Biology & Life Sciences Chapter 16 Homework Simple Immunoglobulin Contains Four Polypeptide Chains two Heavy

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CHAPTER

Adaptive Immunity

Chapter Outline

Overview of Adaptive Immunity (pp. 469–470)

Elements of Adaptive Immunity (pp. 470–485)

The Tissues and Organs of the Lymphatic System

Antigens

Preparation for an Adaptive Immune Response

B Lymphocytes (B Cells) and Antibodies

Immune Response Cytokines

Antibody Immune Responses (pp. 488–491)

Inducement of T-Dependent Antibody Immunity with Clonal Selection

Memory Cells and the Establishment of Immunological Memory

Types of Acquired Immunity (pp. 491–493)

Naturally Acquired Active Immunity

Chapter Summary

Overview of Adaptive Immunity (pp. 469–470)

Adaptive immunity is a vertebrate’s ability to recognize and then mount a defense against spe-

cific invaders and their products. Immunologists, scientists who study the immune system, are

continuing to refine our knowledge.

Chapter 16 Adaptive Immunity

Adaptive immunity has five distinct attributes:

2. Inducibility. Cells of adaptive immunity activate only in response to specific pathogens.

4. Unresponsiveness to self. As a rule, adaptive immunity does not act against normal body

cells.

5. Memory. An adaptive immune response has immunological memory and is able to generate

faster and more effective responses in subsequent encounters with a particular pathogen or

toxin.

These aspects of adaptive immunity involve the activities of lymphocytes, which are a type of

leukocyte (white blood cell) that acts against specific pathogens. Lymphocytes of humans form

in the red bone marrow where blood stem cells (hemopoietic stem cells) produce all types of

Elements of Adaptive Immunity (pp. 470–485)

The following sections present elements of adaptive immunity by describing the “stage” and in-

troducing the “characters” involved in adaptive immunity. These elements include the lymphatic

system, antigens, antibodies, chemical signals, and mediators.

The Tissues and Organs of the Lymphatic System

The lymphatic system is composed of lymphatic vessels, which conduct the flow of lymph. It

also includes lymphoid cells, tissues and organs that are directly involved in adaptive immunity.

The Lymphatic Vessels and the Flow of Lymph

Lymphatic vessels form a one-way system of vessels that carries lymph from local tissues and

returns it to the circulatory system. Lymph is a colorless, watery liquid similar in composition

Instructor’s Manual for Microbiology with Diseases by Taxonomy, 5e

Lymphoid Organs

Lymphocytes arise in the primary lymphoid organs of the bone marrow and thymus. Lympho-

cytes then migrate to secondary lymphoid organs and tissues, including lymph nodes, the

spleen, the tonsils, and mucosa-associated lymphoid tissue (MALT). Each lymph node has nu-

merous afferent (inbound) lymphatic vessels carrying in lymph and one or two efferent (out-

Antigens

Antigens are portions of cells, viruses, and molecules that the body recognizes as foreign.

Properties of Antigens

A molecule’s shape, size, and complexity determine its ability to provoke an immune response.

The three-dimensional shapes of regions of antigens that are recognized by the immune system

are the epitopes (or antigenic determinants). Effective antigen molecules are large, complex,

Types of Antigens

Although immunologists characterize antigens in various ways, one important way is to group

them according to their relationship to the body:

 Exogenous antigens include toxins and other secretions and components of microbial cell

walls, membranes, flagella, and pili.

Preparation for an Adaptive Immune Response

The Roles of the Major Histocompatibility Complex and Antigen-Presenting Cells

Antigens on the surface of cells known as major histocompatibility antigens are how the body

Chapter 16 Adaptive Immunity

Antigen Processing

Antigens must be processed before MHC proteins can display epitopes. Different processes are

involved in endogenous and exogenous antigen presentation.

 Processing Endogenous Antigens. Small amounts of proteins produced inside cells are

catabolized into small peptides. Peptides containing epitopes from endogenous antigens bind

 ANIMATIONS: Antigen Processing and Presentation: Overview; Antigen Processing and

Presentation: Steps, MHC

T Lymphocytes (T Cells)

T lymphocytes act against endogenous antigens, producing cell-mediated immune responses.

An adult’s red bone marrow produces T lymphocytes. Following maturation in the thymus, T

Specificity of the T Cell Receptor (TCR)

Each T cell has a specific TCR type, one of an estimated 109 possible different TCRs. Each T

cell has only one TCR, so it displays one specific epitope-binding site, which recognizes and

binds to a complementary shape. Therefore, unlike MHC, each TCR binds only one unique

epitope.

Types of T Lymphocytes

Immunologists recognize different types of T cells: cytotoxic T cells, helper T cells, and regula-

tory T cells.

 Cytotoxic T cells (Tc or CD8 cells) are distinguished by the presence of the CD8 cell-surface

glycoprotein along with their unique TCR. They directly kill infected cells, as well as abnor-

mal body cells such as cancer cells.

Instructor’s Manual for Microbiology with Diseases by Taxonomy, 5e

Clonal Deletion of T Cells

The huge number of possible TCRs results in some T cells with TCR complementary to autoan-

tigens. When self-tolerance is impaired, the result is an autoimmune disease. Cells with recep-

tors that respond to autoantigens are selectively eliminated via apoptosis in a process known as

 Video Tutor: Clonal Deletion

B Lymphocytes (B Cells) and Antibodies

B lymphocytes are found in the spleen, MALT, and primary follicles of lymph nodes. The major

function of B cells is to secrete soluble antibodies. The specificity of B cell function comes from

the membrane proteins called B cell receptors.

Specificity of the B Cell Receptor (BCR)

The surface of each B cell is covered with about 500,000 identical copies of the B cell recep-

tor (BCR) unique to that cell. A BCR is a type of immunoglobulin (Ig). Simple immunoglobulin

contains four polypeptide chains—two heavy chains and two light chains—linked with disulfide

Specificity and Antibody Structure

Antibodies are similar to BCRs in shape, but are secreted and lack most of the transmembrane

portion of BCRs. Antibodies carry the same specificity for an epitope as the BCR of the activat-

ed B cell. The arms, or Fab region (fragment, antigen-binding), of the antibody contain the anti-

gen-binding sites. The antibody stem is called the Fc region (fragment, crystallizable). There are

five basic types of stems, and these form the five classes of antibodies: IgM, IgG, IgA, IgE, and

IgD.

Chapter 16 Adaptive Immunity

Antibody Function

The binding of antibody to epitope is the central functional feature of antibody-mediated adap-

tive immune responses. Once bound, they function in several ways. These include:

 Activation of Complement and Inflammation. Stems of antibody molecules can bind and

activate complement protein 1 (C1), triggering the complement cascade. IgE antibody stems

Classes of Antibodies

The class of antibody involved in any given antibody immune response depends on the type of

invading foreign antigen, the portal of entry, and the antibody function required:

 Immunoglobulin M (IgM) is the first antibody secreted during the initial stages of an im-

mune response. It is secreted as a pentamer linked by disulfide bonds and a short polypeptide

joining (J) chain. It is most efficient at complement activation, and can neutralize and agglu-

tinate as well.

Clonal Deletion of B Cells

Clonal deletion of B cells occurs in the bone marrow in a manner similar to that in T cells,

though self-reactive B cells may be able to change their BCR. Tolerant B cells leave the bone

marrow, undergo further maturation in the spleen, and then travel in the blood and lymph.

Instructor’s Manual for Microbiology with Diseases by Taxonomy, 5e

Immune Response Cytokines

Cytokines are soluble regulatory proteins that act as intercellular signals when released by cer-

tain body cells such as kidney cells, skin cells, and cells of the immune system. Many cells re-

lease multiple cytokines, some of which are redundant, producing a web of interactions called a

Cell-Mediated Immune Responses (pp. 485–488)

The body uses cell-mediated immune responses to endogenous antigens to fight intracellular

pathogens such as viruses that have invaded body cells, as well as abnormal body cells such as

cancer cells.

Activation of Cytotoxic T Cell Clones and Their Functions

An adaptive immune response is initiated in lymphoid organs where antigen-presenting cells in-

teract with lymphocytes. It occurs through the following series of steps:

1. Antigen presentation. An infected dendritic cell displays endogenous epitope on MHC I and

The Perforin-Granzyme Cytotoxic Pathway

Infected cells display epitope on MHC I, which can be recognized by activated cytotoxic T

cells. In the perforin-granzyme pathway, cytotoxic T cells destroy their targeted cells by secret-

ing perforins and granzymes, toxic protein molecules. Perforins perforate cytoplasmic mem-

Chapter 16 Adaptive Immunity

The CD95 Cytotoxic Pathway

In the CD95/Fas cytotoxic pathway, cytotoxic T cells contact their target cells with their TCR,

and their CD95L protein binds with CD95 on the target cell, thereby activating enzymes that

trigger apoptosis.

Memory T Cells

Some activated T cells become memory T cells and can persist for months or even years in

T Cell Regulation

Antibody Immune Responses (pp. 488–491)

Inducement of T-Dependent Antibody Immunity with Clonal Selection

Activation of T-dependent antibody immunity requires the assistance of type 2 helper T (Th2)

cells. A T-dependent antibody immune response involves the following series of interactions

among antigen-presenting cells, helper T cells, and B cells, which are mediated by cytokines:

1. Antigen presentation for Th activation and cloning. APCs present exogenous epitopes on

3. Activation of B cell. The Th2 cell interacts with B cells displaying epitope on MHC II and

4. The activated B cell proliferates rapidly and the clones differentiate into either memory B

cells or plasma cells.

Plasma cells initially secrete IgM but through class switching secrete IgG, and may switch

Memory Cells and the Establishment of Immunological Memory

A small percentage of the cells produced by B cell proliferation do not secrete antibodies but

survive as memory B cells, long-lived cells with BCRs complementary to the specific antigen

that triggered their production. In a primary immune response, it can take days for relatively

small amounts of antibodies to be produced. Memory B and T cells can survive in lymphoid tis-

sue for years and become active again when there is another exposure to antigen. When an anti-

Instructor’s Manual for Microbiology with Diseases by Taxonomy, 5e

Types of Acquired Immunity (pp. 491–493)

Immunologists categorize specific immunity as either naturally or artificially acquired, and as

either active or passive.

Naturally Acquired Active Immunity

Naturally acquired active immunity occurs when the body responds to exposure to antigens

by mounting specific immune responses. This results in immunological memory.

Naturally Acquired Passive Immunity

Naturally acquired passive immunity occurs when a fetus, newborn, or child receives mater-

nal antibodies across the placenta (IgG) or in breast milk (IgA).

Artificially Acquired Active Immunity

Artificially Acquired Passive Immunotherapy

Artificially acquired passive immunotherapy occurs when the body receives, via injection,

preformed antibodies in antitoxins or antisera, which can destroy fast-acting and potentially fa-