[Structure and biological properties of immunoglobulins and gamma-globulin preparations. I. Structure and function of...

Antibodies serve two different functions: they bind antigens and mediate a series of other functions party as a consequence of antigen binding. These functions are usually termed effector functions. This functional dualism is reflected in the structural arrangement of the immunoglobulin molecules, the basic structure of which consists of 4 polypeptide chains: 2 identical light chains (molecular weight 22 000--23 000) and 2 identical heavy chains (molecular weight 50 000--70 000). These polypeptide chains may be divided into compact globular regions, which are called domains and have different biological functions. The domains which are involved in antigen binding consist of the 110--120 aminoterminal amino acid residues of heavy and light chains. The amino acid sequence within these domains differs from one immunoglobulin molecule to another, and for this reason they are designated variable domains. The remaining domains are of substantially the same structure within groups of immunoglobulins and are therefore called constant domains. The structural variants within the constant domains of the heavy chains are termed classes and subclasses, and those of the light chains, types. The constant domains of the heavy chains mediate the effector functions of the immunoglobulin molecules. This structural division into a variable and a constant part allows the combination of a great number of different antigen binding specifities with a series of different effector functions.