Antibodies perform two important activities: the specific binding to an antigen, which is determined by the VH and VL domains; and participation in various biological effector functions, which is determined by the isotype of the heavy-chain constant domain. As described in Chapter 5, class switching allows any given VH domain to associate with the constant region of any isotype. This enables antibody specificity to remain constant while the biological effector activities of the molecule vary. A number of cytokines affect the decision of what Ig class is chosen when an IgM-bearing cell undergoes the class switch (Figure 11-19). The role of cytokines in class switching is explored further in Chapter 12.
As noted earlier, the humoral response to thymus-dependent antigens is marked by extensive class switching to isotypes other than IgM, whereas the antibody response to thy-mus-independent antigens is dominated by IgM. In the case of thymus-dependent antigens, membrane interaction between CD40 on the B cell and CD40L on the TH cell is essential for the induction of class switching. The importance of the CD40/ CD40L interaction is illustrated by the X-linked hyper-IgM syndrome, an immunodeficiency disorder in which TH cells fail to express CD40L. Patients with this disorder produce IgM but not other isotypes. Such patients fail to generate memory-cell populations, fail to form germinal centers, and their antibodies fail to undergo somatic hypermutation.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.