Anatomy & Physiology

Anatomy & Physiology Study Guide glycoproteins are called MHC proteins. APCs (antigen-presenting cells) are involved in antigen stimulation. • Lymphocytes are not activated by lone antigens, but will respond to an antigen bound to either a Class I or a Class II MHC protein in a process called antigen recognition. • Class I MHC proteins are in all nucleated body cells. Class II MHC proteins are only in antigen- presenting cells (APCs) and lymphocytes. • Whether a T cell responds to antigens held in Class I or Class II MHC proteins depends on the structure of the T cell plasma membrane. T cell plasma membranes contain proteins called CD (cluster of differentiation) markers. CD3 markers are present on all T cells. CD8 markers are on cytotoxic and suppressor T cells. CD4 markers are on all helper T cells. • One type of CD8 cell responds quickly, giving rise to large numbers of cytotoxic T cells and memory T cells. The other type of CD8 cell responds more slowly, giving rise to small numbers of suppressor T cells. • Cytotoxic T cells seek out and destroy abnormal and infected cells, using three different methods, including the secretion of lymphotoxin. • Cell-mediated immunity (cellular immunity) results from the activation of CD8 T cells by antigens bound to Class I MHCs. When activated, most of these T cells divide to generate cytotoxic T cells and memory TC cells, which remain in reserve to guard against future such attacks. Suppressor T cells depress the responses of other T cells and B cells. • Helper, or CD4, T cells respond to antigens presented by Class II MHC proteins. When activated, helper T cells secrete lymphokines that aid in coordinating specific and nonspecific defenses and regulate cell-mediated and antibody-mediated immunity. B cells respond to antigens by producing specific antibodies: • B cells become sensitized when antibody molecules in their membranes bind antigens. The antigens are then displayed on the Class II MHC proteins of the B cells, which become activated by helper T cells activated by the same antigen. • An active B cell may differentiate into a plasma cell or produce daughter cells that differentiate into plasma cells and memory B cells. Antibodies are produced by plasma cells. • An antibody molecule consists of two parallel pairs of polypeptide chains containing constant and variable segments. • When antibody molecules bind to an antigen, they form an antigen-antibody complex. Effects that appear after binding include neutralization (antibody binding such that viruses or bacterial toxins cannot bind to body cells); precipitation (formation of an insoluble immune complex) and agglutination (formation of large complexes); opsonization (coating of pathogens with antibodies and complement proteins to enhance phagocytosis); stimulation of inflammation; and prevention of bacterial or viral adhesion. • The five classes of antibodies (immunoglobulins, Ig) in body fluids are (1) IgG, responsible for resistance against many viruses, bacteria, and bacterial toxins; (2) IgE, which releases chemicals that accelerate local inflammation; (3) IgD, located on the surfaces of B cells; (4)

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