Concept 31.1 Animals Use Innate and Adaptive Mechanisms to Defend Themselves against Pathogens

• Animal defenses against pathogens are based on the body's ability to distinguish between self and nonself.
   
• Innate immunity is a set of nonspecific, inherited mechanisms that protect the body from many kinds of pathogens. These defenses typically act rapidly.
   
• Adaptive immunity is a set of specific mechanisms that respond to specific pathogens. These defenses develop more slowly than nonspecific defenses but are long lasting.
   
• Many of these defenses are implemented by cells and proteins carried in the circulatory and lymphatic systems. Review Figure 31.1 and ANIMATED TUTORIAL 31.1
   

Concept 31.2 Innate Defenses Are Nonspecific

• Innate defenses include physical barriers such as the skin, mucous membranes, and competing resident microorganisms. Review Figure 31.2
   
• Circulating defensive cells, such as phagocytes and natural killer cells, work to eliminate invaders.
   
• The complement system consists of more than 20 different antimicrobial proteins that act to alter membrane permeability and kill targeted cells.
   
• The inflammation response activates several types of cells and proteins that act against invading pathogens. Mast cells release histamine, which increases the permeability of blood vessels to aid in inflammation. Review Figure 31.3 and WEB ACTIVITY 31.1
   

Concept 31.3 The Adaptive Immune Response Is Specific

• The adaptive immune system recognizes specific antigens, responds to an enormous diversity of antigenic determinants (also called epitopes), distinguishes self from nonself, and remembers the antigens it has encountered.
   
• Clonal selection accounts for the specificity and diversity of the immune response and for immunological memory. Review Figure 31.5
   
• Adaptive immunity includes the humoral immune response, which involves antibody production by B cells, and the cellular immune response, mediated by T cells. Both require specific receptors to bind to each antigen. Review Figure 31.6
   

Concept 31.4 The Adaptive Humoral Immune Response Involves Specific Antibodies

• Go to ANIMATED TUTORIAL 31.2
   
• Naïve B cells are activated by binding of the antigen and by stimulation from TH cells with the same specificity, and then form plasma cells. These cells synthesize and secrete specific antibodies.
   
• The basic unit of an immunoglobulin is a tetramer of four polypeptides: two identical light chains and two identical heavy chains, each consisting of a constant region and a variable region. The variable regions determine the specificity of an immunoglobulin, and the constant regions of the heavy chain determine its class. Review Figure 31.7 and WEB ACTIVITY 31.2
   
• There are five classes of immunoglobulins, differing in function and in the type of heavy chain.
   
• B cell genomes undergo recombination events in which the genes that encode specific domains of the immunoglobulin variable regions are randomly selected from large clusters of genes. This DNA rearrangement yields millions of different immunoglobulin proteins. Review Figures 31.8 and 31.9 and ANIMATED TUTORIAL 31.3
   

Concept 31.5 The Adaptive Cellular Immune Response Involves T Cells and Their Receptors

• Go to ANIMATED TUTORIAL 31.4
   
• T cell receptors are somewhat similar in structure to the immunoglobulins, having variable and constant regions. Review Figure 31.10
   
• There are several types of T cells. Cytotoxic T cells (T_C) recognize and kill virus-infected cells or mutated cells. T-helper cells (T_H) direct both the cellular and humoral immune responses.
   
• The genes of the major histocompatibility complex (MHC) encode membrane proteins that bind antigenic fragments and present them to T cells. Review Figure 31.11
   
• Regulatory T cells (Tregs) inhibit the other T cells from mounting an immune response to self antigens. Review Figure 31.12
   
• Acquired immune deficiency syndrome (AIDS) arises from depletion of the TH cells as a result of infection with human immunodeficiency virus (HIV). Review Figure 31.13