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  against helminths Fig. 8.1 Effector functions of antibodies. Antibodies are produced by the activation of B lymphocytes by antigens and other signals (not shown). Antibodies of different heavy-chain classes (isotypes) perform different effector functions, as illustrated schematically in (A) and summarized in (B). (Some properties of antibodies are listed in Chapter 4, Fig. 4.3.) Ig, Immunoglobulin; NK, natural killer. CHAPTER 8 Effector Mechanisms of Humoral Immunity 161 have a half-life of about 3 weeks, much longer than that of other Ig isotypes and most other plasma proteins. This property of Fc regions of IgG has been exploited to increase the half-life of other proteins by coupling the proteins to an IgG Fc region (Fig. 8.3). One of several therapeutic agents based on this principle is the tumor necrosis factor (TNF) receptor–Fc fusion protein, which functions as an antagonist of TNF and is used to treat various inflammatory diseases. By coupling the extracellular domain of the

 protection to the newborn, and FcRn expressed on endothelial cells and phagocytes plays a special role in protecting IgG from intracellular catabolism, thereby prolonging its half-life in the blood (see Chapter 8). Principal effector functions Fc receptordependent phagocyte responses; complement activation; neonatal immunity (placental transfer) Complement activation B cell Immunity against helminths Mast cell degranulation (immediate hypersensitivity) Mucosal immunity (transport of IgA through epithelia) IgG subclasses (IgG1, IgG3) IgE IgA Isotype switching IgM Helper T cells: CD40L, cytokines Various IL-4 Cytokines produced in mucosal tissues, e.g., TGF-ß, BAFF, others Fig. 7.12 Immunoglobulin (Ig) heavy-chain isotype (class) switching. Antigen-stimulated B lymphocytes may differentiate into IgM antibody-secreting cells, or, under the influence of CD40 ligand (CD40L) and cytokines, some of the B cells may differentiate into cells that produce different Ig heavy-chain isotypes. The p