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Secondary Lymphoid Organs

1. Secondary lymphoid organs are the sites at which lymphocytes gather to contact antigens; they facilitate the interactions and transfer of cytokines between the various cells of the immune system.

Primary Lymphoid Organs

1. Primary lymphoid organs are the sites where B cells and T cells mature.

16.3 The Nature of Antigens

1. Antigens are molecules that react specifically with an antibody or lymphocyte; immunogen refers specifically to an antigen that elicits an immune response.

2. The immune response is directed to antigenic determinants, or epitopes, on the antigen. (Figure 16.3)

416 Chapter 16 The Adaptive Immune Response

16.4 The Nature of Antibodies

Structure and Properties of Antibodies (Figure 16.4)

1. Antibody monomers have a Y shape with an antigen-binding site at the end of each arm of the Y. The tail of the Y is the Fc region.

2. The antibody monomer is composed of two identical heavy chains and two identical light chains; each chain forms several domains. The variable region contains the antigen-binding site; the constant region encompasses the entire Fc region as well as part of the Fab regions.

Protective Outcomes of Antibody-Antigen Binding (Figure 16.6)

1. Antibody-antigen binding results in neutralization, immobilization and prevention of adherence, agglutination and precipitation, opsonization, complement activation, and antibody-dependent cytotoxicity.

Immunoglobulin Classes (Table 16.1)

1. There are five major antibody classes, IgM, IgG, IgA, IgD, and IgE, and each has distinct functions.

16.5 Clonal Selection of Lymphocytes (Figure 16.8)

1. When antigen enters a secondary lymphoid organ, only the lymphocytes that specifically recognize the antigen will respond; the antigen receptor they carry on their surface governs this recognition.

2. Lymphocytes may be immature, naive, activated, effector, or memory cells.

16.6 B Lymphocytes and the Antibody Response

The Response to T-Dependent Antigens

1. B cells present antigen to effector T-helper cells for inspection. If an effector T-helper cell recognizes the antigen, it will deliver cytokines to the cell, initiating the process of clonal expansion, which ultimately forms plasma cells that produce antibody. (Figure 16.9)

2. Under the direction of effector T-helper cells, the expanding B-cell population will undergo affinity maturation and class switching, and form memory cells. (Figures 16.12,16.13)

3. In the primary response, a lag period occurs before antibodies can be detected; memory cells are responsible for the swift and effective secondary response, eliminating invaders before they cause noticeable harm. (Figure 16.11)

The Response to T-Independent Antigens

1. T-independent antigens include polysaccharides that have multiple identical evenly spaced epitopes and LPS. (Figure 16.14)

16.7 T Lymphocytes: Antigen Recognition and Response

1. The T-cell receptor recognizes antigen presented by major histocompatibility (MHC) molecules. (Figures 16.15,16.16)

2. T-cytotoxic cells are referred to as CD8 T cells; T-helper are referred to as CD4 T cells.

Functions of Effector T-Cytotoxic (CD8) Cells (Figure 16.18)

1. T-cytotoxic cells induce apoptosis in cells that produce proteins associated with danger; they also produce cytokines that allow neighboring cells to become more vigilant against intracellular invaders.

2. All nucleated cells present peptides from endogenous proteins in the groove of MHC class I molecules. (Figure 16.17)

Functions of Effector T-helper (CD4) Cells

1. T-helper cells respond to exogenous antigens, which are presented by MHC class II molecules.

2. Th1 cells judge antigen presented by macrophages; a responding Th1 cell activates that particular macrophage and secretes cytokines that help orchestrate the immune response. (Figure 16.19)

3. Th2 cells judge antigen presented by B cells; a responding Th2 cell activates that particular B cell and supports actions that enhance its effectiveness.

Activation of T Cells (Figure 16.20)

1. Naive T cells require supporting signals to become activated; upon activation the cell stimulates its own proliferation and then gains its effector functions.

2. Dendritic cells sample material in tissues and then travel to the secondary lymphoid organs to present the antigens to naive T cells. Those that detect molecules associated with danger produce co-stimulatory molecules and are able to activate both subsets of T cells.

3. Activated macrophages that produce co-stimulatory molecules can activate T-helper cells.

16.8 Natural Killer (NK) Cells

1. NK cells mediate antibody-dependent cellular cytotoxicity (ADCC).

2. NK cells kill host cells that are not bearing MHC class I molecules on their surface.

16.9 Lymphocyte Development

Generation of Diversity

1. Mechanisms used to generate the diversity of antigen specificity in lymphocytes include rearrangement of gene segments, imprecise joining of those segments, and combinatorial associations of heavy and light chains. (Figure 16.22)

Negative Selection of Self-Reactive B Cells

1. Negative selection occurs as B cells develop in the bone marrow; cells to which material binds to their B-cell receptor are induced to undergo apoptosis.

Review Questions

Positive and Negative Selection of Self-Reactive T Cells

1. Positive selection permits only those T cells that show moderate recognition of the MHC molecules to develop further. Negative selection also occurs.

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