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Understanding Pathophysiology


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

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





Organization and Content: What’s New in the Sixth Edition

Features to Promote Learning

Art Program

Teaching/Learning Package


Introduction to Pathophysiology
Part One Basic Concepts of Pathophysiology

Unit 1 The Cell

1 Cellular Biology


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


Did You Understand?

Key Terms


2 Genes and Genetic Diseases

DNA, RNA, and Proteins: Heredity at the Molecular Level


Elements of Formal Genetics

Transmission of Genetic Diseases

Linkage Analysis and Gene Mapping

Multifactorial Inheritance

Did You Understand?

Key Terms


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


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


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


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


7 Adaptive Immunity

Third Line of Defense: Adaptive Immunity

Antigens and Immunogens


Immune Response: Collaboration of B Cells and T Cells

Cell-Mediated Immunity

Did You Understand?

Key Terms


8 Infection and Defects in Mechanisms of Defense


Deficiencies in Immunity

Hypersensitivity: Allergy, Autoimmunity, and Alloimmunity

Did You Understand?

Key Terms


9 Stress and Disease

Historical Background and General Concepts

The Stress Response

Stress, Personality, Coping, and Illness

Did You Understand?

Key Terms


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


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


12 Cancer in Children and Adolescents

Incidence, Etiology, and Types of Childhood Cancer


Did You Understand?

Key Terms


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


14 Pain, Temperature, Sleep, and Sensory Function


Temperature Regulation


The Special Senses

Somatosensory Function

Geriatric Considerations

Geriatric Considerations

Did You Understand?

Key Terms


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


16 Disorders of the Central and Peripheral Nervous Systems and Neuromuscular

Central Nervous System Disorders

Peripheral Nervous System and Neuromuscular Junction Disorders

Tumors of the Central Nervous System

Did You Understand?

Key Terms


17 Alterations of Neurologic Function in Children

Development of the Nervous System in Children


Structural Malformations

Alterations in Function: Encephalopathies

Cerebrovascular Disease in Children

Childhood Tumors

Did You Understand?

Key Terms


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


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


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


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


22 Alterations of Hematologic Function in Children

Disorders of Erythrocytes

Disorders of Coagulation and Platelets

Neoplastic Disorders

Did You Understand?

Key Terms


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


24 Alterations of Cardiovascular Function

Diseases of the Veins

Diseases of the Arteries

Disorders of the Heart Wall

Manifestations of Heart Disease


Did You Understand?

Key Terms


25 Alterations of Cardiovascular Function in Children

Congenital Heart Disease

Acquired Cardiovascular Disorders

Did You Understand?

Key Terms


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


27 Alterations of Pulmonary Function

Clinical Manifestations of Pulmonary Alterations

Pulmonary Disorders

Did You Understand?

Key Terms


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


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


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


31 Alterations of Renal and Urinary Tract Function in Children

Structural Abnormalities

Glomerular Disorders


Bladder Disorders

Urinary Incontinence

Did You Understand?

Key Terms


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


33 Alterations of the Female Reproductive System

Abnormalities of the Female Reproductive Tract

Alterations of Sexual Maturation

Disorders of the Female Reproductive System


Disorders of the Female Breast

Did You Understand?

Key Terms


34 Alterations of the Male Reproductive System

Alterations of Sexual Maturation

Disorders of the Male Reproductive System


Disorders of the Male Breast

Sexually Transmitted Diseases

Did You Understand?

Key Terms


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


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


37 Alterations of Digestive Function in Children

Disorders of the Gastrointestinal Tract

Disorders of the Liver

Did You Understand?

Key Terms


Unit 12 The Musculoskeletal and Integumentary

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


39 Alterations of Musculoskeletal Function

Musculoskeletal Injuries

Disorders of Bones

Disorders of Joints

Disorders of Skeletal Muscle

Musculoskeletal Tumors

Did You Understand?

Key Terms


40 Alterations of Musculoskeletal Function in Children

Congenital Defects

Bone Infection

Juvenile Idiopathic Arthritis



Muscular Dystrophy

Musculoskeletal Tumors

Nonaccidental Trauma

Did You Understand?

Key Terms


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


42 Alterations of the Integument in Children

Acne Vulgaris


Infections of the Skin

Insect Bites and Parasites

Cutaneous Hemangiomas and Vascular Malformations

Other Skin Disorders

Did You Understand?

Key Terms




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,

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,

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®),

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


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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


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:


Executive Content Strategist: Kellie White
Content Development Manager: Laurie Gower
Senior Content Development Specialist: Karen C. Turner
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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


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
Clinical Research Coordinator
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
Professor of Pediatrics
Duke University School of Medicine
Durham, North Carolina


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
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
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


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

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

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

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

• Extensive entire chapter revisions and updated epidemiology of cancer (Chapter

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

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


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

The most exciting part of the learning support package is Pathophysiology
Online, a complete set of online modules that provide thoroughly developed lessons

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.

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

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
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.

Basic Concepts of Pathophysiology

Unit 1 The Cell
Unit 2 Mechanisms of Self-Defense
Unit 3 Cellular Proliferation: Cancer

The Cell

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


Cellular Biology
Kathryn L. McCance


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

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

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
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.)

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.

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.

Network of tubular channels (cisternae) that extend throughout outer nuclear membrane. Specializes in synthesis and transport of protein and
lipid components of most organelles.


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


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.)

Plasma Membrane Functions


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

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
Diffusion and exchange diffusion
Endocytosis (pinocytosis, phagocytosis)
Exocytosis (secretion)
Active transport


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).

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,


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.)

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-

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.)

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
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).

Classes of Plasma Membrane Receptors

Type of



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.


Once activated by ligands, function directly as enzymes or associate with enzymes.


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

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


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

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
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 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 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
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

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.

Major Transport Systems in Mammalian Cells

Substance Transported Mechanism of Transport* Tissues
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

Other Organic Molecules
Cholic acid, deoxycholic acid, and taurocholic acid Active: symport with Na+ Intestines
Organic anions (e.g., malate, α-ketoglutarate,

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

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

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,


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.

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
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

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
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

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

3. Define the differences between pinocytosis, phagocytosis, and receptor-mediated

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).

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

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)

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

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).


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,

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?

Characteristics of Epithelial Tissues

Simple Squamous Epithelium
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
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
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
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
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
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
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
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.)

Connective Tissues

Loose or Areolar Tissue
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
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

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
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
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)
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

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
Location and Function
Serves as matrix for blood cells
Macrophages in Tissue, Reticuloendothelial, or Macrophage System
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

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

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-


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

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

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


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

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

5. The mechanisms that control cellular division depend on the integrity of genetic,
epigenetic, and protein growth factors.

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)

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

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

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

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

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

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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.

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.

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.

10. Friedman JR, Nunnari J. Mitochondrial form and function. Nature.

11. Amm I, et al. Protein quality control and elimination of protein waste: the
role of the ubiquitin-proteosome system. Biochim Biophys Acta.

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.

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.


Genes and Genetic Diseases
Lynn B. Jorde


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

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
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

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.

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

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.

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.

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.

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


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

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
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
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.)

Characteristics of Various Chromosome Disorders

Disease/Disorder Features
Down Syndrome
Trisomy of Chromosome 21
IQ Usually ranges from 20 to 70 (intellectual disability)

Virtually all males are sterile; some females can reproduce

Face Distinctive: low nasal bridge, epicanthal folds, protruding tongue, low-set ears

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

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

Found only in females


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

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).

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

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.

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.

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.

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
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
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
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,

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


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
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
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


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

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

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 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
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

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

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

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
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

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

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

Quick Check 2-3

1. Define linkage analysis; cite an example.

2. Why is “threshold of liability” an important consideration in multifactorial

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

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

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

13. During translation, mRNA interacts with transfer RNA (tRNA), a molecule that
has an attachment site for a specific amino acid.

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

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

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

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

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

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

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RNAs in health and disease. Cell. 2013;152(6):1308–1323.
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on genomics. Cell. 2013;155(1):27–38.


Epigenetics and Disease
Diane P. Genereux, Lynn B. Jorde


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

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, 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).

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

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,


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
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
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

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
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


Molecular Approaches to Understand Epigenetic
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

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

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

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?
1. Why are pairs of identical twins especially useful in the study of epigenetic

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

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

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

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

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

3. Hypermethylation also is seen in microRNA genes and is associated with

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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Altered Cellular and Tissue Biology
Kathryn L. McCance, Todd Cameron Grey


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 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 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,


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
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 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

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 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.

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.

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
oxygen species

Lack of oxygen is key in progression of cell injury in ischemia (reduced blood supply); activated oxygen species (ROS, , H


, 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


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)


Early loss of selective membrane permeability found in all forms of cell injury, lysosomal membrane damage with release of enzymes
causing cellular digestion

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,


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

• 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.

Biologically Relevant Free Radicals

Reactive oxygen species (ROS)

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)

Oxidases present in peroxisomes


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−)



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


Ischemic brain injury

Alzheimer disease



Cardiac myopathy

Chronic granulomatous disease

Diabetes mellitus

Eye disorders

Macular degeneration


Inflammatory disorders

Iron overload

Lung disorders


Oxygen toxicity


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).

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,

Chemical or Toxic Injury
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
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,


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

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

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


• 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

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.

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
Methaqualone (Quaalude)
Glutethimide (Doriden)
Ethchlorvynol (Placidyl)

Psychomotor stimulants Dopamine transporter (antagonist)
Serotonin receptors (toxicity)

3,4-Methylenedioxymethamphetamine (MDMA, ecstasy)

Phencyclidine-like drugs NMDA glutamate receptor channel (antagonist) Phencyclidine (PCP, angel dust)

Cannabinoids CB1 cannabinoid receptors (agonist) Marijuana

Hallucinogens Serotonin 5-HT2 receptors (agonist) Lysergic acid diethylamide (LSD)

CB1, Cannabinoid receptor type 1; GABA, γ-aminobutyric acid; 5-HT2, 5-hydroxytryptamine; NMDA, N-

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.

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


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
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,

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

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
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 (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

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 Accessed September 24, 2012.

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

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

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

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

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

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,

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

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

Data from Adams PF et al: Vital Health Stat 10(255), 2012; available from; 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);

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-

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 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.

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
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
Fracture: Blunt-force blows
or impacts can cause bone to
break or shatter (see Chapter

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,
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
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
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
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,
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, 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 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

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 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
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.

Mechanisms of Cellular Injury

Mechanism Characteristics Examples

Alter cell’s nucleus and plasma membrane’s structure, shape, receptors, or transport

Sickle cell anemia, Huntington disease, muscular
dystrophy, abetalipoproteinemia, familial


Induction of mitotically heritable alterations in gene expression without changing DNA Gene silencing in cancer


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

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

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


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


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,


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:
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.

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,

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 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 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.

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 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 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 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.

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

Release from red blood cells, liver, kidney, skeletal muscle

Creatine kinase (CK) (CK

Release from skeletal muscle, brain, heart

Aspartate aminotransferase

Release from heart, liver, skeletal muscle, kidney, pancreas

Alanine aminotransferase

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

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.


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


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 (“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,


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

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
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

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-


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


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 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

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

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

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

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

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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Fluids and Electrolytes, Acids and
Sue E. Huether


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.

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.

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).

Representative Distribution of Electrolytes in Body Compartments

Electrolytes ECF (mEq/L) ICF (mEq/L)
Sodium 142 12
Potassium 4.2 150
Calcium 5 0
Magnesium 2 24
TOTAL 153.2 186
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-

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
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

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 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.

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
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
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.

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)
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)
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 electro​cardiogram (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

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,

Ureterosigmoidoscopy (chloride absorbed in excess of sodium in small
Renal failure (loss of bicarbonate)
Proximal renal tubular acidosis (loss of more renal sodium in relation to
chloride)Decreased renal H+ excretion

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

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 1. Day RE, et al. Human aquaporins: regulators of transcellular water flow. 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. 18. Berend K, de Vries AP, Gans RO. Physiological approach to assessment of 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. 21. Soifer JT, Kim HT. Approach to metabolic alkalosis. Emerg Med Clin North Am. 2014;32(2):453–463. 22. Bruno CM, Valenti M. Acid-base disorders in patients with chronic obstructive pulmonary disease: a pathophysiological review. J Biomed Biotechnol. 2012;2012:915150. 23. Palmer BF. Evaluation and treatment of respiratory alkalosis. Am J Kidney Dis. 2012;60(5):834–838. 24. Madias NE. Renal acidification responses to respiratory acid base disorders. 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 References 1. Peterson LW, Artis D. Intestinal epithelial cells: regulators of barrier function and immune homeostasis. Nat Rev Immunol. 2014;14(3):141–153. 2. Human Microbiome Project Consortium. Structure, function and diversity of the healthy human microbiome. Nature. 2012;486(7402):207–214. 3. Mayer EA, et al. Gut/brain axis and the microbiota. J Clin Invest. 2015;125(3):926–938. 4. Moal VL-L, Servin AL. Anti-infective activities of Lactobacillus strains in the human intestinal microbiota: from probiotics to gastrointestinal anti- infectious biotherapeutic agents. Clin Microbiol Rev. 2014;27(2):167–199. 5. Ricklin D, Lambris JD. Complement in immune and inflammatory disorders: therapeutic interventions. J Immunol. 2013;190(8):3839–3847. 6. Sekine H, et al. The role of MASP-1/3 in complement activation. Adv Exp Med Biol. 2013;735:41–53. 7. O'Neil LA, et al. The history of Toll-like receptors—redefining innate immunity. Nat Rev Immunol. 2013;13(6):453–460. 8. Qian C, Cao X. 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Updated scar management practical guidelines: non- invasive and invasive measures. J Plast Reconstr Aesthet Surg. 2014;67(8):1017–1025. 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 imme​diately, 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. 15. Jiang S, Dong C. A complex issue on CD4+ T-cell subsets. Immunol Rev. 2013;252(1):5–11. 16. Ramachandran G. Gram-positive and gram-negative bacterial toxins in sepsis. Virulence. 2014;5(1):213–218. 17. Avalos AM, Ploegh HL. Early BCR events and antigen capture, processing, and loading on MHC class II on B cells. Front Immunol. 2014;5:92. 18. Njau MN, Jacob J. The CD28/B7 pathway: a novel regulator of plasma cell function. Adv Exp Med Biol. 2013;785(2013):67–75. 19. Farber DL, et al. Human memory T cells: generation, compartmentalization and homeostasis. Nat Rev Immunol. 2014;14(1):24–35. 20. Annunziato F, et al. Main features of human T helper 17 cells. Ann N Y Acad Sci. 2013;1284(2013):66–70. 21. Singer BD, et al. Regulatory T cells as immunotherapy. Front Immunol. 2014;5:46. 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: 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 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: 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 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 the NK cells, which release toxic substances that destroy the target cell. The last mechanism does not destroy the target cell but rather causes the cell to malfunction (see Figure 8-13, E). The antibody is usually directed against antigenic determinants associated with specific cell surface receptors. The antibody changes the function of the rec