects, some of which appear to be unrelated to the immune system yet may be life-
threatening by themselves. These associated symptoms can be useful diagnostically
and can clarify the pathophysiology of the disease. WAS results from a mutation in
the WAS gene that affects the actin cytoskeleton, which is important for platelet
function. Thus, WAS has an associated major defect in platelet function and is
classified as a combined deficiency with nonimmune defects. Clinical
manifestations include bleeding secondary to thrombocytopenia (low platelet
counts), eczema, and recurrent infections (e.g., otitis media, pneumonia, herpes
simplex, cytomegalovirus).
DiGeorge syndrome (congenital thymic aplasia or hypoplasia and diminished
parathyroid gland development) is caused by the lack or partial lack of the thymus,
resulting in greatly decreased T-cell numbers and function. Defective development
of the third and fourth pharyngeal pouches during embryonic development results
in the thymic defects and the lack of the parathyroid gland (causing an inability to
regulate calcium concentration). Low blood calcium levels cause the development
of tetany or involuntary rigid muscular contraction. DiGeorge syndrome is
frequently associated with abnormal development of facial features that are
controlled by the same embryonic pouches; these include low-set ears, fish-shaped
mouth, and other altered features (Figure 8-6). Other examples of combined
immune deficiencies include defects in CD3 resulting in the loss of T-cell receptor
intracellular signaling, defective somatic gene rearrangement of variable region
genes or constant region genes, IL-2 receptor defects, and defects in DNA repair.
FIGURE 8-6 Facial Anomalies Associated with DiGeorge Syndrome. Note the wide-set eyes,
low-set ears, and shortened structure of the upper lip. (From Male D et al: Immunology, ed 8, Philadelphia, 2013,
Mosby.)
Predominantly Antibody Deficiencies
Predominantly antibody deficiencies result from defects in B-cell maturation or
function and are the most common of immune deficiencies.33 T-cell immune
responses are not affected in pure B-lymphocyte deficiencies. The results are lower
levels of circulating immunoglobulins (hypogammaglobulinemia) or occasionally
totally or nearly absent immunoglobulins (agammaglobulinemia).
Some defects may involve a particular class of antibody, such as selective IgA
deficiency, in which only IgA is suppressed. This occurs in 1 in 700 to 1 in 400
individuals and may result from a failure to class-switch to IgA and mature into
IgA-producing plasma cells. Many individuals are asymptomatic, although others
have a history of recurring sinus, pulmonary, and gastrointestinal infections.
Individuals with IgA deficiency often have chronic intestinal candidiasis (infection
with C. albicans). Complications of IgA deficiency include severe allergic disease
and autoimmune diseases. Secretory IgA normally may prevent the uptake of
allergens from the environment; therefore IgA deficiency may lead to a more
intense challenge to the immune system by environmental antigens.
Bruton agammaglobulinemia is caused by blocked development of mature B
cells in the bone marrow. There are few or no circulating B cells, although T-cell
number and function are normal, resulting in repeated bacterial infections, such as
otitis media, streptococcal sore throat, and conjunctivitis, and more serious
conditions, such as septicemia.
Other predominantly antibody deficiencies include severe reduction in particular
classes or subclasses of antibody; defects in B-cell surface receptors, such as CD21
and CD40; and defects in class-switch, which may result in a hyper-IgM syndrome.
Phagocyte Defects
Phagocyte defects range from inadequate numbers of phagocytes (e.g., severe
congenital neutropenia) to defects in phagocyte function that can result in
recurrent infections with the same group of microorganisms (encapsulated bacteria)
associated with antibody, and complement deficiencies. Chronic granulomatous
disease (CGD) is a severe defect in the myeloperoxidase–hydrogen peroxide
system—a major means of bacterial destruction using the enzyme myeloperoxidase,
halides (e.g., chloride ion), and hydrogen peroxide (H2O2).
34 As a result of
phagocytosis, neutrophils and other phagocytes switch much of their glucose
metabolism to the hexose-monophosphate shunt. A byproduct of this pathway is the
conversion of molecular oxygen by nicotinamide adenine dinucleotide phosphate
(NADPH) oxidase into highly reactive oxygen derivatives, including hydrogen
peroxide. Mutations in NADPH oxidase result in deficient production of hydrogen
peroxide and other oxygen products needed for phagocytic killing. Thus, affected
individuals have adequate myeloperoxidase and halide but lack the necessary
hydrogen peroxide. This results in recurrent severe pneumonias; tumor-like
granulomata in lungs, skin, and bones; and other infections with some opportunistic
microorganisms, such as Staphylococcus aureus, Serratia marcescens, and
Aspergillus species. Other phagocytic deficiencies include defects in various
leukocyte adhesion molecules, defects in the phagocytosis process or bacterial
killing, and defects in cytokine receptors.
Defects in Innate Immunity
Some immune deficiencies are characterized by a defect in the capacity to produce
an immune response against a particular antigen. In chronic mucocutaneous
candidiasis, interaction between the Th17 lymphocytes and macrophages is
ineffective related to a specific infectious agent, C. albicans. Thus the macrophage
cannot be activated and these individuals usually have mild to extremely severe
recurrent Candida infections involving the mucous membranes and skin. Other
defects in innate immunity include defects in Toll-like receptors and natural killer
cells.
Complement Deficiencies
Many complement deficiencies have been described. C3 deficiency is the most
severe defect because of its central role in the complement cascade. Loss of C3b and
C3a production and the inability to activate C5 result in recurrent life-threatening
infections with encapsulated bacteria (e.g., Haemophilus influenzae and
Streptococcus pneumoniae) at an early age. Deficiencies of any of the terminal
components of the complement cascade (C5, C6, C7, C8, or C9 deficiencies) are
associated with increased infections with only one group of bacteria—those of the
genus Neisseria (Neisseria meningitides or N. gonorrhoeae). Neisseria bacteria
usually cause localized infections (meningitis or gonorrhea), but terminal pathway
defects result in an 8000-fold increased risk for systemic infections with atypical
strains of these microorganisms.
Mannose-binding lectin (MBL) deficiency is the primary defect of the lectin
pathway of complement activation. This defect, as well as defects in the alternative
pathway, results in increased risk of infection with microorganisms that have
polysaccharide capsules rich in mannose, particularly the yeast Saccharomyces
cerevisiae and encapsulated bacteria such as N. meningitidis and S. pneumoniae.
Other complement deficiencies include defects in components C1, C4, C2, C5, C1
inhibitor, factor B, factor D, properdin, complement control factors, MASP, or
complement receptors.
Secondary (Acquired) Immune Deficiencies
Secondary, or acquired, immune deficiencies are far more common than primary
deficiencies. These deficiencies are complications of other physiologic or
pathophysiologic conditions. Some conditions that are known to be associated with
acquired deficiencies are summarized in Box 8-1.
Box 8-1
Some Conditions Known to Be Associated with
Acquired Immunodeficiencies
Normal Physiologic Conditions
Pregnancy
Infancy
Aging
Psychologic Stress
Emotional trauma
Eating disorders
Dietary Insufficiencies
Malnutrition caused by insufficient intake of large categories of nutrients, such as
protein or calories
Insufficient intake of specific nutrients, such as vitamins, iron, or zinc
Infections
Congenital infections, such as rubella, cytomegalovirus, hepatitis B
Acquired infections, such as AIDS
Malignancies
Malignancies of lymphoid tissues, such as Hodgkin disease, acute or chronic
leukemia, or myeloma
Malignancies of nonlymphoid tissues, such as sarcomas and carcinomas
Physical Trauma
Burns
Medical Treatments
Stress caused by surgery
Anesthesia
Immunosuppressive treatment with corticosteroids or antilymphocyte antibodies
Splenectomy
Cancer treatment with cytotoxic drugs or ionizing radiation
Other Diseases or Genetic Syndromes
Diabetes
Alcoholic cirrhosis
Sickle cell disease
Systemic lupus erythematosus (SLE)
Chromosome abnormalities, such as trisomy 21
Although secondary deficiencies are common, many are not clinically relevant.
In many cases, the degree of the immune deficiency is relatively minor and without
any apparent increased susceptibility to infection. Alternatively, the immune system
may be substantially suppressed, but only for a short duration, thus minimizing the
incidence of clinically relevant infections. Some secondary immune deficiencies
(e.g., AIDS or immunosuppression by cancer), however, are extremely severe and
may result in recurrent life-threatening infections.
Evaluation and Care of Those with Immune
Deficiency
A review of clinical characteristics can help select the appropriate tests. A basic
screening test is a complete blood count (CBC) with a differential. The CBC
provides information on the numbers of red blood cells, white blood cells, and
platelets, and the differential indicates the quantities of lymphocytes, granulocytes,
and monocytes in the blood. Quantitative determination of immunoglobulins (IgG,
IgM, IgA) is a screening test for antibody production, and an assay for total
complement (total hemolytic complement, CH50) is useful if a complement defect is
suspected. Further testing is described in Table 8-11.
TABLE 8-11
Laboratory Evaluation of Immune Deficiencies
Function
Tested
Laboratory Test Significance of Test
Tests of Humoral Immune Function
Antibody
production
Total immunoglobulin levels, including IgG,
IgM, and IgA
Decrease or absence of total antibody production or of specific classes of antibody, which
is associated with many B-cell and combined deficiencies
Levels of isohemagglutinins Production of specific IgM antibodies, which is decreased in some combined deficiencies;
not useful with persons who are blood type AB and do not have naturally occurring
isohemagglutinins
Levels of antibodies against vaccines—especially
diphtheria and tetanus toxoids
Production of specific IgG antibodies, which is decreased when B cells are deficient or
class-switch is blocked
B-cell numbers Numbers of lymphocytes with surface
immunoglobulin
Production of circulating B cells, which is decreased in many severe B-cell or combined
deficiencies
Antibody
subclasses
Level-specific subclasses, particularly IgG1,
IgG2, and IgG3
Decrease or absence of a particular subclass, which is characteristic of several immune
deficiencies
Tests of Cellular Immune Function
Delayed
hypersensitivity
skin test
Skin test reaction against previously encountered
antigens, especially Candida albicans or
tetanus toxoid
Defects in antigen-responsive T cells and skin test cellular interactions (e.g., lymphokine
activity and macrophage function)
T-cell numbers Numbers of T cells expressing characteristic
membrane antigens (CD3 or CD11)
Defects in production of circulating T cells
T-cell
proliferation in
vitro
Proliferative response to nonspecific mitogens
(e.g., phytohemagglutinin)
General T-cell defects in response to nonspecific stimulation (mitogens)
Proliferative response to antigens (e.g., tetanus
toxoid)
Defects in response of T cells to specific antigens
T-cell
subpopulations
Quantify percentage of T cells with specific
markers for total T cells (CD3), Th cells (CD4),
Tc cells (CD8)
Decrease in numbers of CD4 cells, which is related to AIDS progression
Replacement Therapies for Immune Deficiencies
Many immune deficiencies can be successfully treated by replacing the missing
component of the immune system. Individuals with B-cell deficiencies that cause
hypogammaglobulinemia or agammaglobulinemia usually are treated by
administration of intravenous immune globulin (IVIg), antibody-rich fractions
prepared from plasma pooled from large numbers of donors.35 Administration of
IVIg replaces the individual's antibodies temporarily; these antibodies have a half-
life of 3 to 4 weeks. Thus individuals must be treated repeatedly to maintain a
protective level of antibodies in the blood.
Defects in lymphoid cell development in the primary lymphoid organs (e.g.,
SCID, Wiskott-Aldrich syndrome) can sometimes be treated by replacement of stem
cells through transplantation of bone marrow, umbilical cord cells, or other cell
populations that are rich in stem cells. Thymic defects (e.g., DiGeorge syndrome,
chronic mucocutaneous candidiasis) may be treated by transplantation of fetal
thymus tissue or thymic epithelial cells (the cells that produce thymic hormones).
However, in most cases improvement is only temporary.
Enzymatic defects that cause SCID (e.g., adenosine deaminase deficiency) have
been treated successfully with transfusions of glycerol frozen-packed erythrocytes.
The donor erythrocytes contain the needed enzyme and can, at least temporarily,
provide sufficient enzyme for normal lymphocyte function.
Bone marrow transplants containing hematopoietic stem cells are routinely used
to treat SCID. However, as discussed later in this chapter, the donor and recipient
should be matched as closely as possible for HLA antigens. Individuals with SCID
are at risk for graft-versus-host disease (GVHD). This occurs if T cells in a
transplanted graft (e.g., transfused blood, bone marrow transplants) are mature and
therefore capable of cell-mediated immunity against the recipient's HLA. The
primary targets for GVHD are the skin (e.g., rash, loss or increase of pigment,
thickening of skin), liver (e.g., damage to bile duct, hepatomegaly), mouth (e.g., dry
mouth, ulcers, infections), eyes (e.g., burning, irritation, dryness), and
gastrointestinal tract (e.g., severe diarrhea), and the disease may lead to death from
infections. The risk of GVHD can be diminished by removing mature T cells from
tissue used to treat individuals with immune deficiencies.36
Injection of mesenchymal stem cells (MSCs) may be useful in these individuals.
Stem cells are relatively undifferentiated cells and can be obtained from a variety of
sources (e.g., embryos, bone marrow, adult tissues). MSCs are present in all adult
tissues. These particular stem cells undergo differentiation into other cell types and,
more importantly, have potent immunosuppressive properties.37 Several clinical
trials have demonstrated complete suppression of GVHD in a large number of
recipients of MSCs.38
The first successful therapeutic replacement of defective genes was performed in
two girls with SCID caused by an ADA deficiency.39 The normal gene for ADA was
cloned and inserted into a retroviral vector.40 The gene for ADA replaced some
retroviral genes, resulting in a virus that carried the normal human gene but did not
cause disease. The virus was used to infect bone marrow stem cells from these
children. The retrovirus inserted the normal ADA gene into the individuals' genetic
material. The genetically altered stem cells were infused into the children, resulting
in reconstitution of their immune systems. Gene therapy trials have verified immune
reconstitution in individuals with ADA deficiency, X-linked SCID, CGD, and WAS.41
However, the treatment trials have not been without some major complications, such
as leukemia, that raise questions concerning the use of retroviral vectors for the
insertion of new genes.
Acquired Immunodeficiency Syndrome (AIDS)
Acquired immunodeficiency syndrome is a secondary immune deficiency that
develops in response to viral infection. The human immunodeficiency virus (HIV)
infects and destroys the CD4-positive (CD4+) Th cells, which are necessary for the
development of both plasma cells and cytotoxic T cells. Therefore HIV suppresses
the immune response against itself and secondarily creates a generalized immune
deficiency by suppressing the development of immune responses against other
pathogens and opportunistic microorganisms, leading to the development of
acquired immunodeficiency syndrome (AIDS).
Despite major efforts by healthcare agencies around the world, the number of
cases and deaths from HIV infection and AIDS (HIV/AIDS) remains a major health
concern. The WHO estimated that at the end of 2013, 35.3 million people were
living with HIV/AIDS worldwide and more than 2.5 million were newly infected.42
Approximately 3 million deaths occur each year from AIDS. Since 1980 it is
estimated that more than 36 million individuals have died from AIDS worldwide.
The majority of cases are in sub–Saharan Africa where about 1 in 20 adults is living
with HIV, but the epidemic is worldwide and the number of new cases is increasing
rapidly, particularly in Asia.
In the United States the spread of HIV/AIDS remains somewhat stable. The CDC
estimated in 2013 (the most recent data) that approximately 47,352 people were
newly infected with HIV.43 Although new infections remain at about 50,000 per year,
both encouraging and discouraging trends were apparent. Although new HIV
infections in black women decreased by 12% between 2008 and 2010, new
infections in young gay and bisexual men increased by 21%. Men who had sex with
men accounted for 78% of new HIV infections in 2010. Heterosexual transmission
accounted for about 25% of new HIV infections. with two thirds of those cases
occurring in women, with the highest number among black women. Deaths related
to HIV/AIDS were 13,712 in 2012, and appear to be decreasing. The cumulative
number of HIV/AIDS-related deaths in the United States is in excess of 658,507, and
more than 1,201,100 persons age 13 years and older are currently living with
HIV/AIDS.
Before the implementation of massive public health campaigns and the use of
antiviral drugs in the United States, the progression from HIV infection to AIDS and
death was unrelenting. In 1995 AIDS became the number one killer of individuals
between the ages of 25 and 44 years and remains the eighth most common cause of
death in that age group. With the advent of effective therapy to stabilize progression
of the disease in the mid-1990s, HIV infection has become a chronic disease in the
United States, with many fewer deaths.
Epidemiology of AIDS
HIV is a blood-borne pathogen with the typical routes of transmission: blood or
blood products, intravenous drug abuse, both heterosexual and homosexual activity,
and maternal-child transmission before or during birth. Although the disease first
gained attention in the United States related to sexual transmission between males,
the most common route worldwide is through heterosexual activity (see Health
Alert: Risk of HIV Transmission Associated with Sexual Practices). Worldwide,
women constitute more than half of those living with HIV/AIDS. In the United
States, as in the rest of the world, the predominant means of transmission to women
is through heterosexual contact. Hundreds of thousands of cases of HIV/AIDS have
been reported in children who contracted the virus from their mothers across the
placenta, through contact with infected blood during delivery, or through the milk
during breast-feeding.
Health Alert
Risk of HIV Transmission Associated with Sexual Practices
High Risk (in descending order of risk)
Receptive anal intercourse with ejaculation (no condom)
Receptive vaginal intercourse with ejaculation (no condom)
Insertive anal intercourse (no condom)
Insertive vaginal intercourse (no condom)
Receptive anal intercourse with withdrawal before ejaculation
Insertive anal intercourse with withdrawal before ejaculation
Receptive vaginal intercourse (with spermicidal foam but no
condom)
Insertive vaginal intercourse (with spermicidal foam but no
condom)
Receptive anal or vaginal intercourse (with a condom)
Insertive anal or vaginal intercourse (with a condom)
Some Risk (in descending order of risk)
Oral sex with men with ejaculation
Oral sex with women
Oral sex with men with preejaculation fluid (precum)
Oral sex with men, no ejaculation or precum
Oral sex with men (with a condom)
Some Risk (depending on situation, intactness of mucous membranes, etc.)
Mutual masturbation with external or internal touching
Sharing sex toys
Anal or vaginal fisting
No Risk
Masturbating with another person without touching one
another
Hugging/massage/dry kissing
Frottage (rubbing genitals while remaining clothed)
Masturbating alone
Abstinence
Unresolved Issues
The role of precum in transmission
The protection offered by covering female genitals with a
dental dam during oral sex on the women
The risk of transmission from wet kissing
Pathogenesis of AIDS
HIV is a member of a family of viruses called retroviruses, which carry genetic
information in the form of RNA rather than DNA (Figure 8-7). Retroviruses use a
viral enzyme, reverse transcriptase, to convert RNA into double-stranded DNA.
Using a second viral enzyme, HIV integrase, the new DNA is inserted into the
infected cell's genetic material, where it may remain dormant. If the cell is activated,
translation of the viral information may be initiated, resulting in the formation of
new virions, lysis and death of the infected cell, and shedding of infectious HIV
particles. During that process, HIV protease is essential in processing proteins
needed from the viral internal structure (capsid). If, however, the cell remains
relatively dormant, the viral genetic material may remain latent for years and is
probably present for the life of the individual.
FIGURE 8-7 The Structure and Genetic Map of HIV-1. The HIV-1 virion consists of a core of two
identical strands of viral RNA molecules of viral enzymes (reverse transcriptase [RT], protease
[PR], integrase [IN]) encoated in a core capsid structure consisting primarily of the structural
viral protein p24. The capsid is further encased in a matrix consisting primarily of viral protein
p17. The outer surface is an envelope consisting of the plasma membrane of the cell from
which the virus budded (lipid bilayer) and two viral glycoproteins: a transmembrane
glycoprotein, gp41, and a noncovalently attached surface protein, gp120. The HIV-1 genome
contains regions that encode the structural proteins (gag), the viral enzymes (pol), and the
envelope proteins (env). The genome of complex retroviruses, like HIV-1, often contains a
variety of small regions that regulate expression of the virus. (Modified from Kumar V et al, editors: Robbins and
Cotran pathologic basis of disease, ed 9, Philadelphia, 2015, Saunders.)
The primary surface receptor on HIV is the envelope protein gp120, which binds
to the molecule CD4 on the surface of Th cells. Several other necessary co-
receptors, particularly the chemokine receptor CCR5, have been identified on target
cells. Thus the major immunologic finding in AIDS is the striking decrease in the
number of CD4+ Th cells (Figure 8-8).
FIGURE 8-8 Life Cycle and Possible Sites of Therapeutic Intervention of Human
Immunodeficiency Virus (HIV). The HIV virion consists of a core of two identical strands of viral
RNA encoated in a protein structure with viral proteins gp41 and gp120 on its surface
(envelope). HIV infection begins when a virion binds to CD4 and chemokine co-receptors on a
susceptible cell and follows the process described here. The provirus may remain latent in the
cell's DNA until it is activated (e.g., by cytokines). The HIV life cycle is susceptible to blockage at
several sites (see the text for further information), including entrance inhibitors, reverse
transcriptase inhibitors, integrase inhibitors, and protease inhibitors. (Modified from Kumar V et al, editors:
Robbins and Cotran pathologic basis of disease, ed 9, Philadelphia, 2015, Saunders.)
Clinical Manifestations of AIDS
Depletion of CD4+ cells has a profound effect on the immune system, causing a
severely diminished response to a wide array of infectious pathogens and cancers
(Box 8-2). At the time of diagnosis, the individual may present with one of several
different conditions: serologically negative (no detectable antibody), serologically
positive (positive for antibody against HIV proteins) but asymptomatic, early stages
of HIV disease, or AIDS (Figure 8-9).
Box 8-2
AIDS-Defining Opportunistic Infections and
Neoplasms Found in Individuals with HIV
Infection
Infections
Protozoal and Helminthic Infections
Cryptosporidiosis or isosporiasis (enteritis)
Pneumocystosis (pneumonia or disseminated infection)
Toxoplasmosis (pneumonia or CNS infection)
Fungal Infections
Candidiasis (esophageal, tracheal, or pulmonary)
Coccidioidomycosis (disseminated)
Cryptococcosis (CNS infection)
Histoplasmosis (disseminated)
Bacterial Infections
Mycobacteriosis (“atypical,” e.g., Mycobacterium avium-intracellulare,
disseminated or extrapulmonary
M. tuberculosis, disseminated or extrapulmonary)
Nocardiosis (pneumonia, meningitis, disseminated)
Salmonella infections (septicemia, recurrent)
Viral Infections
Cytomegalovirus (pulmonary, intestinal, retinitis, or CNS)
Herpes simplex virus (localized or disseminated)
Progressive multifocal leukoencephalopathy
Varicella-zoster virus (localized or disseminated)
Neoplasms
Invasive cancer of the uterine cervix
Kaposi sarcoma
Non-Hodgkin lymphomas (Burkitt, immunoblastic)
Primary lymphoma of brain
CNS, Central nervous system.
From Kumar V et al: Robbins and Cotran pathologic basis of disease, ed 9, Philadelphia, 2015, Saunders.
FIGURE 8-9 Typical Progression from HIV Infection to AIDS in Untreated Persons. A, Clinical
progression begins within weeks after infection; the person may experience symptoms of
acute HIV syndrome. During this early period, the virus progressively infects T cells and other
cells and spreads to the lymphoid organs, with a sharp decrease in the number of circulating
CD4+ T cells. During a period of clinical latency, the virus replicates and T-cell destruction
continues, although the person is generally asymptomatic. The individual may develop HIV-
related disease (constitutional symptoms)—a variety of symptoms of acute viral infection that
do not involve opportunistic infections or malignancies. When the number of CD4+ cells is
critically suppressed, the individual becomes susceptible to a variety of opportunistic infections
and cancers with a diagnosis of AIDS. The length of time for progression from HIV infection to
AIDS may vary considerably from person to person. B, Laboratory tests are changing
throughout infection. Antibody and Tc cell (cytotoxic T lymphocytes [CTLs]) levels change during
the progression to AIDS. During the initial phase antibodies against HIV-1 are not yet detectable
(window period), but viral products, including proteins and RNA, and infectious virus may be
detectable in the blood a few weeks after infection. Most antibodies against HIV are not
detectable in the early phase. During the latent phase of infection antibody levels against p24
and other viral proteins, as well as HIV-specific CTLs, increase, and then remain constant until
the development of AIDS. (A redrawn from Fauci AS, Lane HC: Human immunodeficiency virus disease: AIDS and related conditions.
In Fauci AS et al, editors: Harrison's principles of internal medicine, ed 14, New York, 1997, McGraw-Hill; B from Kumar V et al: Robbins and Cotran
pathologic basis of disease, ed 9, Philadelphia, 2015, Saunders.)
The presence of circulating antibody against the HIV protein p24 followed by
more complex tests for antibodies against additional HIV proteins (e.g., Western
blot analysis) or for HIV DNA (e.g., polymerase chain reaction) indicates infection
by the virus, although many of these individuals are asymptomatic. Antibody
appears rather rapidly after infection through blood products, usually within 4 to 7
weeks, although some individuals have been seronegative for longer periods. The
period between infection and the appearance of antibody is referred to as the
window period. Although a person does not have antibody against HIV, he or she
may have virus growing, have virus in the blood and body fluids, and be infectious
to others.
Those with the early stages of HIV disease (early-stage disease) usually initially
present with relatively mild and nonspecific symptoms resembling influenza, such
as headaches, fever, or fatigue. These symptoms disappear after 1 to 6 weeks, and
although individuals appear to be in clinical latency the virus is actively
proliferating in lymph nodes.
The currently accepted definition of AIDS relies on both laboratory tests and
clinical symptoms. If the individual is positive for antibodies against HIV, the
diagnosis of AIDS is made in association with various clinical symptoms (Figure 8-
10; also see Box 8-2). The symptoms include atypical or opportunistic infections
and cancers, as well as indications of debilitating chronic disease (e.g., wasting
syndrome, recurrent fevers). Most commonly, new cases of AIDS are diagnosed
initially by decreased CD4+ T cell numbers. Individuals who are not HIV infected
typically have 800 to 1000 CD4+ cells per cubic millimeter of blood, with a range
from 600/mm3 to 1200/mm3. A diagnosis of AIDS can be made if the CD4+ T cell
numbers decrease to less than 200/mm3. Without treatment, the average time from
infection to development of AIDS is just over 10 years. Some estimates are that
approximately 99% of untreated HIV-infected individuals would eventually progress
to AIDS.
FIGURE 8-10 Clinical Symptoms of AIDS. A, Severe weight loss and anorexia. B, Kaposi
sarcoma lesions. C, Perianal lesions of herpes simplex infection. D, Deterioration of vision from
cytomegalovirus retinitis leading to areas of infection, which can lead to blindness. (A and D from
Taylor PK: Diagnostic picture tests in sexually transmitted diseases, London, 1995, Mosby; B and C from Morse SA et al, editors: Atlas of sexually
transmitted diseases and AIDS, ed 4, London, 2011, Saunders.)
Treatment and Prevention of AIDS
Approved AIDS medications are classified by mechanism of action: nucleoside and
non-nucleoside inhibitors of reverse transcriptase (reverse transcriptase
inhibitors), inhibitors of the viral protease (HIV protease inhibitors), inhibitors of
the viral integrase (HIV integrase inhibitors), inhibitors of viral entrance into the
target cell (HIV fusion inhibitors), and a CCR5 antagonist (inhibitor of viral
attachment) (see Figure 8-8). The current regimen for treatment of HIV infection is
a combination of drugs, termed antiretroviral therapy (ART). ART protocols
require a combination of synergist drugs from different classes and specific
regimens (e.g., timing of drug administration, doses, drug combinations) are
adapted based on age of the individual, secondary clinical symptoms (renal or
hepatic insufficiency), CD4+ T cell levels, viral load, specific coinfections, pre-
existing cardiac risk factors, past history of treatment failure, suspected drug
resistance, and other parameters.44,45 The clinical benefits of ART are profound.
Death from AIDS-related diseases has been reduced significantly since the
introduction of ART. However, resistant variants to these drugs have been identified.
Drug therapy for AIDS is not curative because HIV incorporates into the genetic
material of the host, particularly CD4+ T memory cells, and may never be removed
by antimicrobial therapy.46 Therefore drug administration to control the virus may
have to continue for the lifetime of the individual. Additionally, HIV may persist in
regions where the antiviral drugs are not as effective, such as the CNS.
The chronic nature of HIV/AIDS resulting from successful ART has led to
additional concerns. Long-term toxicity of ART drugs has resulted in increased risk
for cardiovascular disease, metabolic disorders, and organ failure. Treated
individuals frequently fail to reconstitute their immune system and develop chronic
immune activation characterized by activation of monocytes and T cells, production
of pro-inflammatory cytokines (e.g., interferon- γ [IFN-γ], interleukin-6 [IL-6]), and
depletion of Th17 cells.47 Chronic immune activation tends to exacerbate clinical
disease in adults and neonates.48
Vaccine development should be the most effective means of preventing HIV
infection and may be useful in treating preexisting infection. Most of the common
viral vaccines (e.g., rubella, mumps, influenza) induce protective antibodies that
block the initial infection. Only one vaccine (rabies) is used after the infection has
occurred. The rabies vaccine is successful because the rabies virus proliferates and
spreads very slowly. However, the ability of an HIV vaccine to either successfully
prevent or treat HIV infection is questionable for several reasons.49 First, the AIDS
virus is genetically and antigenically variable, like the influenza virus, so that a
vaccine created against one variant may not provide protection against another
variant. Second, although individuals with HIV/AIDS have high levels of circulating
antibodies against the virus, these antibodies do not appear to be protective.
Therefore even if a circulating antibody response can be induced by vaccination,
that response might not be effective. A vaccine may have to induce both circulating
and secretory (to prevent initial infection of the mucosal T cell) antibody and Tc
cells.
Pediatric AIDS and Central Nervous System Involvement
HIV can be transmitted from mother to child during pregnancy, at the time of
delivery, or through breast-feeding, although the risk of mother-to-child
transmission has dropped precipitously since the use of anti-retroviral drugs in
pregnant women. The clinical diagnosis of HIV infection in young children born of
HIV-infected mothers is very often a difficult task because the presence of maternal
antibodies may result in a misleading false-positive test for antibodies against HIV
for as long as 18 months after birth. Testing for antibody against HIV can be
performed recurrently from birth until 18 months; if the test results become
negative and remain so after 12 months, the child can be considered uninfected.
The 2008 revised surveillance case definition for HIV infection in children
younger than 18 months, which remains in effect today, recommends testing for
HIV or viral components in two separate specimens, not including cord blood.50
These include detection of HIV nucleic acid or p24 antigen, or direct isolation of
HIV in viral cultures.
HIV infection of babies is generally more aggressive than in adults; on average
an untreated child will die by his or her second birthday. Neurologic involvement
occurs more commonly in children than in adults and results from CNS
involvement, rather than effects on peripheral portions of the nervous system. HIV
encephalopathy occurs with varying degrees of severity and is a clinical component
in the diagnosis of AIDS in children. Most HIV-infected newborns appear normal,
but may progressively develop signs of CNS involvement. These usually appear as
failure to attain, or loss of, developmental milestones or loss of intellectual ability,
verified by standard developmental scale or neuropsychologic tests; acquired
symmetric motor deficits, seen in children older than age 1 month; impaired brain
growth or acquired microcephaly, demonstrated by head circumference
measurements; or brain atrophy, demonstrated by computed tomography (CT) or
magnetic resonance imaging (MRI) with serial imaging and required in children
younger than 2 years of age.
It may be difficult to completely differentiate the effect of HIV infection on the
CNS from other risk factors, including prenatal drug exposure, prematurity,
chronic illness, and a chaotic social atmosphere. The pathogenesis of HIV
encephalopathy in children is poorly understood, but the presence of inflammatory
mediators may be a contributing factor.
Because HIV infection in infants progresses very rapidly, treatment must begin at
the diagnosis of infection. In older children the criteria for treatment are similar to
those used in adults. A growing number of investigational protocols are available
for treatment of children with HIV. In general, treatment is focused on the
preservation and maintenance of the immune system, aggressive response to
opportunistic infections, support and relief of symptomatic occurrences, and
administration of ART.
Quick Check 8-2
1. Why is the development of recurrent or unusual infections the clinical hallmark
of immunodeficiency?
2. Compare and contrast the most common infections in individuals with defects in
cell-mediated immune response and those with defects in humoral immune
response.
3. What are the new treatments for HIV?
Hypersensitivity: Allergy, Autoimmunity,
and Alloimmunity
Allergy, autoimmunity, and alloimmunity are classified as hypersensitivity
reactions. Hypersensitivity is an altered immunologic response to an antigen that
results in disease or damage to the individual. Allergy, autoimmunity, and
alloimmunity (also termed isoimmunity) can be most easily understood in
relationship to the source of the antigen against which the hypersensitivity response
is directed (Table 8-12). Allergy refers to a hypersensitivity to environmental
antigens. These can include medicines, natural products (e.g., pollens, bee stings),
infectious agents, and any other antigen that is not naturally found in the individual.
TABLE 8-12
Relative Incidence and Examples of Hypersensitivity Diseases*
Target
Antigen
MECHANISM
Type I (IgE
Mediated)
Type II (Tissue
Specific)
Type III (Immune Complex Mediated) Type IV (Cell
Mediated)
Allergy ++++ + + ++
Environmental
antigens
Hay fever Hemolysis in drug
allergies
Gluten (wheat) allergy Poison ivy
allergy
Autoimmunity + ++ +++ ++
Self-antigens May contribute to some
type III reactions
Autoimmune
thrombocytopenia
Systemic lupus erythematosus Hashimoto
thyroiditis
Alloimmunity + ++ + ++
Another
person's
antigens
May contribute to some
type III reactions
Hemolytic disease
of the newborn
Individuals who do not make their own IgA may have an anaphylactic
response against IgA in human immune globulin
Graft rejection
*The frequency of each reaction is indicated in a range from rare (+) to very common (++++). An example
of each reaction is given.
Autoimmunity is a disturbance in the immunologic tolerance of self-antigens.
The immune system normally does not strongly recognize the individual's own
antigens. Healthy individuals of all ages, but particularly the elderly, may produce
low quantities of antibodies against their own antigens (autoantibodies) without
developing overt autoimmune disease. Therefore the presence of low quantities of
autoantibodies does not necessarily indicate a disease state. Autoimmune diseases
occur when the immune system reacts against self-antigens to such a degree that
autoantibodies or autoreactive T cells damage the individual's tissues. Many clinical
disorders are associated with autoimmunity and are generally referred to as
autoimmune diseases (Table 8-13). Autoimmune diseases are more prevalent in
women and the overall prevalence is rising.51
TABLE 8-13
Examples of Autoimmune Disorders
System Disease Organ or Tissue Probable Self-Antigen
Endocrine System
Hyperthyroidism (Graves disease) Thyroid gland Receptors for thyroid-stimulating hormone on plasma membrane of thyroid cells
Hashimoto hypothyroidism Thyroid gland Thyroid cell surface antigens, thyroglobulin
Insulin-dependent diabetes Pancreas Islet cells, insulin, insulin receptors on pancreatic cells
Addison disease Adrenal gland Surface antigens on steroid-producing cells; microsomal antigens
Male infertility Testis Surface antigens on spermatozoa
Skin
Pemphigus vulgaris Skin Intercellular substances in stratified squamous epithelium
Bullous pemphigoid Skin Basement membrane
Vitiligo Skin Surface antigens on melanocytes (melanin-producing cells)
Neuromuscular Tissue
Multiple sclerosis Neural tissue Surface antigens of nerve cells
Myasthenia gravis Neuromuscular junction Acetylcholine receptors; striations of skeletal and cardiac muscle
Rheumatic fever Heart Cardiac tissue antigens that cross-react with group A streptococcal antigen
Cardiomyopathy Heart Cardiac muscle
Gastrointestinal System
Ulcerative colitis Colon Mucosal cells
Pernicious anemia Stomach Surface antigens of parietal cells; intrinsic factor
Primary biliary cirrhosis Liver Cells of bile duct
Chronic active hepatitis Liver Surface antigens of hepatocytes, nuclei, microsomes, smooth muscle
Eye
Sjögren syndrome Lacrimal gland Antigens of lacrimal gland, salivary gland, thyroid, and nuclei of cells
Connective Tissue
Ankylosing spondylitis Joints Sacroiliac and spinal apophyseal joint
Rheumatoid arthritis Joints Collagen, IgG
Systemic lupus erythematosus Multiple sites Numerous antigens in nuclei, organelles, and extracellular matrix
Renal System
Immune complex glomerulonephritis Kidney Numerous immune complexes
Goodpasture syndrome Kidney Glomerular basement membrane
Hematologic System
Idiopathic neutropenia Neutrophil Surface antigens on polymorphonuclear neutrophils
Idiopathic lymphopenia Lymphocytes Surface antigens on lymphocytes
Autoimmune hemolytic anemia Erythrocytes Surface antigens on erythrocytes
Autoimmune thrombocytopenic purpura Platelets Surface antigens on platelets
Respiratory System
Goodpasture syndrome Lung Septal membrane of alveolus
Alloimmune diseases occur when the immune system of one individual produces
an immunologic reaction against tissues of another individual. Alloimmunity can be
observed during immunologic reactions against transfusions, transplanted tissue, or
the fetus during pregnancy.
The mechanism that initiates the onset of hypersensitivity, whether allergy,
autoimmunity, or alloimmunity, is not completely understood. It is generally
accepted that genetic, infectious, and possibly environmental factors contribute to
the development of hypersensitivity reactions.
Mechanisms of Hypersensitivity
Hypersensitivity reactions can be characterized also by the particular immune
mechanism that results in the disease (Table 8-14). These mechanisms are apparent
in most hypersensitivity reactions and have been divided into four distinct types:
type I (IgE-mediated reactions), type II (tissue-specific reactions), type III (immune
complex–mediated reactions), and type IV (cell-mediated reactions). This
classification is artificial and seldom is a particular disease associated with only a
single mechanism. The four mechanisms are interrelated, and in most
hypersensitivity reactions several mechanisms can be functioning simultaneously or
sequentially.
TABLE 8-14
Immunologic Mechanisms of Tissue Destruction
Type Name
Rate of
Development
Class of
Antibody
Involved
Principal Effector
Cells Involved
Participation of
Complement Examples of Disorders
I IgE-mediated
reaction
Immediate IgE Mast cells No Seasonal allergic rhinitis
Asthma
II Tissue-specific
reaction
Immediate IgG
IgM
Macrophages in
tissues
Frequently Autoimmune thrombocytopenic purpura, Graves
disease, autoimmune hemolytic anemia
III Immune complex–
mediated reaction
Immediate IgG
IgM
Neutrophils Yes Systemic lupus erythematosus
IV Cell-mediated
reaction
Delayed None Lymphocytes
Macrophages
No Contact sensitivity to poison ivy, metals (jewelry),
and latex
As with all immune responses, hypersensitivity reactions require sensitization
against a particular antigen that results in a primary immune response. Disease
symptoms appear after an adequate secondary immune response occurs.
Hypersensitivity reactions are immediate or delayed, depending on the time
required to elicit clinical symptoms after reexposure to the antigen. Reactions that
occur within minutes to a few hours after exposure to antigen are termed immediate
hypersensitivity reactions. Delayed hypersensitivity reactions may take several
hours to appear and are at maximal severity days after reexposure to the antigen.
Generally, immediate reactions are caused by antibody, whereas delayed reactions
are caused by cells (e.g., T cells, NK cells, macrophages).
The most rapid and severe immediate hypersensitivity reaction is anaphylaxis.
Anaphylaxis occurs within minutes of reexposure to the antigen and can be either
systemic (generalized) or cutaneous (localized). Symptoms of systemic anaphylaxis
include pruritus, erythema, vomiting, abdominal cramps, diarrhea, and breathing
difficulties, and the most severe reactions may include contraction of bronchial
smooth muscle, edema of the throat, and decreased blood pressure that can lead to
shock and death.52 Examples of systemic anaphylaxis are allergic reactions to bee
stings (see p. 206), peanuts, shellfish, or eggs. Cutaneous anaphylaxis results in
local symptoms, such as pain, swelling, and redness, which occur at the site of
exposure to an antigen (e.g., a painful local reaction to an injected vaccine or drug).
Type I: IgE-Mediated Hypersensitivity Reactions
Type I hypersensitivity reactions are mediated by antigen-specific IgE and the
products of tissue mast cells (Figure 8-11). Most common allergic reactions are
type I reactions. In addition, most type I reactions occur against environmental
antigens and are therefore allergic. Because of this strong association, many
healthcare professionals use the term allergy to indicate only IgE-mediated
reactions. However, IgE can contribute to some autoimmune and alloimmune
diseases, and many common allergies (e.g., poison ivy) are not mediated by IgE.
FIGURE 8-11 Mechanism of Type I, IgE-Mediated Reactions. First exposure to an allergen leads
to antigen processing and presentation of antigen by an antigen-presenting cell (APC) to B
lymphocytes, which is under the direction of T-helper 2 (Th2) cells. Th2 cells produce specific
cytokines (e.g., IL-4, IL-13, and others) that favor maturation of the B lymphocytes into plasma
cells that secrete IgE. The IgE is adsorbed to the surface of the mast cell by binding with IgE-
specific Fc receptors. When an adequate amount of IgE is bound the mast cell is sensitized.
During a reexposure, the allergen cross-links the surface-bound IgE and causes degranulation
of the mast cell. Contents of the mast cell granules, primarily histamine, induce local edema,
smooth muscle contraction, mucous secretion, and other characteristics of an acute
inflammatory reaction. (See Chapter 6 for more details on the role of mast cells in
inflammation.)
IgE has a relatively short life span in the blood because it rapidly binds to Fc
receptors on mast cells.53 Unlike Fc receptors on phagocytes, which bind IgG that
has previously reacted with antigen, the Fc receptors on mast cells specifically bind
IgE that has not previously interacted with antigen. After a large amount of IgE has
bound to the mast cells, an individual is considered sensitized. Further exposure of a
sensitized individual to the allergen results in degranulation of the mast cell and the
release of mast cell products (see Chapter 6).
Mechanisms of IgE-mediated hypersensitivity.
The most potent mediator of IgE-mediated hypersensitivity is histamine, which
affects several key target cells. Acting through H1 receptors, histamine contracts
bronchial smooth muscles (bronchial constriction), increases vascular permeability
(edema), and causes vasodilation (increased blood flow) (see Chapter 6). The
interaction of histamine with H2 receptors results in increased gastric acid
secretion. Blocking histamine receptors with antihistamines can control some type I
responses.
Clinical manifestations of IgE-mediated hypersensitivity.
The clinical manifestations of type I reactions are attributable mostly to the biologic
effects of histamine. The tissues most commonly affected by type I responses
contain large numbers of mast cells and are sensitive to the effects of histamine
released from them. These tissues are found in the gastrointestinal tract, the skin,
and the respiratory tract (Figure 8-12 and Table 8-15).
FIGURE 8-12 Type I Hypersensitivity Reactions. Manifestations of allergic reactions as a result
of type I hypersensitivity include pruritus, angioedema (swelling caused by exudation), edema of
the larynx, urticaria (hives), bronchospasm (constriction of airways in the lungs), hypotension
(low blood pressure), and dysrhythmias (irregular heartbeat) because of anaphylactic shock,
and gastrointestinal cramping caused by inflammation of the gastrointestinal mucosa.
Photographic inserts show a diffuse allergic-like eye and skin reaction on an individual. The skin
lesions have raised edges and develop within minutes or hours, with resolution occurring after
about 12 hours. (Inserts from Male D et al: Immunology, ed 8, St Louis, 2013, Mosby.)
TABLE 8-15
Causes of Clinical Allergic Reactions
Typical Allergen Mechanism of Hypersensitivity Clinical Manifestation
Ingestants
Foods Type I Gastrointestinal allergy
Drugs Types I, II, III Urticaria, immediate drug reaction, hemolytic anemia, serum sickness
Inhalants
Pollens, dust, molds Type I Allergic rhinitis, bronchial asthma
Aspergillus fumigatus Types I, III Allergic bronchopulmonary aspergillosis
Thermophilic actinomycetes* Types III, IV Extrinsic allergic alveolitis
Injectants
Drugs Types I, II, III Immediate drug reaction, hemolytic anemia, serum sickness
Bee venom Type I Anaphylaxis
Vaccines Type III Localized Arthus reaction
Serum Types I, III Anaphylaxis, serum sickness
Contactants
Poison ivy, metals Type IV Contact dermatitis
Latex Type I, IV Contact dermatitis, anaphylaxis
*An order of fungi that grows best at high temperatures (between 45° and 80° C [113° and 176° F]).
Modified from Bellanti JA: Immunology III, Philadelphia, 1985, Saunders.
Gastrointestinal allergy is caused by allergens that enter through the mouth—
usually foods or medicines. Symptoms include vomiting, diarrhea, or abdominal
pain. Foods most often implicated in gastrointestinal allergies are milk, chocolate,
citrus fruits, eggs, wheat, nuts, peanut butter, and fish.54 The most common food
allergy in adults is a reaction to shellfish, which may initiate an anaphylactic
response and death.55 When food is the source of an allergen, the active immunogen
may be an unidentifiable product of how the food is processed during manufacture
or broken down by digestive enzymes.56 Sometimes the allergen is a drug, an
additive, or a preservative in the food. For example, cows treated for mastitis with
penicillin yield milk containing trace amounts of this antibiotic. Thus
hypersensitivity apparently caused by milk proteins may instead be the result of an
allergy to penicillin.
Urticaria, or hives, is a dermal (skin) manifestation of allergic reactions (see
Figure 8-12). The underlying mechanism is the localized release of histamine and
increased vascular permeability, resulting in limited areas of edema. Urticaria is
characterized by white fluid-filled blisters (wheals) surrounded by areas of redness
(flares). This wheal and flare reaction is usually accompanied by pruritus. Not all
urticarial symptoms are caused by immunologic reactions. Some, termed
nonimmunologic urticaria, result from exposure to cold temperatures, emotional
stress, medications, systemic diseases, or malignancies (e.g., lymphomas).
Effects of allergens on the mucosa of the eyes, nose, and respiratory tract include
conjunctivitis (inflammation of the membranes lining the eyelids) (see Figure 8-12),
rhinitis (inflammation of the mucous membranes of the nose), and asthma
(constriction of the bronchi). Symptoms are caused by vasodilation, hypersecretion
of mucus, edema, and swelling of the respiratory mucosa. Because the mucous
membranes lining the respiratory tract are continuous, they are all adversely
affected. The degree to which each is affected determines the symptoms of the
disease; most anaphylactic reactions are type I hypersensitivities.
The central problem in allergic diseases of the lung is obstruction of the large
and small airways (bronchi) of the lower respiratory tract by bronchospasm
(constriction of smooth muscle in airway walls), edema, and thick secretions. This
leads to ventilatory insufficiency, wheezing, and difficult or labored breathing (see
Chapter 27).
Certain individuals are genetically predisposed to develop allergies and are
called atopic. In families in which one parent has an allergy, allergies develop in
about 40% of the offspring. If both parents have allergies, the incidence may be as
high as 80%. Atopic individuals tend to produce higher quantities of IgE and have
more Fc receptors for IgE on their mast cells. The airways and the skin of atopic
individuals have increased responsiveness to a wide variety of both specific and
nonspecific stimuli.
Evaluation and treatment of IgE hypersensitivity.
Allergic reactions can be life-threatening; therefore it is essential that severely
allergic individuals be informed of the specific allergen against which they are
sensitized and instructed to avoid contact with that material. Several tests are
available to evaluate allergic individuals. These include food challenges, skin tests
with allergens, and laboratory tests for total IgE and allergen-specific IgE.
Type II: Tissue-Specific Hypersensitivity Reactions
Type II hypersensitivities are generally reactions against a specific cell or tissue.
Cells express a variety of antigens on their surfaces, some of which are called
tissue-specific antigens because they are expressed on the plasma membranes of
only certain cells. Platelets, for example, have groups of antigens that are found on
no other cells of the body. The symptoms of many type II diseases are determined
by which tissue or organ expresses the particular antigen. Environmental antigens
(e.g., drugs or their metabolites) may bind to the plasma membranes of specific
cells (especially erythrocytes and platelets) and function as targets of type II
reactions. The five general mechanisms by which type II hypersensitivity reactions
can affect cells are shown in Figure 8-13. Each mechanism begins with antibody
binding to tissue-specific antigens or antigens that have attached to particular
tissues.
FIGURE 8-13 Mechanisms of Type II, Tissue-Specific Reactions. Antigens on the target cell bind
with antibody and are destroyed or prevented from functioning by one of the following
mechanisms: (A) complement-mediated lysis (an erythrocyte target is illustrated here); (B)
clearance (phagocytosis) by macrophages in the tissue; (C) neutrophil-mediated immune
destruction; (D) antibody-dependent cell-mediated cytotoxicity (ADCC) (apoptosis of target cells
is induced by natural killer [NK] cells by two mechanisms: by the release of granzymes and
perforin, which is a molecule that creates pores in the plasma membrane, and enzymes
[granzymes] that enter the target through the perforin pores; by the interactions of Fas ligand
[FasL; a molecule similar to TNF-α] on the surface of NK cells with Fas [the receptor for FasL] on
the surface of target cells); or (E) modulation or blocking of the normal function of receptors by
antireceptor antibody. This example of mechanism (E) depicts myasthenia gravis in which
acetylcholine receptor antibodies block acetylcholine from attaching to its receptors on the
motor end plates of skeletal muscle, thereby impairing neuromuscular transmission and
causing muscle weakness. C1, Complement component C1; C3b, complement fragment
produced from C3, which acts as an opsonin; C5a, complement fragment produced from C5,
which acts as a chemotactic factor for neutrophils; Fcγ receptor, cellular receptor for the Fc
portion of IgG; FcR, Fc receptor.
The cell may be destroyed by antibody and complement (see Figure 8-13, A). The
antibody (IgM or IgG) reacts with an antigen on the surface of the cell, causing
activation of the complement cascade through the classical pathway. Formation of
the membrane attack complex (C5-9) damages the membrane and may result in lysis
of the cell. For example, erythrocytes are destroyed by complement-mediated lysis
in individuals with autoimmune hemolytic anemia (see Chapters 21 and 22) or as a
result of an alloimmune reaction to mismatched transfused blood cells.
Antibody may cause cell destruction through phagocytosis by macrophages (see
Figure 8-13, B). The antibody may additionally activate complement, resulting in the
deposition of C3b on the cell surface. Receptors on the macrophage recognize and
bind opsonins (e.g., antibody or C3b) and increase phagocytosis of the target cell.
For example, antibodies against platelet-specific antigens or against red blood cell
antigens of the Rh system cause their removal by phagocytosis in the spleen.
Tissue damage may be caused by toxic products produced by neutrophils (see
Figure 8-13, C). Soluble antigens such as medications, molecules released from
infectious agents, or molecules released from an individual's own cells may enter
the circulation. In some instances, the antigens are deposited on the surface of
tissues, where they bind antibody. The antibody may activate complement, resulting
in the release of C3a and C5a, which are chemotactic for neutrophils, and the
deposition of complement component C3b. Neutrophils are attracted, bind to the
tissues through receptors for the Fc portion of antibody (Fc receptor) or for C3b,
and release their granules onto the healthy tissue. The components of neutrophil
granules, as well as the toxic oxygen products produced by these cells, will damage
the tissue.
Antibody-dependent cell-mediated cytotoxicity (ADCC) involves natural killer
(NK) cells (see Figure 8-13, D). Antibody on the target cell is recognized by Fc
receptors on t