Antibody
Structure
Immunoglobulin Molecules - There are five classes of immunoglobulin molecules, which carry out various functions in the immune system (Table 7.3). All are built from the same basic immunoglobulin pattern (Figure 7.32). Different kinds of antibodies may contain from one to five immunoglobulin molecules. When more than one is present, the monomers are linked by a second type of polypeptide, called a J chain (see Table 7.3).
Immunoglobulin Structure - Each immunoglobulin monomer consists of four chains, two heavy chains (M = 53,000 each) and two light chains (M = 23,000 each), held together by disulfide bonds. In each chain are constant domains (identical in all antibodies of a given class) and a variable domain (Figure 7.33). Variations in the amino acid sequence (and therefore the tertiary structure) of the variable domains of the light and heavy chains confer the multitudinous specificities of antigens to different determinants.
Precipitating Antigens - A large protein, a virus, or a bacterial cell has many different potential antigenic determinants on its surface that antibody molecules can bind, thereby precipitating the antigen (see Figure 7.29a). Antigens with only one determinant, will bind to an antibody, but not precipitate. Precipitation also requires the antibody to be bivalent (to have two binding sites). By careful proteolysis, it is possible to cleave antibodies at the hinge region (see Figure 7.32) to produce Fab fragments with only one binding site each. Such fragments will bind, but not precipitate antigen.
Antigen Binding Sites - The antigen binding site lies at the extreme end of the variable domains and involves amino acid residues from the variable regions of both heavy and light chains. The antigen and antibody surfaces fit together in a highly complementary fashion.
Signaling Macrophages - The constant domains of the heavy chains in the base of the Y-shaped immunoglobulin molecule hold the chains together and function as effectors, to signal macrophages in the circulatory system to attack particles or cells that have been labeled by antibody binding. Macrophages are large white blood cells that are specially adapted to engulf and digest foreign particles. Differences in heavy chains identify immunoglobulin types for delivery to different tissues or for secretion (see Table 7.3).