Prescott's Microbiology,4/e

		Microbiology, 4/e Prescott, Harley, Klein
		Instructor Resources

30 The Immune Response: Antigens and Antibodies

CHAPTER OVERVIEW

This chapter introduces specific immunity, the defense mechanism that involves the recognition of infectious agents, their products, some tumor cells, and certain macromolecules as foreign materials. The body must produce specific defensive responses that will destroy or neutralize these materials. This complex system involves several different types of responses and may lead to several different problems of various system malfunctions. In this chapter, some aspects of the immune system are introduced; other aspects are discussed in subsequent chapters.

CHAPTER OBJECTIVES

After reading this chapter you should be able to:

! explain the differences between specific and nonspecific immunity, artificial and natural immunity, and active and passive immunity

! describe the types of lymphocytes involved in immune responses and the different ways they respond to foreign substances

! describe the basic structure of antibody molecules and the different classes (isotypes) of these molecules

! describe the specificity and diversity of antibody molecules

! discuss the use of hybridomas to produce highly specific antibody molecules

! discuss the production and potential uses of catalytic antibodies

CHAPTER OUTLINE

I. Introduction

A. ImmunityCthe general ability of a host to resist a particular disease

B. Immune responseCa specific and complex series of events throughout the animal=s body that helps it defend against disease-causing organisms or substances

C. ImmunologyCthe science that deals with these immune responses

D. ImmunobiologyCa more current term used to describe the biological basis for host defenses, growth and development, hypersensitivity, heredity, aging, cancer and transplantation

II. Nonspecific ResistanceCgeneral, physical, chemical, and biological barriers against disease (discussed in chapter 29); also referred to as innate or natural immunity

III. Specific ImmunityCcertain cells (lymphocytes) recognize the presence of foreign substances (antigens or immunogens) and act to eliminate them either by direct action or by producing specialized proteins (antibodies) that destroy the antigen or target it for destruction by other cells; also referred to as acquired or adaptive immunity

A. Naturally acquired active immunityCan individual comes in contact with an antigen and produces sensitized lymphocytes and/or antibodies that inactivate the antigen

B. Naturally acquired passive immunityCtransfer (e.g. transplacentally or in breast milk) of antibodies from one individual (where they were actively produced) to another (where they are passively received)

C. Artificially acquired active immunityCdeliberate exposure of an individual to a vaccine (an inactivated or weakened form of the antigen) with subsequent development of an immune response

D. Artificially acquired passive immunityCdeliberate introduction of antibodies (produced elsewhere) into an individual

IV. Origin of Lymphocytes

A. Derived from bone marrow stem cells

B. May differentiate in thymus to become T lymphocytes (occurs during early childhood)

C. May differentiate in fetal liver or adult bone marrow to become B lymphocytes (continues throughout life)

D. Natural killer (NK) cells are probably derived from prethymic lymphocytes but do not have the characteristics of either B-cells or T-cells

V. Function of Lymphocytes

A. B cells

1. When exposed to antigen, they become plasma cells and secrete antibodies

2. This is referred to as humoral immunity or antibody-mediated immunity

3. Humoral immunity defends against bacteria, toxins, and viruses in body fluids

B. T cells

1. When exposed to antigens, they attack infected cells and release chemical mediators (cytokines) that augment the body=s defenses

2. Must be in close contact with foreign or infected cells

3. This is referred to as cell-mediated immunity

4. Cell-mediated immunity defends against

a. Host cells parasitized by a microorganism

b. Tissue cells transplanted from one host to another

c. Cancer cells

C. Natural killer (NK) cells

1. Nonspecifically kill tumor cells, virus-infected cells, and other parasite-infected cells

2. Play a role in regulating the immune response

3. Exhibit antibody dependent cytotoxicity

VI. Antigens

A. Prior to birth, the immune system removes most T cells specific for self-recognition determinants

B. Antigens (immunogens) are foreign substances, such as proteins, nucleoproteins, polysaccharides, and some glycolipids, to which lymphocytes may respond

C. Epitopes (antigenic determinant sites) are areas of an antigen that can stimulate production of specific antibodies and that can combine with them

D. ValenceCthe number of epitopes on an antigen; determines number of antibody molecules an antigen can combine with at one time

E. HaptenCa small organic molecule that is not itself antigenic but that may become antigenic when bound to a larger carrier molecule

VII. Antibodies (Immunoglobulins)Ca group of glycoproteins in the blood, serum, and tissue fluids of mammals; they are produced in response to an antigen and can combine specifically with that antigen

A. Immunoglobulin structure

1. Multiple antigen-combining sites (usually two; some can form multimeric antibodies with up to ten combining sites)

2. Composed of four polypeptide chains (two heavy or long, and two light or short) that form a flexible Y with a hinge region

3. The stalk of the Y (called the Fc) is constant in amino acid sequence (i.e., the amino acid sequences of antibodies of the same subclass do not vary significantly)

4. The arms of the Y have variable regions and constitute the antigen-binding domains (Fab)

5. Within the Fab segment are hypervariable (or complementarity determining) regions; these regions are responsible for the diversity of antibodies

6. There are five types of heavy chains that determine the five classes (isotypes) of immunoglobulins (IgG, IgA, IgM, IgD, and IgE)

7. In IgG there are four subclasses, and in IgA there are two subclasses

8. Categories of immunoglobulin types

a. IsotypesCvariations in the constant regions of heavy chains that are associated with different classes and subclasses

b. AllotypesCgenetically controlled allelic forms of the immunoglobulin molecule

c. IdiotypesCindividual-specific immunoglobulin molecules that differ in the hypervariable regions of the Fab segments

B. Immunoglobulin function

1. Fab region binds to antigen

2. Fc region mediates binding to host tissue, to various cells of the immune system, to some phagocytic cells, or to the first component of the complement system

3. Binding of antibody to an antigen does not destroy the antigen, but marks (targets) the antigen for immunological attack or attack by the nonspecific defense mechanisms that do destroy it

4. OpsonizationCcoating a bacterium with antibodies to stimulate phagocytosis

C. Immunoglobulin classes (Isotypes)

1. IgGCmonomeric protein, 70% to 75% of Ig pool

a. Antibacterial and antiviral

b. Enhances opsonization; neutralizes toxins

c. Only IgG is able to cross placenta (naturally acquired passive immunity for newborn)

d. Activates the complement system by the classical pathway

e. Four subclasses with some differences in function

2. IgMCpentameric protein, 10% of Ig pool

a. First antibody made during B-cell maturation

b. First antibody secreted into serum during primary antibody response

c. Never leaves the bloodstream

d. Activates complement by classical pathway

e. Enhances phagocytosis of target cells

f. Agglutinates bacteria and foreign red blood cells

g. Up to 5% may be hexameric which is better able to activate the complement system than pentameric IgM

3. IgACdimeric protein, 15% of Ig pool

a. Associated with secretory mucosal surfaces

b. Protects gastrointestinal tract, respiratory tract, and genitourinary tract

c. Also found in saliva, tears, and breast milk (protects nursing newborns)

d. Secretory form (sIgA) helps rid the body of antibody-antigen complexes by excretion into the gut lumen and subsequent excretion from the body

4. IgDCmonomeric protein, trace amounts in serum

a. Abundant on surface of B cells

b. May play a role in B-cell recognition of antigens

c. Does not activate the complement system

d. Cannot cross the placenta

5. IgECmonomeric protein, less than 1% of Ig pool

a. Skin-sensitizing and anaphylactic antibodies

b. When an antigen cross-links two molecules of IgE on the surface of a mast cell or basophil, it triggers release of histamine, and it increases intestinal motility, which helps to eliminate helminthic parasites

VIII. Diversity of AntibodiesCa number of mechanisms contribute to the generation of antibody diversity

A. Combinatorial joining

1. Ig genes are interrupted or split genes for segments of the variable regions of heavy and light chains

2. During differentiation of B cells, these genes are rearranged on the chromosome to form various combinations

3. The number of different antibodies possible is the product of the number of light chains possible and the number of heavy chains possible

B. Imprecise joiningCduring combinations, the same segments can be joined at different nucleotides, thus increasing the number of codons and the possible diversity

C. Somatic mutationsCthe V regions of germ-line DNA are susceptible to a high rate of somatic mutation during B-cell development

D. The total diversity produces more than 2x10⁸ different antibody molecules

IX. Specificity of AntibodiesCclonal selection theory

A. Because of combinatorial joining and somatic mutation, there are a small number of B cells capable of responding to any given antigen; each group of cells is derived asexually from a parent cell and is referred to as a clone; there is a large, diverse population of B-cell clones that collectively are capable of responding to many possible antigens

B. The surface receptor molecules of the B cells bind to the appropriate antigen

C. The cell is then stimulated to divide and differentiate into two populations of cells: plasma cells and memory cells

D. Plasma cells are protein factories that produce about 2,000 antibodies per second for their brief life span (5B7 days)

E. Memory cells, like the original B cells, can differentiate into plasma cells if they are stimulated by being bound to the antigen; because there are more memory cells than original B cells, the secondary (anamnestic) response can be (and usually is) faster and larger than the primary response; they have long life spans (years or decades)

X. Sources of Antibodies (pure homogeneous preparations)

A. Immunization

1. Procedure

a. Inject animal with antigen to stimulate primary immune response

(1) Initial lag phase of several days

(2) Log phase; antibody titer rises logarithmically

(3) Plateau phase; antibody titer stabilizes

(4) Decline phase; antibody titer decreases because the antibodies are metabolized or cleared from the circulation

(5) Mostly IgM; low-affinity antibodies

b. After a period of time, give animal a series of booster injections with same antigen to stimulate secondary immune response, or anamnestic response

(1) Shorter lag phase, higher antibody titer

(2) Mostly IgG; high-affinity antibodies (affinity maturation)

c. Withdraw blood from animal; allow it to clot; and remove fluid (serum), which is referred to as antiserum since it is from a specifically immunized host

2. Limitations

a. This method results in polyclonal antibodies which have different epitope specificities; thus sensitivity is lower, and the antibodies often cross-react with closely related antigens

b. Repeated injections with antiserum from one species into another can cause serious allergic reactions

c. Antiserum contains a mixture of antibodies, not all of which are of interest

B. HybridomasCovercome some of the limitations of antisera

1. Inject animals with antigen

2. Separate spleen cells (which contain plasma cells)

3. Fuse spleen cells with myeloma cells (tumor cells of the immune system that produce large quantities of antibodies and that are easy to culture)

4. Culture fused cells (hybridomas) so that each grows into a separate colony

5. Some plasma cells that fused with a myeloma cell will produce the desired antibody

6. Screen colonies for those producing desired antibody

7. Can grow many desired colonies to obtain large amounts of antibody

8. Antibodies produced by this method are monoclonal (react with only one epitope) since they come from the fusion of a single plasma cell with the tumor cell

9. Have a variety of uses in which high specificity is required

a. Tissue typing for transplants

b. Identification and epidemiological study of infectious microorganisms

c. Identification of tumor and other surface antigens

d. Classification of leukemias and T-cell populations

e. Sensitive diagnostic procedures

f. ImmunotoxinsCused in the targeted delivery of a toxic substance to a particular cell type; chemotherapeutic agents

XI. Catalytic Antibodies

A. They are monoclonal antibodies made by subjecting an animal to an enzyme-substrate transition-state analogue and then producing a hybridoma

B. The binding pocket on the antibody lowers the energy of activation of a reaction by ensuring the proper orientation of the reactant(s)

C. Currently, catalytic antibodies have been produced that can transform relatively simple compounds, but the potential is great if antibodies that act on proteins and nucleic acids can be produced

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