Ebook Elseviers integrated review immunology and microbiology with student consult online access (2nd edition) Part 1

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ELSEVIER’S INTEGRATED REVIEW IMMUNOLOGY AND

MICROBIOLOGY

ELSEVIER’S INTEGRATED REVIEW IMMUNOLOGY AND

MICROBIOLOGY

SECOND EDITION

Jeffrey K Actor, PhD

ProfessorDepartment of Pathology and Laboratory MedicineUniversity of Texas-Houston Medical School

Houston, Texas

ELSEVIER’S INTEGRATED REVIEW IMMUNOLOGY ISBN: 978-0-323-07447-6 AND MICROBIOLOGY, SECOND EDITION

Copyright #2012 by Saunders, an imprint of Elsevier Inc.

Copyright # 2007 by Mosby, Inc., an affiliate of Elsevier Inc.

No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein).

Notices Knowledge and best practice in this field are constantly changing As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary.

Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility.

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Library of Congress Cataloging-in-Publication Data

Actor, Jeffrey K.

Elsevier’s integrated review immunology and microbiology / Jeffrey K.

Actor – 2nd ed.

p ; cm.

Integrated review immunology and microbiology

Rev ed of: Elsevier’s integrated immunology and microbiology / Jeffrey K Actor c2007.

Includes index.

ISBN 978-0-323-07447-6 (pbk : alk paper)

I Actor, Jeffrey K Elsevier’s integrated immunology and microbiology II Title III Title: Integrated review immunology and microbiology.

[DNLM: 1 Immune System Phenomena 2 Microbiological Phenomena QW 540]

Acquisitions Editor: Madelene Hyde

Developmental Editor: Andrew Hall

Publishing Services Manager: Patricia Tannian

Team Manager: Hemamalini Rajendrababu

Project Manager: Antony Prince

Designer: Steven Stave

Printed in China

Last digit is the print number: 9 8 7 6 5 4 3 2 1

To my father, Paul Actor, PhD, who instilled in me a sense of excitement about thewonders of science and the curiosity to ask questions about biological systems

Immunology represents a rapidly changing field with new

the-ories actively evolving as molecular techniques broaden our

scientific perspective on interactions between pathogens and

the human host The immune cells and organs of the body

comprise the primary defense system against invasion by

mi-croorganisms A functional immune system confers a state

of health through effective immune surveillance and

elimina-tion of infectious agents The study of immunologic and

he-matologic principles, as applied toward understanding host

protection against pathogenic assault, integrates well with

mi-crobiology and the study of basic concepts underlying the

na-ture of foreign pathogens The goal of the first half of the book

is to present immune system components, both innate and

adaptive, in a concise manner to elucidate their intertwined

relationships that culminate in effective host protection and

health The remaining chapters present the world of

microbi-ology, with a concise overview of clinically relevant bacteria,

viruses, fungi, and parasites, to allow an understanding of

in-fectious organisms as the causative agents underlying human

disease

This book is aimed at students of human health and those in

the medical profession; it is written to simplify concepts and

encourage inquisitive individuals to explore further medically

relevant topics Indeed, the purpose of the integrated textseries is to encourage cross-disciplinary thought across mul-tiple sciences Integration boxes promote cross-disciplinethinking and allow the reader to build bridges between relatedideas in other medical fields The clinical vignettes (Case Studies)and associated questions at the end of the book are organized toprovide perspectives into molecular aspects underlying clinicaldisease manifestation These scenarios are aimed to assist inunderstanding consequences of ineffective, inappropriate, over-active, or nonregulated responses and their relationship to im-munologic disorders and deficiencies as well as to responsesoccurring during infection The associated USMLE format ques-tions available at www.StudentConsult.com will also testknowledge in a clinical context, with succinct explanations to al-low increased application of immunologic and microbiologicconcepts to medically related disease states

Overall, the text attempts to present information in a ically relevant and focused manner that outlines concepts forfurther exploration, creating a base of knowledge for thosewith a desire to understand how the healthy individual com-bats disease

clin-Jeffrey K Actor, PhD

Editorial Review Board

Chief Series Advisor

J Hurley Myers, PhD

Professor Emeritus of Physiology and Medicine

Southern Illinois University School of Medicine;

President and CEO

DxR Development Group, Inc

Carbondale, Illinois

Anatomy and Embryology

Thomas R Gest, PhD

University of Michigan Medical School

Division of Anatomical Sciences

Office of Medical Education

Ann Arbor, Michigan

Biochemistry

John W Baynes, MS, PhD

Graduate Science Research Center

University of South Carolina

Columbia, South Carolina

Marek Dominiczak, MD, PhD, FRCPath, FRCP(Glas)

Clinical Biochemistry Service

NHS Greater Glasgow and Clyde

Gartnavel General Hospital

Glasgow, United Kingdom

Woodland Hills Family Medicine Residency Program

Woodland Hills, California

Genetics

Neil E Lamb, PhD

Director of Educational Outreach

Hudson Alpha Institute for Biotechnology

University of Maryland at BaltimoreBaltimore, Maryland

James L Hiatt, PhDProfessor EmeritusDepartment of Biomedical SciencesBaltimore College of Dental SurgeryDental School

University of Maryland at BaltimoreBaltimore, Maryland

Immunology

Darren G Woodside, PhDPrincipal Scientist

Drug DiscoveryEncysive Pharmaceuticals, Inc

Houston, Texas

Microbiology

Richard C Hunt, MA, PhDProfessor of Pathology, Microbiology, and ImmunologyDirector of the Biomedical Sciences Graduate ProgramDepartment of Pathology and Microbiology

University of South Carolina School of MedicineColumbia, South Carolina

Neuroscience

Cristian Stefan, MDAssociate ProfessorDepartment of Cell BiologyUniversity of Massachusetts Medical SchoolWorcester, Massachusetts

Pathology

Peter G Anderson, DVM, PhDProfessor and Director of Pathology UndergraduateEducation, Department of Pathology

University of Alabama at BirminghamBirmingham, Alabama

Michael M White, PhD

Professor Department of Pharmacology and Physiology

Drexel University College of Medicine

Philadelphia, Pennsylvania

Physiology

Joel Michael, PhDDepartment of Molecular Biophysics and PhysiologyRush Medical College

Chicago, Illinois

I would like to thank Robert L Hunter Jr, MD, PhD; Steven

J Norris, PhD; and Gailen D Marshall, MD, PhD, who

sup-ported and encouraged me as I strove to reach my academic

and research goals Special thanks go to Alexandra Stibbe

at Elsevier for her vision and insights that made the Integrated

Series possible, to Kate Dimock for her excellent leadership in

the project, and to Andrew C Hall for his attention to detail,his humor, and his positive guidance Finally, I could not havecompleted this endeavor without the support of my wife Loriand her continued faith in my abilities and encouragement tofollow my dreams

SECTION I IMMUNOLOGY

1 Introduction to Immunity and Immune Systems 3

2 Cells and Organs of the Immune System 7

3 Humoral Immunity: Antibody Recognition of Antigen 17

Series Preface

How to Use This Book

The idea for Elsevier’s Integrated Series came about at a

seminar on the USMLE Step 1 Exam at an American Medical

Student Association (AMSA) meeting We noticed that the

discussion between faculty and students focused on how the

exams were becoming increasingly integrated—with case

sce-narios and questions often combining two or three science

disciplines The students were clearly concerned about how

they could best integrate their basic science knowledge

One faculty member gave some interesting advice: "read

through your textbook in, say, biochemistry, and every time

you come across a section that mentions a concept or piece of

information relating to another basic science—for example,

immunology—highlight that section in the book Then go to

your immunology textbook and look up this information, and

make sure you have a good understanding of it When you have,

go back to your biochemistry textbook and carry on reading."

This was a great suggestion—if only students had the time, and

all of the books necessary at hand, to do it! At Elsevier we thought

long and hard about a way of simplifying this process, and

eventually the idea for Elsevier’s Integrated Series was born

The series centers on the concept of the integration box

These boxes occur throughout the text whenever a link to

an-other basic science is relevant They’re easy to spot in the—

with their color-coded headings and logos Each box contains

a title for the integration topic and then a brief summary of the

topic The information is complete in itself—you probably

won’t have to go to any other sources—and you have the basic

knowledge to use as a foundation if you want to expand your

knowledge of the topic

You can use this book in two ways First, as a review book

When you are using the book for review, the integration boxes

will jog your memory on topics you have already covered You’ll

be able to reassure yourself that you can identify the link, and

you can quickly compare your knowledge of the topic with

the summary in the box The integration boxes might highlight

gaps in your knowledge, and then you can use them to determine

what topics you need to cover in more detail

Second, the book can be used as a short text to have at hand

while you are taking your course

You may come across an integration box that deals with a

topic you haven’t covered yet, and this will ensure that you’re

one step ahead in identifying the links to other subjects

(espe-cially useful if you’re working on a PBL exercise) On a

sim-pler level, the links in the boxes to other sciences and to

clinical medicine will help you see clearly the relevance of

the basic science topic you are studying You may already

to confident in the subject matter of many of the integration

boxes, so they will serve as helpful reminders

At the back of the book we have included case study tions relating to each chapter so that you can test yourself asyou work your way through the book

ques-Online Version

An online version of the book is available on our Student sult site Use of this site is free to anyone who has bought theprinted book Please see the inside front cover for full details

Con-on Student CCon-onsult and how to access the electrCon-onic versiCon-on ofthis book

In addition to containing USMLE test questions, fullysearchable text, and an image bank, the Student Consult siteoffers additional integration links, both to the other books inElsevier’s Integrated Series and to other key Elseviertextbooks

Books in Elsevier’s Integrated SeriesThe nine books in the series cover all of the basic sciences Themore books you buy in the series, the more links that are madeaccessible across the series, both in print and online

Anatomy and Embryology

SECTION I

Immunology

Introduction to Immunity

CONTENTS

CHIEF FUNCTION OF IMMUNITY

INNATE IMMUNE SYSTEM

ADAPTIVE IMMUNE SYSTEM

SPECIFICITY OF ADAPTIVE RESPONSE BY

LYMPHOCYTE RECEPTORS

TIGHT REGULATION OF THE IMMUNE SYSTEM AND

ASSOCIATED RESPONSES

The immune cells and organs of the body make up the primary

defense system against invasion by microorganisms and foreign

pathogens A functional immune system confers a state of health

through effective elimination of infectious agents (bacteria,

viruses, fungi, and parasites) and through control of

malignan-cies by protective immune surveillance In essence, the process

is based in functional discernment between self and nonself, a

process that begins in utero and continues through adult life

Immune responses are designed to interact with the

envi-ronment to protect the host against pathogenic invaders

The goal of the chapters in Section I is to provide an

appreci-ation of the components of the human immune response that

work together to protect the host In addition, a working

clin-ical understanding of the concept of immune-based diseases

resulting from either immune system component deficiencies

or excess activity will be presented

The immune system consists of two overlapping

compart-ments representing interactions between innate and adaptive

components and associated responses The innate immune

mechanisms provide the first line of defense against infectious

disease (Table 1-1) Innate immune components are present

from birth and consist of nonspecific components available

before the onset of infection Innate immune recognition uses

preformed effector molecules to recognize broad structural

motifs that are highly conserved within microbial species

Engagement of innate components leads to triggering of signal

pathways to promote inflammation, ensuring that invadingpathogens remain in check while the specific immuneresponse is either generated or upregulated

The adaptive (also called acquired) immune response counts for specificity in recognition of foreign substances, orantigens, by functional receptors residing on the surface of

ac-B and T lymphocytes (Table 1-2 and Fig 1-1) The B-cellantigen receptor (BCR) is the surface immunoglobulin, an in-tegral glycosylated membrane protein with unique regionsthat bind specific antigens There can be thousands of identicalcopies present on the surface of a single cell B-cell activationoccurs upon interaction of the BCR with antigen, leading tocell activation and differentiation into plasma cells, whichsecrete soluble immunoglobulins, or antibodies B cells andantibodies together make up the humoral immune response.The T cell has a surface receptor structurally similar to theantibody, which also recognizes specific antigenic determi-nants (epitopes) T cells control the cellular arm of the immuneresponse Unlike the antibody, the T-cell receptor is presentonly on its surface and is not secreted The process of T-cellactivation requires a third group of cells called antigen-presenting cells (APCs) APCs contain surface molecules,the human leukocyte antigens, that are encoded within a generegion known as the major histocompatibility complex.Together these groups of molecules form a regulated pathway

to present foreign antigens for subsequent recognition andtriggering of specific responses to protect against disease.BIOCHEMISTRY

Mediators of Acute InflammationMetabolism of phospholipids is required for production of prostaglandins and leukotrienes, both of which enhance inflammatory response by promoting increased vascular permeability and vasodilation Prostaglandins are 20-carbon fatty acid derivatives containing a cyclopentane ring and

an oxygen-containing functional group; leukotrienes are 20-carbon fatty acid derivatives containing three conjugated double bonds and hydroxyl groups.

l ll SPECIFICITY OF ADAPTIVE

RESPONSE BY LYMPHOCYTE RECEPTORS

Each B and T lymphocyte expresses a unique antigen receptor.The generation of antigen-binding specificity occurs before an-tigen exposure through a DNA rearrangement process that cre-ates receptors of high diversity and binding potential During

an active immune response, a small number of B- and T-cellclones bind to the antigen with high affinity and then undergoactivation, proliferation, and differentiation (Fig 1-2) Thisprocess is called clonal selection and leads both to the

TABLE 1-1 Innate Defensive Components

Temperature, acidic

pH, lactic acid Chemical mediators Inflammatory

mediators

Complement Direct lysis of

pathogen or infected cells Cytokines and

interferons

Activation of other immune components Lysozymes Bacterial cell wall

destruction Acute-phase

proteins and lactoferrin

Mediation of response Leukotrienes and

prostaglandins

Vasodilation and increased vascular permeability Cellular

components

Polymorphonuclear cells

l Neutrophils, eosinophils

l Basophils, mast cells

Phagocytosis and intracellular destruction of microorganisms

endocytic cells

Phagocytic-l Monocytes and macrophages

l Dendritic cells

Presentation of foreign antigen

to lymphocytes

TABLE 1-2 Key Elements of the Innate and Adaptive

Immune Systems

Rapid response (minutes to

hours)

Slow response (days to weeks)

PMNs and phagocytes B cells and T cells

Preformed effectors with

limited variability

B-cell and T-cell receptors with a diverse array of highly selective specificities Pattern recognition molecules

recognizing structural motifs

Antibodies (humoral response) Soluble activators Cytokines (cellular response)

Surface Immunoglobulin

Antigen-Presenting Cell

Light chain Heavy chain

Variable regions

Constant regions Transmembrane region

Transmembrane region

Antigen-binding site

T lymphocyte

B lymphocyte

T-Cell Receptor

Figure 1-1 Basic structure of antigen receptors on the surface

of a B cell (the immunoglobulin B-cell receptor), a T cell (theT-cell receptor), and major histocompatibility complex (MHC)molecules

production of multiple cells all with the same antigen

recogni-tion capability and to the generarecogni-tion of immunologic memory

Long-term good health requires continued discrimination

against foreign agents and depends on immunologic memory,

which allows the adaptive immune system to respond more

efficiently to previously encountered antigens (Fig 1-3)

The specific adaptive response against an antigen is much

greater during secondary exposure This principle accounts

for the clinical utility of vaccines, which have done more to

improve mortality rates worldwide than any other medical

discovery in recorded history

IMMUNE SYSTEM AND

ASSOCIATED RESPONSES

Immunologic diseases can be grouped into two large

catego-ries: deficiency and dysfunction (Fig 1-4)

Immunodefi-ciency diseases occur as the result of the absence of one or

more elements of the immune system; this can either be

con-genital or acquired after birth Immune dysfunction occurs

when a particular immune response develops that is

detri-mental to the host This deleterious response may be against

a foreign antigen or a self-antigen It may also be an

inappro-priate regulation of an effector response that serves to

pre-vent a protective response Notwithstanding the cause, the

host is adversely affected A healthy immune system occurs

as a result of balance between innate and adaptive immunity,

cellular and humoral immunity (see Chapters 3 and 4), flammatory and regulatory networks (see Chapter 6), andsmall biochemical mediators (cytokines) (see Chapter 7).Disease occurs when the balance is altered by eitherdeficiency or dysfunction

in-Antigen

Processing

Presentation MHC TCR

Antigen-presenting cell

T cell

Phagocyte

Activation for enhanced control of pathogens Effector

function 1

Effector function 2

Memory B cell

B cell

Plasma cell (antibody secretion)

Release of cytokines and chemokines Accessory signals

Figure 1-2 Cellular interactions drive adaptive immune functions Antigen-presenting cells present foreign substances to activate Tcells T cells differentiate to become effectors to help phagocytes control pathogen infection or to assist B cells in production andsecretion of immunoglobulins MHC, major histocompatability complex; TCR, T-cell receptor

100,000 10,000 1000 100 10 1 0

42 35 28

21 14 7

0

Duration (days)

Primary response

Primary antigen challenge

Secondary antigen challenge

Secondary response

KEY POINTS

▪ The chief function of the immune system is to distinguish

be-tween self and nonself.

▪ The immune system consists of two overlapping compartments:

the innate immune system and the adaptive immune system.

▪ The specificity of the adaptive immune system is due to

antigen-specific receptors (immunoglobulins and T-cell receptors) The

generation of antigen-binding diversity inherent in these receptors

occurs before antigen exposure through DNA rearrangement.

▪ Clonal selection occurs after immune recognition of an antigen A small number of lymphocytes bind antigen with high affinity and undergo activation, proliferation, and differentiation into plasma cells (for B cells) or activated T cells.

▪ The adaptive immune system has memory, meaning that the sponse against a foreign substance is much greater after the first exposure Tight regulation ensures appropriate and directed activation.

re-Self-assessment questions can be accessed at www.StudentConsult.com

Immunodeficiency (hyporeactivity)

Neutrophil disorders Antibody deficiency Complement deficiency T-cell dysfunction

Systemic autoimmunity Organ-specific autoimmunity Allergies and asthma Pathogen-induced pathology

Immunopathology (hyperreactivity)

Immune homeostasis

Figure 1-4 Immunodeficiency and dysfunction Immune-based diseases can be caused by lack of specific functions ciency) or by excessive activity (hypersensitivity)

(immunodefi-Cells and Organs of the

CONTENTS

PLURIPOTENT HEMATOPOIETIC STEM CELLS

MYELOID CELLS: FIRST LINE OF DEFENSE

Neutrophils

Eosinophils

Basophils and Mast Cells

Monocytes and Macrophages

Dendritic Cells

Platelets and Erythrocytes

LYMPHOID CELLS: SPECIFIC AND LONG-LASTING

IMMUNITY

B Lymphocytes

T Lymphocytes

Natural Killer T Cells

Natural Killer Cells

PRIMARY AND SECONDARY LYMPHOID ORGANS

Primary Lymphoid Organs

Secondary Lymphoid Organs

The first line of defense against infection includes natural

physical barriers that limit the entry of microorganisms into

the body The skin, mucosal epithelia, and cilia lining the

respiratory tract represent effective mechanical barriers

Bio-chemical mechanisms also support innate processes to ward

off potential pathogens; these include sebaceous gland

secre-tions containing fatty acids, hydrolytic enzymes, and

antibac-terial defensins Enzymes in saliva, intestinal secretions that

are capable of digesting bacterial cell walls, and the acidic

pH of the stomach lumen all represent innate barriers to

infec-tion Once a pathogen has compromised these barriers and

gained access to the body, cellular components must be

evoked to combat invading organisms

STEM CELLS

Immune system cells are derived from pluripotent

hematopoi-etic stem cells in the bone marrow These cells are functionally

grouped into two major categories of immune response:

innate (natural) and acquired (adaptive) Innate immunity is

present from birth and consists of nonspecific components.Acquired immunity by definition requires recognition speci-ficity to foreign (nonself) substances (antigens) The majorproperties of the acquired immune response are specificity,memory, adaptiveness, and discrimination between self andnonself

The acquired immune response is further classified as humoral

or cellular immunity, based on participation of two major celltypes Humoral immunity involves B lymphocytes that synthe-size and secrete antibodies to neutralize pathogens and toxins.Cell-mediated immunity (CMI) involves effector T lympho-cytes, which can lyse infected target cells or secrete immunoreg-ulatory factors following interaction with antigen-presentingcells (APCs) to combat intracellular viruses and organisms

An intricate communication system allows components ofinnate and acquired immunity to work in concert to combatinfectious disease Leukocytes provide either innate or specificadaptive immunity and are derived from myeloid or lymphoidlineage (Fig 2-1) The production of leukocytes is induced byhematopoietic growth factor glycoproteins that exert criticalregulatory functions in the processes of proliferation, differen-tiation, and functional activation of hematopoietic progenitorsand mature blood cells Myeloid cells include highly phago-cytic, motile polymorphonuclear neutrophils, monocytes, andmacrophages (tissue-resident monocytes) that provide a firstline of defense against most pathogens The other myeloid cells,including polymorphonuclear eosinophils, basophils, and theirtissue counterparts—mast cells—are involved in defenseagainst parasites and in the genesis of allergic reactions Cellsfrom the lymphoid lineage are responsible for humoral immu-nity (B lymphocytes) and CMI (T lymphocytes) (Table 2-1).HISTOLOGY

Pluripotent Hemopoietic Stem CellsPluripotent hemopoietic stem cells differentiate into myeloid and lymphoid lineages The myeloid lineage gives rise to histologically distinct neutrophils with a characteristic multilobed nucleus, basophils with azurophilic granules and an S-shaped nucleus, and eosinophils with red-orange specific granules and a bilobed nucleus as well as to erythrocytes, monocytes, and platelets Lymphoid lineages give rise to B and

T lymphocytes.

l ll MYELOID CELLS: FIRST LINE

OF DEFENSE

Neutrophils

Granulocyte neutrophils are the most highly abundant

my-eloid cell type, comprising 40% to 70% of total white blood

cells They are motile phagocytic leukocytes that are the

first cells recruited to acute inflammatory sites

Neutro-phils are short lived and are produced within the bone

mar-row through stimulation with granulocyte colony-stimulating

factor They ingest, kill, and digest microbial pathogens,with their functions dependent upon special proteins found

in primary granules (containing cationic defensins and loperoxidase) and secondary granules (iron chelators,lactoferrin, and digestive enzymes) Neutrophilic granulescontain multiple antimicrobial agents, including oxygen-independent lysozyme (peptidoglycan degradation) and lac-toferrin (iron chelator) In addition, respiratory burst andgranule oxidases can reduce molecular oxygen to superox-ide radicals and reactive oxygen species to produce toxicmetabolites (hydrogen peroxide) that limit bacterial growth

mye-Leukocytes Granulocytes Mononuclear cells

Mast cell Macrophage(histiocyte)

T lymphocyte

Lymphocytes

Plasma cell

NK cell

Figure 2-1 Nomenclature of immune system cells NK, natural killer; RBC, red blood cell

TABLE 2-1 Myeloid Leukocytes and Their Properties

Neutrophil PMN granulocyte 2 to 7.5  109/L Phagocytosis and digestion of microbes Eosinophil PMN granulocyte 0.04 to 0.44  10 9 /L Immediate hypersensitivity (allergic) reactions,

defense against helminths Basophil PMN granulocyte 0 to 0.1  109/L Immediate hypersensitivity (allergic) reactions Mast cell PMN granulocyte Tissue specific Immediate hypersensitivity (allergic) reactions

antigen presentation to T cells Dendritic cell Monocytic Tissue specific Antigen presentation to naı¨ve T cells, initiation

of adaptive responses PMN, polymorphonuclear.

*Normal range for 95% of population 2 standard deviations.

Neutrophils dying at the site of infection contribute to the

formation of the whitish exudate called pus

Eosinophils

Eosinophils are polymorphonuclear granulocytes that defend

against parasites and participate in hypersensitivity reactions

via antibody-dependent, cell-mediated cytotoxicity

mecha-nisms Their cytotoxicity is mediated by large cytoplasmic

granules, which contain eosinophilic basic and cationic

pro-teins Small granules within the cellular cytoplasm contain

chemical mediators such as histamine and eosinophil

peroxi-dase, deoxyribonuclease, ribonuclease, lipase, and major

basic protein Eosinophils are involved in manifestation of

allergy and asthma via low affinity immunoglobulin E (IgE)

receptors for immunoglobulins with specificity for allergen

antigens Eosinophils are specifically geared to combat

multi-cellular parasites

Basophils and Mast Cells

Basophils and their tissue counterpart, mast cells, are

poly-morphonuclear granulocytes that produce cytokines in

de-fense against parasites These cells are also responsible for

allergic inflammation Basophils and mast cells display

high-affinity surface membrane receptors for IgE antibodies These

cells degranulate when cell-bound IgE antibodies are

cross-linked by antigens secreting low-molecular-weight mediators

that regulate vascular tone and capillary permeability from

both primary (histamine, serotonin, and platelet-activating

factor) and secondary granules (leukotrienes C4, D4, and B4;

prostaglandin D2; and bradykinin) Mast cells arise from

independent myeloid progenitor cells and also regulate

IgE-mediated hypersensitivity responses via high-affinity IgE

receptors

Monocytes and Macrophages

Monocytes and macrophages are involved in phagocytosis

and intracellular killing of microorganisms Monocytes

com-prise up to 10% of circulating white blood cells

Macro-phages are terminally differentiated, long-lived monocytes

residing in reticular connective tissue that comprises the

re-ticuloendothelial system (RES; also referred to as the

mono-nuclear phagocytic system or lymphoreticular system)

Monocytes and macrophages are motile, yet become highly

adherent upon phagocytic activity They provide natural

im-munity against microorganisms by a coupled process of

phagocytosis and intracellular killing, recruiting other

in-flammatory cells through the production of cytokines and

chemokines, and presenting peptide antigens to T

lympho-cytes for the production of antigen-specific immunity The

cells of the RES include circulating monocytes as well as

tissue-resident macrophages in the spleen, lymph nodes,

thymus, submucosal tissues of the respiratory and

alimen-tary tracts, bone marrow, and connective tissues

Special-ized macrophages include Kupffer cells in the liver,

Langerhans cells in skin, and glial cells in the central vous system

ner-BIOCHEMISTRYVasoactive Mediators

A hallmark of inflammation includes the four cardinal signs of tumor (swelling), rubor (redness), calor (heat), and dolor (pain), resulting from biochemical actions of the vasoactive mediators prostaglandins and leukotrienes, produced from arachidonic acid precursors, and bradykinin, a peptide of the kinin group of proteins formed in response to activation of factor XII (Hageman factor) In general, these potent vasodilators cause contraction of nonvascular smooth muscle, increased vascular permeability, and pain.

Dendritic Cells

Dendritic cells (DCs) are bone marrow-derived differentiatedmacrophages that act as APCs to activate helper T cells andcytotoxic T cells as well as B cells They are found in epitheliaand in most organs and are important initiators for adaptiveimmune recognition of foreign (nonself) proteins Subsets ofDCs exist, characterized according to their location andimmunological function For example, monocytoid DCs areclassical immunosurveillance cells that endocytose and enzy-matically digest antigen to subsequently present to adaptive

T lymphocytes Plasmacytoid DCs are type I producing cells important in antiviral responses

interferon-Platelets and Erythrocytes

Platelets and erythrocytes (red blood cells) arise from myeloidmegakaryocyte precursors Platelets and erythrocytes are in-volved in blood clotting and release inflammatory mediatorsinvolved in innate immune activation

KEY POINTS ABOUT MYELOID CELLS

▪ Myeloid cells are from pluripotent hematopoietic stem cells in the bone marrow and represent the first line of defense against invad- ing pathogens.

▪ Neutrophils are the most abundant of the myeloid populations They phagocytose pathogens and fight infections using primary and secondary granules containing enzymes and molecules regulating reactive oxygen-mediated defense mechanisms.

▪ Eosinophils are polymorphic granulocytes that are critical for defense against large multicellular pathogens.

▪ Basophils and mast cells are the least common of the cytes They are important in asthma and allergic responses.

granulo-▪ DCs, monocytes, and macrophages are critical for mediation of proinflammatory function and are important APCs that can show foreign proteins to adaptive lymphocyte populations.

Myeloid cells: first line of defense 9

l ll LYMPHOID CELLS: SPECIFIC

AND LONG-LASTING IMMUNITY

Lymphoid cells are present in healthy adults at concentrations

of approximately 1.3 to 3.5109

/L Lymphocytes ate into three separate lines: (1) thymic-dependent cells or T

differenti-lymphocytes that operate in cellular and humoral immunity,

(2) B lymphocytes that differentiate into plasma cells to

secrete antibodies, and (3) natural killer (NK) cells that can

lyse infected target cells T and B lymphocytes produce and

express specific receptors for antigens whereas NK cells do

not (Table 2-2)

B Lymphocytes

B lymphocytes differentiate into plasma cells to secrete

anti-bodies, which are Ig glycoproteins that bind antigens with a

high degree of specificity The genesis of mature B cells from

pluripotent progenitor stem cells occurs in the bone marrow

and is antigen independent The activation of B cells into

antibody-producing and antibody-secreting cells (plasma

cells) is antigen dependent Mature B cells can have 1 to

1.5105

immunoglobulin receptors for antigen embedded

within their plasma membrane Once specific antigen binds

to surface Ig molecule, the B cells differentiate into plasma

cells that produce and secrete these antibodies Because all

the antibodies produced by the individual plasma cell have

the same antigenic specificity, the antibodies are referred to

as monoclonal If B cells also interact with T helper cells, they

proliferate and switch the isotype (class) of immunoglobulin

that is produced while retaining the same antigen-binding

specificity T helper cells are thought to be required for

switching from IgM to IgG, IgA, or IgE isotypes B cells also

process and present protein antigens In this case,

immuno-globulin receptors recognize and internalize antigen, which

then is degraded and presented to T lymphocytes

T Lymphocytes

Immature thymocytes differentiate in the thymus, where

rear-rangement of antigen-specific T-cell receptor (TCR) genes

gives rise to a diverse set of clonotypic T lymphocytes In

the thymus, cells are selected for maturation only if theirTCRs do not interact with self-peptides presented in thecontext of self–major histocompatibility complex (MHC)molecules on APCs T lymphocytes are involved in the regu-lation of the immune response and in CMI and help B cellsproduce antibody Mature lymphocytes also display one oftwo accessory molecules—CD4 or CD8—that define whether

a T cell will be a CD4-expressing helper T lymphocyte or aCD8-expressing cytotoxic T lymphocyte (CTL) Every T lym-phocyte also expresses CD3, a multisubunit cell-signalingcomplex noncovalently associated with the antigen-specificTCR The TCR/CD3 complex recognizes antigens associatedwith the MHC molecules on APCs or target cells (e.g.,virus-infected cells) Upon recognition of presented antigen,the T lymphocyte becomes activated and secretes cytokines(interleukins)

HISTOLOGYCluster of Differentiation

CD (cluster of differentiation) designates cell surface proteins Each unique molecule is assigned a different number designation Surface expression of a particular CD molecule may not be specific for just one cell or even a cell lineage; however, many are useful for characterization of cell phenotypes.

T Helper Lymphocytes

T helper lymphocytes are the primary regulators of type hypersensitivity responses They express the CD4 mole-cule and regulate antigen-directed effector functions involved

delayed-in CMI to pathogens They also assist delayed-in the stimulation of

B lymphocytes to proliferate and differentiate to becomeantibody-producing cells T helper lymphocytes recognizeforeign antigen complexed with MHC class II molecules onDCs, B cells, and macrophages or other APCs that canexpress MHC class II

Cytotoxic T LymphocytesCTLs are cytotoxic against tumor cells and host cells infectedwith intracellular pathogens These cells usually express CD8and destroy infected cells in an antigen-specific manner that is

TABLE 2-2 Lymphoid Leukocytes and Their Properties

B cell

regulation

NK, natural killer; NKT cell, natural killer T cell.

dependent upon the expression of MHC class I molecules that

are expressed on almost all nucleated cells in the body T

sup-pressor cells express CD8 molecules and are thought to be

related to CTLs The suppressor cells function to suppress

and limit T- and B-lymphocyte–specific responses

Natural Killer T Cells

NK T cells (NKTs) are a heterogeneous group of T cells that

share properties of both T cells and NK cells They recognize

foreign lipids and glycolipids and constitute only 0.2% of all

pe-ripheral blood T cells It is now generally accepted that the term

NKT cells refers to a restricted population of T cells

coexpres-sing a heavily biased, semi-invariant TCR and NK cell markers

NKT cells should not be confused with NK cells

Natural Killer Cells

NK cells are large granular lymphocytes that kill tumor cells

and virus-infected targets NK cells do not express

antigen-specific receptors such as the TCR/CD3 complex Instead,

NK cells express a variety of killer immunoglobulin-like

re-ceptors, which can bind MHC class I molecules and stress

molecules on target cells and send either a positive or negative

signal for NK cell activation NK cells adhere to infected target

cells and induce their cell death via delivery of apoptic signals

mediated by perforins, granzymes, and tumor necrosis

factor-a, or by effector interactions via their surface Fas ligand

molecule with Fas on the target cell NK cells can also

kill through antibody-dependent, cell-mediated cytotoxicity

mechanisms via cell surface receptors for constant domains

present on immunoglobulins

KEY POINTS ABOUT LYMPHOID CELLS

▪ Lymphoid cells are from pluripotent hematopoietic stem cells in

the bone marrow and represent the secondary line of defense

against invading pathogens.

▪ B lymphocytes make antibodies that specifically recognize

anti-genic determinants Activated B lymphocytes are called plasma

cells.

▪ T lymphocytes are involved in cell-mediated immune function.

They are subdivided into helper and cytotoxic populations.

▪ NKT cells recognize foreign glycolipids They share properties of

both T cells and NK cells.

▪ NK cells do not have a specific antigen receptor but do have the

ability to kill tumor and virally infected cells.

LYMPHOID ORGANS

The lymphatic organs are tissues in which leukocytes of

mye-loid and lymphoid origin mature, differentiate, and proliferate

(Fig 2-2) Lymphoid organs are composed of epithelial and

stromal cells arranged either into discretely capsulated organs

or accumulations of diffuse lymphoid tissue The primary

(central) lymphoid organs are the major sites of lymphopoiesis,for example, where B and T lymphocytes differentiate fromstem cells into mature antigen-recognizing cells The secondarylymphoid organs are those tissues in which antigen-driven pro-liferation and differentiation occur

Historically, the primary lymphoid organ was discovered inbirds, in which B lymphocytes undergo maturation in thebursa of Fabricius, an organ situated near the cloaca Humans

do not have a cloaca, nor do they possess a bursa of Fabricius

In embryonic life, B lymphocytes mature and differentiatefrom hematopoietic stem cells in the fetal liver After birth,

B cells differentiate in the bone marrow Maturation of T phocytes occurs in a different manner Progenitor cells fromthe bone marrow migrate to the thymus, where they differen-tiate into T lymphocytes The T lymphocytes continue to dif-ferentiate after leaving the thymus, and are driven to do so byencounter with APCs presenting trapped circulating antigen

lym-in the secondary lymphoid organs

ANATOMY AND EMBRYOLOGYOntogenic Development of Lymphoid CellsLymphocyte stem cells are produced first by the omentum and later by the yolk sac or fetal liver In older fetuses and adults, the bone marrow is the major source of lymphocytes derived from pluripotent hematopoietic stem cells.

Adenoid Tonsil Right subclavian vein Lymph node Kidney Appendix

Lymphatics

Left subclavian vein

Thymus Heart Thoracic duct Spleen

Peyer patch in small intestine Large intestine

Bone marrow

Figure 2-2 Distribution of lymphoid tissues

Primary and secondary lymphoid organs 11

Primary Lymphoid Organs

Fetal Liver and Adult Bone Marrow

Islands or foci of hemopoietic progenitor cells in the fetal liver

and in the adult bone marrow give rise directly to

polymor-phonuclear cells, monocytes, DCs, B lymphocytes, and

precursor T lymphocytes Although the bone marrow is

tech-nically a primary lymphoid organ, recirculation due to

vascu-larization enables entry of circulating leukocytes from

peripheral tissue, thereby also allowing bone marrow to serve

a secondary lymphoid organ function

Thymus GlandThe principal function of the thymus gland is to educate T lym-phocytes to differentiate between self and nonself antigens.The thymus reaches maturity before puberty and slowly losesfunction thereafter This lymphoepithelial organ is primarilycomposed of stroma (thymic epithelium) and cells of theT-lymphocyte lineage (Fig 2-3) The thymus is divided intolobules containing cortex and medulla regions Precursor

T lymphocytes (thymocytes) differentiate to express specificreceptors for antigen The cortex contains immature thymo-cytes with few macrophages The maturing lymphocytes pass

Cortical epithelial cell

Medullary epithelial cell

Dendritic cell Macrophage Hassall’s corpuscle

Thymocyte

Major Organs of the Immune System (Thymus)

Figure 2-3 The thymus Visible are lobules with a darker staining cortex and a lighter staining medulla The medulla is characterized

by the presence of Hassall’s corpuscles

through the medulla, interacting with epithelial cells, DCs,

and macrophages Only 5% to 10% of thymocytes leave

the thymus for final maturation in secondary organs Hassall

corpuscles are a characteristic morphologic feature located

within the medullary region of the thymus

Secondary Lymphoid Organs

The secondary lymphoid organs provide localized

environ-ments where lymphocytes may respond to pathogens and

for-eign antigens The spleen and lymph nodes are the major

secondary lymphoid organs Additional secondary lymphoid

organs include the mucosa-associated lymphoid tissue (MALT),

which is composed of cellular aggregates in the lamina propria

of the digestive tract lining and respiratory tract

SpleenThe spleen is a filter for blood that is histologically composed

of two tissue types, red pulp and white pulp The red pulp ismade up of vascular sinusoids containing large numbers ofmacrophages, and is actively involved in the removal of dyingand dead erythrocytes and of infectious agents (Fig 2-4) Thewhite pulp contains the lymphoid tissue, which is arrangedaround a central arteriole as a periarteriolar lymphoid sheathand is composed of T- and B-cell areas and follicles containinggerminal centers Dendritic reticular cells and phagocytic

Marginal zone Marginal sinus Germinal center

Central arteriole

Major Organs of the Immune System (Spleen)

Periarteriolar lymphoid sheath

Figure 2-4 The spleen The white pulp of the spleen contains a central artery and associated follicle (germinal center, marginal zone,and periarteriolar lymphoid sheath)

Primary and secondary lymphoid organs 13

macrophages found in germinal centers present antigen to

lymphocytes The germinal centers are where B cells are

stim-ulated to become plasma cells that produce and secrete

antibodies

Lymph Node

Lymph nodes (Fig 2-5) form part of the lymphatic network

that filters antigen and debris from lymph during its passage

from the periphery to the thoracic duct The lymphatic vessels

(or lymphatics) are a network of thin tubes that branch into

tissues throughout the body Lymphatic vessels carry a

color-less, watery fluid called lymph, which originates from

intersti-tial fluid Plasma and leukocytes from vascular tissue and

capillary beds continuously circulate via afferent lymphatics,

eventually arriving at draining lymph nodes Histologically,the lymph node is composed of an outer sinuous connec-tive capsule Subcapsular afferent vessels deliver antigen-containing fluid and draining pathogens into discrete lobules,where they are phagocytosed by antigen-presenting macro-phages and DCs Naive T lymphocytes enter the lymph nodethrough specialized high endothelial venules and travel tothe paracortex, where they encounter the APCs A B-lymphocyte–rich cortex contains both primary and secondaryfollicles and active germinal centers representing regions oflymphocyte division, differentiation, and expansion Acti-vated and memory lymphocytes eventually migrate throughthe central medullary sinus, leaving through efferent lym-phatic vessels

Afferent lymphatic vessel T-cell area

Germinal center

Lymphoid follicle (mostly B cells) Medullary sinus

Artery Vein

Efferent lymphatic vessel

Marginal sinus

Major Organs of the Immune System (Lymph Node)

Figure 2-5 Lymph node The normal architecture of a lymph node containing afferent and efferent lymphatic vessels, T-cell–richparacortex, and germinal centers of activation

Mucosa-Associated Lymphoid Tissue

Multiple nodules of partially encapsulated lymphoid tissue

and loosely associated lymphoid aggregates are found in

epi-thelia and lamina propria of mucosal surfaces (Fig 2-6)

Collectively, these aggregates located along the

gastrointesti-nal and respiratory tracts are called MALT, which can be

further classified as gut-associated lymphoid tissue (GALT)

or bronchus-associated lymphoid tissue (BALT) The tonsils,

appendix, and Peyer patches are representative of lymphoid

tissue found in and around mucosal epithelia In particular,

specialized microfold (M) cells of the follicle-associated

epi-thelium of the MALT in gut and the respiratory system play

a critical role in the genesis of immune responses by delivering

foreign (nonhost) material via transcytosis to underlying

lym-phoid tissue Specialized resident intraepithelial lymphocytes

with potent cytolytic and immunoregulatory capacities

moni-tor mucosal tissue to help defend against pathogenic infection

KEY POINTS ABOUT LYMPHOID ORGANS

▪ The thymus and bone marrow represent primary lymphoid organs; they are the sites of lymphopoiesis where cells develop and learn to differentiate self from nonself.

▪ The spleen is a filter for blood and is composed of parenchyma that allows both innate and adaptive cells to interact and respond

to presented antigens.

▪ The lymph nodes are connected via lymphatics and represent a local environment for antigen drainage and interaction with pre- senting populations to engage adaptive lymphocytes.

▪ MALT represents cellular aggregates loosely associated with the epithelia and lamina propria of mucosal parenchyma, delivering foreign materials to underlying lymphocytes The tonsils, appen- dix, and Peyer patches represent a more formal association of ac- cumulated tissue that shares this same functional parameter.

Primary and secondary lymphoid organs 15

KEY POINTS

▪ Physical barriers and biochemical mechanisms comprise the first

line of defense against foreign pathogens Once inside the body,

innate and cellular components are required to fight against

infectious agents.

▪ Innate cells and components are present from birth and

repre-sent a nonspecific first line of defense to foreign substances.

▪ Stem cell progenitors within the bone marrow are the precursors

to myeloid and lymphoid progenitors, giving rise to cells

in-volved in either innate or adaptive immune responses Myeloid

cells include neutrophils, eosinophils, basophils, mast cells,

monocytes, macrophages, and DCs Lymphoid progenitors

give rise to B and T lymphocytes, NKT cells, and NK cells.

▪ Acquired (adaptive) immune responses discriminate between self and nonself and demonstrate specificity, memory, and adaptive- ness Humoral immunity refers to B lymphocytes, which produce antibodies that neutralize pathogens and toxins Cellular immunity encompasses T lymphocytes to eradicate intracellular organisms.

▪ Primary lymphoid organs, such as the bone marrow and thymus, are the major sites of lymphopoiesis where lymphocytes differen- tiate Secondary lymphoid organs, such as the spleen and lymph nodes, are locations within the body where antigen-driven proliferation and maturation of lymphocytes occur The MALT represents a loosely associated lymphoid aggregate where APCs located in the mucosal epithelia present antigens to lymphocytes.

Self-assessment questions can be accessed at www.StudentConsult.com

Physiochemical Forces in Antigen-Antibody Interactions

ANTIGENS AND IMMUNOGENS

Recognition of Sequential and Conformational Epitopes

Cross-Reactivity

GENETIC BASIS OF ANTIBODY STRUCTURE

GENERATION OF ANTIBODY DIVERSITY

Multiple Variable Gene Segments

Combinatorial Diversity

Heavy and Light Chain Combinations

Junctional and Insertional Diversity

ISOTYPE SWITCHING AND AFFINITY MATURATIONS

Humoral immunity refers to the arm of the acquired immune

system that is mediated by antibody (immunoglobulin)

recog-nition of antigens associated with foreign substances or

path-ogens Antigen recognition is coupled with the ability to

initiate biologic responses that protect against

microorgan-isms and neutralize viruses The specific recognition of foreign

antigen by B lymphocytes occurs through membrane-bound

receptors and triggers proliferation and differentiation into

antibody-producing plasma cells

AND FUNCTION

The antibody is a tetrameric polypeptide structure with

dis-tinct biologic activity attributed to each end of the molecule

(Fig 3-1) Antibodies are composed of two identical heavy

chain and two identical light chain polypeptides Both heavy

and light chain molecules have variable and constant domains

and interact via intradisulfide and interdisulfide linkages The

variable region, termed F(ab0)2(fragment, antibody binding),

confers antigen recognition The constant region, termed Fc

(fragment, crystalline), interacts with cell surface receptors

The heavy chain contains a hinge domain that confers

flexi-bility to allow optimal binding to antigen

The constant regions of the heavy chain confer antibody tion representing five different classes, or isotypes (Fig 3-2).Each class of heavy chain has a characteristic amino acid sequencethat distinguishes it from the other four classes, but all fiveclasses have a significant percentage of amino acid sequence sim-ilarities The isotypes, immunoglobulin (Ig) M, IgD, IgG, IgE, andIgA, have characteristic properties (Table 3-1) The differentheavy chains corresponding to their class are given Greek letterdesignations:g, a, m, E, and d In many species, there are two ormore subclasses of some heavy chains that differ from oneanother by only a few amino acids; humans have nine possibleheavy chains each with unique biologic functions There are foursubclasses of the IgG isotype, called IgG1, IgG2, IgG3, and IgG4, aslisted inTable 3-2 IgA has two subclasses called IgA1and IgA2.Light chains come in two varieties, called kappa (k) andlambda (l) The difference between the two types of lightchains is in the amino acid sequence of the constant regiondomain The overall ratio of the two light chain types inhuman immunoglobulin is approximately 60%k and 40% l.Both heavy and light chains have functional domains—aminoacid sequences giving regularity to structure by way of disulfide-bridged loops There are two domains on bothk and l light chainsand either four or five domains on heavy chains The amino acid se-quences in the first domain on both light and heavy chains varygreatly from molecule to molecule and are referred to as the vari-able light domain (VL) or variable heavy domain (VH) The lightchain domain, which is constant in its amino acid sequencefor thek or l type of chain, is referred to as the CLdomain Theconstant domains of heavy chains are numbered from theamino terminal to the carboxyl terminal as CH1, CH2, CH3, and

func-CH4 (for IgM and IgE)

IgG, IgA, and IgD genes each have an exon coding for ashort span of amino acids that occupy the space betweenthe CH1 and CH2 domains This segment is rich in cysteineand proline and permits significant flexibility between thetwo arms of the antibody; the area is called the hinge region.This stretch is highly susceptible to protease cleavage.Vertebrates and invertebrates express a large number ofclosely related cell-surface proteins, many of which appear

to have evolved from common gene sequences Collectively,

they are called the immunoglobulin gene superfamily All

members of the immunoglobulin gene superfamily contain

Ig domains, which share the primary amino acid sequence

and physical structure of b-pleated sheets with intrachain

disulfide bonds Members of the Ig gene superfamily include

surface receptors and adhesion molecules

KEY POINTS ABOUT ANTIBODY STRUCTURE

AND FUNCTION

n The basic immunoglobulin (antibody) unit consists of two light

and two heavy chains.

n Each set of chains are covalently linked by disulfide bridges,

allowing for unique generation of antigen binding regions.

n The specific domains within the constant chains allow for unique

biologic functions.

n Five subclasses of heavy chains (g, a, m,E, and d) correspond to

distinct heavy chains (IgG, IgA, IgM, IgD, and IgE).

n There are two types of light chains in humans (k and l), which differ

in amino acid content with their respective constant domains.

BIOCHEMISTRY

Amino Acid Protein Structure

The combination of disulfide links between cysteine residues and

the proline-rich hinge region gives the antibody molecule the

unique structure and flexibility necessary to interact with antigens.

an-Physiochemical Forces in Antibody Interactions

Antigen-Antibodies can bind a wide variety of molecules with highspecificity, ranging from large macromolecules to smallchemical moieties The molecular region on the antigen rec-ognized by immune components is called an epitope, orantigenic determinant Antibody binding to antigen doesnot involve covalent chemical bonds The strength ofbinding to epitopes on the antigen, or the interaction affinity,

is based on multiple forces present within the binding site(Fig 3-4)

S S

S

S S

S

S S S S

S S

S S S

S S S

S

S

S

S S

S

S S

S S

IgM (pentamer)

Hinge region

Secretory component

Binding to mast cells and

basophils

Additional properties Effective

agglutinator of particulate antigens, bacterial opsonization

Found on surface

of mature B ells, signaling via cytoplasmic tail

dependent cell cytotoxicity

Antibody-Mediation of allergic response, effective against parasitic worms

Monomer in secretory fluid, active as dimer on epithelial surfaces

Ig, immunoglobulin.

TABLE 3-2 Unique Biologic Properties of Human IgG Subclasses

Hydrophobic bonding

Electrostatic interactions

Van der Waal forces

Major antigen (Ag) classes

Carbohydrates Polysaccharides Proteins Glycoproteins Nucleic acids Lipids

Requirements for immunogenicity

Physiochemical complexity Molecular weight > 6 kDa Foreignness (non-self) Degradability

Figure 3-4 Noncovalent forces contribute to avidity of antibody-antigen interactions Binding of antigen to antibody is a noncovalentinteraction The major forces involved are depicted

l ll ANTIGENS AND IMMUNOGENS

An antigen is any substance that can be recognized by the

immune system Major classes of antigens include proteins,

car-bohydrates, lipids, and nucleic acids In contrast, an

immuno-gen is any substance that can evoke an immune response

Not all antigens are immunogenic For example, haptens are

low-molecular-weight compounds that are nonimmunogenic

by themselves but are antigenic; haptens become immunogenic

after conjugation to high-molecular-weight immunogenic

car-riers The rule to remember is that all immunogens are

anti-gens, but not all antigens are immunogens This concept is

important when considering rational design of vaccines

BIOCHEMISTRY

Noncovalent Bond Forces

The weak interactions represented by noncovalent bond

forces are important in multiple biologic systems and allow for

fluidity of information, as is seen with ligands interacting with

cellular receptors One of these interactions is the van der

Waals forces, which are relatively weak electric forces that

attract neutral atoms and molecules to one another in gases, in

liquefied and solidified gases, and in almost all organic liquids

and solids These forces arise from polarization induced in

each particle by the presence of other particles.

Recognition of Sequential and

Conformational Epitopes

Two general classes of epitopes can be distinguished on

gens They are best described as they exist on protein

anti-gens, but other classes of antigens (e.g., carbohydrates and

nucleic acids) can exhibit both kinds of epitopes under some

circumstances Sequential epitopes are short stretches of

amino acids (4 to 7 inches length) that can be recognized by

antibodies when the short peptide exists free in solution or

when it is chemically coupled to another protein molecule

Conformational epitopes require the native

three-dimensional configuration of the molecule to be intact, and

antigenic determinants need not be contiguous; denaturation

of the molecule destroys these kinds of epitopes

Cross-Reactivity

The forces mediating antigen-antibody recognition allow a high

degree of specificity That is, antibodies specific for one epitope

or hapten can easily distinguish that epitope or hapten from

other similar structures This specificity is not absolute;

anti-bodies specific for one epitope can bind with structurally

sim-ilar nonidentical epitopes with a lower affinity Cross-reactivity

refers to the situation in which an antibody can react with two

similar molecules because they share one or more identical

epi-tope or the epiepi-tope in question is similar enough in sequence or

shape to bind with weaker affinity For example, antibodies

elicited with toxoids react with native toxins, allowing clinical

application for vaccination with nonpathogenic antigens such

as tetanus toxoid and diphtheria toxoid

KEY POINTS ABOUT ANTIBODY-ANTIGENINTERACTIONS

n Antibodies recognize antigenic determinants (epitopes) via ing pockets found within their combined heavy and light chain sequences.

bind-n Major classes of antigens include proteins or glycoproteins, nucleic acids, carbohydrates, and lipids.

n The binding site for antigen occurs in hypervariable regions The binding is not a covalent interaction Rather, multiple weak forces stabilize the binding These forces include electrostatic interac- tions, hydrogen bonding, hydrophobic interactions, and van der Waals forces.

n Cross-reactivity refers to reactivity of an antibody with two similar molecules that share one or more physically similar epitopes.

MICROBIOLOGYAntibacterial VaccinesGroup B streptococcus causes invasive infections of newborns and adults, with commonly reported bacteremia, meningitis, and pneumonia The associated polysaccharide capsule antigens are being targeted for use in vaccines with methods that conjugate carbohydrates to a protein carrier to create T-dependent antigens.

STRUCTUREThe generation of antigen-binding capability of the antibodyB-cell receptor occurs before antigen exposure through DNArearrangement involving combinations of multiple genes toachieve high diversity needed for immune responses Recom-bination occurs for both heavy and light chain genes by way

of an enzyme complex known as V(D)J recombinase Therecombinase is the product of two genes (RAG-1 andRAG-2; recombinase-activating genes); defects in eitherRAG-1 or RAG-2 can cause a spectrum of severe immuno-deficiencies with devastating clinical complications

The organization of heavy chain genes shows three ent regions contributing to production of the variable region.These gene segments are referred to as VH(variable), DH(di-versity), and JH(joining) (Fig 3-5) There are approximately

differ-50 VHgenes, 20 DHgenes, and up to 6 JHgenes present ingermline DNA The V(D)J recombinase mediates joining ofdifferent gene segments through mechanisms by which inter-vening segments are spliced out of the genome within that par-ticular B cell This process requires pairing of 7-base-pair and9-base-pair gene sequences, after which the intervening DNA

is “looped out” and deleted permanently from the some To complete the process, alternative splicing bringsthe rearranged VDJ sequence together with distinct gene seg-ments coding for the CH(constant) region

chromo-Similar events occur for rearrangement of the light chain cus In humans (and most other mammals) there are two light

lo-Genetic basis of antibody structure 21

chain loci—k and l—located on different chromosomes The

germline arrangement for light chains is similar to that of the

heavy chain locus, except there are no D gene segments

In addition, for thek locus there is only one constant region,

whereas thel locus has multiple constant regions, each with

its own J gene segment

BIOCHEMISTRY

Gene Transcription and RNA Translation

The production of protein involves gene transcription of DNA

into primary RNA transcripts by polymerases, under the control

of transcription factors The primary transcript is further

processed by splicing out noncoding introns, to produce full

messenger RNA (mRNA), which is eventually translated into

protein In B lymphocytes, the zinc finger transcription factor

early growth response 1 (Egr-1) is one of the many

immediate-early genes induced upon B-cell antigen receptor

engagement.

DIVERSITY

The portions of the variable region that participate in

antigen-binding are called complementarity-determining regions

(CDRs) Amino acids in these hypervariable regions contact

residues for antigen; the CDRs form the region of structural

complementarity for antigenic epitopes, and differences in

antigen-binding are due to differences in these sequences.Two of the CDRs (CDR1 and CDR2) are “hard wired” intothe V gene segment and thus depend upon the V segment se-lected during rearrangement CDR3 consists of the junction ofthe V, D, and J gene segments and hence has a high degree ofvariability The CDRs of both the heavy and light chain par-ticipate in the formation of the antigen-binding pocket(paratope)

Four major mechanisms for generating antibody diversity

in humans occur during B-cell development, before antigenexposure These mechanisms give rise to a repertoire ofantibodies that in theory have the capability of recognizingapproximately 1014different epitopes

Multiple Variable Gene Segments

There are more than 50 V gene segments in the heavy chain cus; there are approximately 40 V gene segments each in theklight chain loci and nearly as many V gene segments in thel loci

lo-Combinatorial Diversity

The V, D, and J regions in heavy chains (and the V and J gions in light chains) are selected randomly during V(D)J rear-rangement (“joining”) Thus, 50 VHgenes 26 DHgenes

re-6 JHgenes yield more than 6000 possible heavy chain VDJloci combinations A lesser degree of variation occurs in thelight chain recombination because there are no D regions;there are about 200 and about 160 different VJ combinations

Translation, processing

Mature heavy chain (IgM)

Primary RNA transcript

Similarly processed light chain

Figure 3-5 Genetic organization and recombination events Antibody diversity is generated by DNA recombination events thatrandomly fuse variable, diversity, and joining regions The recombination is accomplished in a defined order by enzymes RAG-1and RAG-2 The first events culminate in transcribing mRNA coding for immunoglobulin M and D; differential translationdetermines whether mature polypeptide will be one or the other RAG, recombinant activating genes; L, leader sequence;

V, variable; D, diversity; J, joining; C, constant region

Heavy and Light Chain Combinations

Because the heavy and light chain loci recombine

indepen-dently, each B cell will contain a different combination of

ran-domly assorted heavy and light chains

Junctional and Insertional Diversity

The recombination between V and D-J segments and between

D and J segments is not precise, often staggering the junctional

location between recombined segments by a few base pairs

This “sloppiness” causes differences in the amino acid

se-quence and leads to junctional diversity Insertional diversity

results from the activity of terminal deoxynucleotide

transfer-ase, an enzyme that is expressed during heavy chain

rearran-gement This enzyme adds nucleotides randomly at the V-D

and D-J junctions

AFFINITY MATURATION

Secretion of antibodies occurs only after antigenic stimulation

of membrane-anchored immunoglobulin on the B-cell surface

Differentiation into plasma cells usually occurs in germinal

centers of lymph nodes, the spleen, or mucosa-associated

lym-phoid tissue The process requires both antigenic recognition

and help given in the form of T-cell cytokines During

differ-entiation, two processes can occur to change the biologic

properties of the secreted antibody The first is class switching,

or isotype switching, in which deletion of intervening DNA

sequences occurs, allowing antibodies to switch from IgM

and IgD to another isotype (Fig 3-6)

Isotype switching results when antigen-stimulated B cellsreceive a cytokine signal from T helper cells The V regiondoes not change during isotype switching; therefore, the sameantigenic specificity is retained Switching involves the dele-tion of intervening DNA between specific recombination sitescalled switch regions Because the intervening DNA is lost,the B cell cannot “switch back” to an isotype that has alreadybeen deleted The V region and C regions are transcribed to-gether, and RNA splicing and translation results in expression

of the new isotype

The second process is termed affinity maturation, in whichgermline DNA is subject to mutational change, allowing codingfor antibodies with increased affinity for binding to antigen.This process is a result of somatic hypermutation in which V re-gions of the antibody heavy and light chain genes undergomore than 10,000 times higher rate of mutation than “regular”DNA Somatic hypermutation occurs only after antigen stimu-lation Some of these mutations increase the affinity of anti-body for antigen, and those B cells expressing antibody withhigher affinity will be selectively stimulated, increasing the pro-portion of high-affinity antibody in secondary responses

KEY POINTS ABOUT ANTIBODY DIVERSITY

n The V(D)J recombination that occurs during B-cell development, along with somatic mutation after antigenic stimulation, leads to the generation of antigen-binding diversity.

n Each individual B cell and all of its progeny express only one heavy chain and one light chain V region sequence; thus all have the same antigenic specificity.

IL-13 Influence of T-cell cytokines

Transcription

Translation,

processing

Mature heavy chain (IgG1)

Mature heavy chain (IgE)

Mature heavy chain (IgA2)

Figure 3-6 Isotype class switching During plasma cell differentiation, the antibody isotype may be changed where the samevariable region is recombined with a different constant region gene sequence Intervening DNA is excised, allowing generation ofmRNA for different isotypes Subsequent switching may occur to any downstream remaining coding sequence; once done,however, the change is irreversible INF, interferon; Ig, immunoglobulin; L, leader sequence; V, variable; D, diversity; J, joining;

C, constant region

Isotype switching and affinity maturation 23