Immunoglobulins, or antibodies, are a complex, heterogeneous mixture of proteins that exhibit two fundamental types of structural variation. Subtle structural differences in their antigen combining sites, or variable regions, account for their unique antigen binding specificities (see Figure 4-3). Structural differences outside their antigen combining sites, in the so-called constant regions, correlate with the different effector functions mediated by antibodies, such as complement activation or binding to one or more of the antibody Fc receptors expressed on monocytes and granulocytes. The variable and constant regions of antibodies arise from distinct structural domains as illustrated here in an animated overview of the three-dimensional structure of IgG antibodies.

IgG antibodies are among one of the five major classes of immunoglobulins that also include IgA, IgD, IgE, and IgM antibodies (see Figure 5-15). Each antibody class is distinguished by certain effector functions and structural features including a unique heavy (H) chain isotype, designated a (IgA), d (IgD), e (IgE), g (IgG), or m (IgM). H chains of all isotypes associate with light (L) chains of two isotypes -- k and l. Thus, the basic H₂L₂ composition of an antibody can be specified in terms of its H and L isotypes; e.g., e₂k₂, (m₂l₂)₅, etc.

In some species, the immunoglobulin classes are further differentiated according to subclasses, adding another layer of complexity to antibody structure. In humans, for example, IgG antibodies comprise four IgG subclasses -- IgG1, IgG2, IgG3, and IgG4 (see Figure 5-16) -- while there are two IgA subclasses -- IgA1 and IgA2. Each subclass corresponds to a different heavy chain isotype, designated g1 (IgG1), g2 (IgG2), g3 (IgG3), g4 (IgG4), a1 (IgA1) or a2 (IgA2). In all, nine heavy chain genes in the human haploid genome account for the subclasses and classes of human immunoglobulins.

While some of the heterogeneity of antibodies derives from the different classes and subclasses, most of the heterogeneity stems from the highly polymorphic nature of their variable regions. Estimates of the total number of distinct antibody variable regions or antigen combining sites that an individual's B cells can theoretically produce range from 10 million upward. It is not surprising then that antibody responses to even simple antigens are generally very heterogeneous in terms of the numbers of different antibodies made, a bonus for the individual as far as immune protection goes but a hindrance to the immunologist trying to sort out and manipulate this intricate process. However, each different antibody is produced only by a single clone of B cells, the "biological key" for creating hybridomas producing monoclonal antibodies. The fine-specificity of antigen recognition by monoclonal antibodies coupled with the relative ease of producing them has resulted in widespread use of monoclonal antibodies in both research and medicine.

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