Immune System Genetics

The antibody is formed from four polypeptides that link up in the shape of a Y. There are two identical long heavy (H) chains and two identical short light (L) chains. The tips of each branch of the Y form a pocket, and it is here the antibody binds antigen. Thus, these twin pockets are called the antigen-binding regions of the antibody.

By comparing the amino acid sequences of antibodies from different B cell clones, several important features can be discovered. Light chains, for instance, have a constant region, with amino acid sequences that differ little from clone to clone, and a variable region, with sequences that differ considerably. The constant region comes in two different forms, termed "kappa" and "lambda." The amino acid sequence of one kappa constant sequence differs little or not at all from clone to clone; similarly, all lambda constant sequences are essentially identical. The variable region does differ considerably between clones. The heavy chain also has a constant region (of which there are five forms) and a variable region.

The constant regions of all the chains are found toward the bottom of the Y, while the variable regions are found toward the tip. Furthermore, within each variable region, there are three hypervariable regions, whose five to ten amino acids differ even more than the other portions of the variable region. These hypervariable regions form the actual points of contact between antibody and antigen.