Antibodies

"Antibodies of the immunoglobulin G class are Y-shaped glycoproteins that circulate in the bloodstream. They bind to and inactivate foreign molecules—the antigens—and mark them for destruction. Each IgG molecule consists of two light chains and two heavy chains. The heavy chains have carbohydrates attached. The regions of the antibody that bind to antigens are located at the very tips of the two arms. Each arm of the antibody is composed of four domains. Two are called the variable domains, contributed by the heavy and light chains, and hence called VH and VL. The variable domains are attached to two constant domains, again one each from the heavy and light chains, and hence called CH and CL. Variable and constant domains share a similar structure, called the Ig fold. Each domain consists of a pair of beta sheets, one with three strands and one with five. A single covalent disulfide bridge holds the two sheets together, which results in a rigid and very stable domain. As their name implies, the variable domains vary in amino acid sequence from one antibody molecule to another, thus providing the vast diversity in structure required by the immune system. The antigen-binding site in the variable domains is composed of hypervariable loops that are especially susceptible to sequence variations. Sequence variations in the hypervariable loops are responsible for the specificity of antibodies to particular antigens. Antigens bind to the tip of each antibody arm, generally two molecules per antibody. In the example shown here, the antigen binds to the antibody via a large contact surface, providing a tight and highly specific association." Essential Cell Biology, Second Edition by Alberts, Bray, Hopkin, Johnson, Lewis, Raff, Roberts, Walter copyright 2004 by Garland Science Publishing