Chapter Summary

• Animal defenses against pathogens are based on the body’s ability to distinguish between self and nonself.

• Innate (nonspecific) defenses are inherited mechanisms that protect the body from many kinds of pathogens. They typically act rapidly.

• Adaptive (specific) defenses respond to specific pathogens. They develop more slowly than innate defenses but are longer-lasting.

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• Innate defenses evolved before adaptive defenses, and in animals there is a common signaling pathway that sets innate defenses in motion. Review Figure 41.1

• Many defenses are implemented by cells and proteins carried in the blood plasma and lymph. Review Figure 41.2, Activity 41.1

• White blood cells fall into two broad groups. Phagocytes engulf pathogens by phagocytosis. Lymphocytes, which include B cells and T cells, participate in adaptive responses. Review Figure 41.3, Activity 41.2

• An animal’s innate defenses include physical barriers such as the skin, and competing resident microorganisms known as normal flora. Review Figure 41.4

• The complement system consists of more than 20 different antimicrobial proteins that act to alter membrane permeability and kill targeted cells.

• Circulating defensive cells, such as phagocytes and natural killer cells, eliminate invaders.

• Inflammation involves activation of several types of cells and proteins that act against invading pathogens. Mast cells release histamine, which causes blood vessels to dilate and become “leaky.” Review Figure 41.5, Activity 41.2

• The adaptive immune response recognizes specific antigens, responds to an enormous diversity of antigenic determinants (epitopes), distinguishes self from nonself, and remembers the antigens it has encountered. Review Animation 41.1

• Each antibody and each T cell is specific for a single antigenic determinant. T cell receptors and antibodies bind to antigens and initiate an adaptive immune response.

• The humoral immune response is directed against pathogens in the blood, lymph, and tissue fluids. The cellular immune response is directed against an antigen established within a host cell. Both responses are mediated by antigenic fragments being presented on a cell surface. Review Focus: Key Figure 41.6

• Clonal selection accounts for the specificity and diversity of the immune response and for immunological memory. Review Figure 41.7

• An activated B or T lymphocyte produces effector cells that attack the antigen, and memory cells that are long-lived and rarely divide. Effector B cells are called plasma cells and secrete specific antibodies.

• Vaccination is inoculation with modified pathogens or antigens that provoke an immune response but are not pathogenic. Review Investigating Life: What Are the Mechanisms and Implications of Long-Lasting Immunity?

• B cells are the basis of the humoral immune response. Unexposed B cells are activated by binding of antigen and by stimulation by T_H cells with the same specificity, and then form plasma cells. These cells synthesize and secrete specific antibodies.

• An antibody is an immunoglobulin, a tetramer of four polypeptides: two identical light chains and two identical heavy chains, each consisting of a constant region and a variable region. Review Figure 41.8, Activity 41.4

• The variable regions determine the specificity of an immunoglobulin, and the constant regions of the heavy chain determine its class. There are five classes of immunoglobulins with different body locations and functions. Review Table 41.2

• B cell genomes undergo random recombination of genes coding for regions of the immunoglobulin polypeptide chains so that each cell can produce a specific antibody protein. The immunoglobulin chains derive from “supergenes” that are constructed from different combinations of V, D, J, and C genes. This DNA rearrangement and rejoining yields millions of different immunoglobulin chains. Review Figures 41.9, 41.10, Animation 41.3

• Once a B cell becomes a plasma cell, it may undergo class switching, in which a deletion of one or more constant region genes results in the production of an immunoglobulin with a different constant region and a different function.

• T cells are the effectors of the cellular immune response. T cell receptors are somewhat similar in structure to the immunoglobulins, having variable and constant regions. Review Figure 41.11

• The genes of the major histocompatibility complex (MHC) encode membrane proteins that bind antigenic fragments and present them to T cells. Review Figures 41.12, 41.13

• There are three types of T cells. Cytotoxic T cells (T_C cells) recognize and kill virus-infected or mutated cells. T-helper cells (T_H cells) assist both the cellular and humoral immune responses. Regulatory T cells (Tregs) inhibit the other T cells from mounting an immune response to self antigens. Review Figure 41.14

• An allergic reaction is an inappropriate immune response caused by immediate hypersensitivity or delayed hypersensitivity to certain antigens. Review Figure 41.15

• Autoimmune diseases result when the immune system produces B and T cells that attack self antigens.

• Immune deficiency disorders result from failure of some part of the immune system. Acquired immune deficiency syndrome (AIDS) is a disorder that arises from depletion of the T_H cells as a result of infection with human immunodeficiency virus (HIV). Review Figure 41.16