B

IgG2, y2; IgG3, y3; IgG4, y4; and IgM, p.. In human IgM and IgE antibodies, the C regions contain four tandem Ig domains (see Fig. 5-1). The C regions of IgG, IgA, and IgD contain only three Ig domains. These domains are generically designated CH domains and are numbered sequentially from amino terminus to carboxyl terminus (e.g., CH1, Ch2, and so on). In each isotype, these regions may be designated more specifically (e.g., Cy1, Cy2 in IgG).

Different isotypes and subtypes of antibodies perform different effector functions. The reason for this is that most of the effector functions of antibodies are mediated by the binding of heavy chain C regions to Fc receptors on different cells, such as phagocytes, NK cells, and mast cells, and to plasma proteins, such as complement proteins. Antibody isotypes and subtypes differ in their C regions and therefore in what they bind to and what effector functions they perform. The effector functions mediated by each antibody isotype are listed in Table 5-2 and are discussed in more detail later in this chapter and in Chapter 12.

Antibody molecules are flexible, permitting them to bind to different arrays of antigens. Every antibody contains at least two antigen-binding sites, each formed by a pair of VH and VL domains. Many Ig molecules can orient these binding sites so that two antigen molecules on a planar (e.g., cell) surface may be engaged at once (Fig. 5-7). This flexibility is conferred, in large part, by a hinge region located between CH1 and CH2 in certain isotypes. The hinge region varies in length from 10 to more than 60 amino acid residues in different isotypes. Portions of this sequence assume an unfolded and flexible conformation, permitting molecular motion between k-1 ( B-1

Widely spaced cell Closely spaced cell surface determinants surface determinants

FIGURE 5-7 Flexibility of antibody molecules. The two antigen-binding sites of an Ig monomer can simultaneously bind to two determinants separated by varying distances. In A, an Ig molecule is depicted binding to two widely spaced determinants on a cell surface, and in B, the same antibody is binding to two determinants that are close together. This flexibility is mainly due to the hinge regions located between the CH1 and CH2 domains, which permit independent movement of antigen-binding sites relative to the rest of the molecule.

FIGURE 5-7 Flexibility of antibody molecules. The two antigen-binding sites of an Ig monomer can simultaneously bind to two determinants separated by varying distances. In A, an Ig molecule is depicted binding to two widely spaced determinants on a cell surface, and in B, the same antibody is binding to two determinants that are close together. This flexibility is mainly due to the hinge regions located between the CH1 and CH2 domains, which permit independent movement of antigen-binding sites relative to the rest of the molecule.

the CH1 and CH2 domains. Some of the greatest differences between the constant regions of the IgG subclasses are concentrated in the hinge. This leads to different overall shapes of the IgG subtypes. In addition, some flexibility of antibody molecules is due to the ability of each VH domain to rotate with respect to the adjacent CH1 domain.

There are two classes, or isotypes, of light chains, called Kand X, that are distinguished by their carboxyl-terminal constant (C) regions. An antibody molecule has either two identical k light chains or two identical X light chains.

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