Anatomy & Physiology

Anatomy & Physiology Study Guide chemical forces lock them together. Antibodies bind not to the entire antigen, but to specific portions of its exposed surface—regions called antigenic determinant sites . The specificity of the binding depends initially on the three-dimensional “fit” between the variable segments of the antibody molecule and the corresponding antigenic determinant sites. A complete antigen is an antigen with at least two antigenic determinant sites, one for each of the antigen binding sites on an antibody molecule. Exposure to a complete antigen can lead to B cell sensitization and subsequent immune response. Most environmental antigens have multiple antigenic determinant sites; entire microorganisms may have thousands. Haptens , or partial antigens, do not ordinarily cause B cell activation and antibody production. Body fluids have five classes of antibodies, or immunoglobulins (Igs): IgG, IgE, IgD, IgM, and IgA. The classes are determined by differences in the structure of the heavy-chain constant segments and do not affect the antibody’s specificity, which is defined by the antigen binding sites. Primary and Secondary Responses to Antigen Exposure The initial response to exposure to an antigen is called the primary response. It is not until a second exposure to the antigen does it trigger a more extensive and prolonged secondary response. During the primary response, the antibody titer, or level of antibody activity, in the plasma does not peak until one to two weeks after the initial exposure. If the individual is no longer exposed to the antigen, the antibody concentration declines. During the secondary response, antibody titers increase more rapidly and reach levels many times higher than they did in the primary response. Immunological competence is the ability to produce an immune response after exposure to an antigen. 21.9 Hormones of the Immune System Interleukins are the most diverse and essential chemical messengers in the immune system. Nearly 20 types of interleukins have been identified. Lymphocytes andmacrophages are the primary sources of interleukins. They have four primary functions: Increasing T Cell sensitivity to antigens exposed on macrophage membranes (heightened sensitivity accelerates the production of cytotoxic and regulatory T cells); stimulating B cell activity, plasma cell formation, and antibody production; enhancing nonspecific defenses, and moderating the immune response. Several cytokines coordinate immune defenses by adjusting the operations of phagocytic cells. These cytokines include factors that attract free microphages and macrophages and prevent their premature departure from the site of an injury. Colony-stimulating factors ( CSFs ) are produced by active T cells, cells of the monocyte–macrophage group, endothelial cells, and fibrocytes. CSFs stimulate the production of blood cells in bone marrow and lymphocytes in lymphoid tissues and organs.

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