Antibodies are part of a class of proteins called immunoglobulins. As one of the human body's most important defenses against disease, these naturally occurring proteins are produced by the immune system in response to antigens, which are substances that appear to be foreign to the human body.

Immunoglobulins are usually "Y" shaped and consist of two light chains and two heavy chains. There are two classes or isotypes of the light chain called kappa and lambda. The heavy chains have five different isotypes (gamma, alpha, delta, mu and epsilon), each with different biological functions.

The light and heavy chains are composed of two distinct domains: the variable region and the constant region. The variable region is located at the tip of the arms of the "Y" and serves as the antigen-binding site. Antibodies achieve their diversity through rearrangement of the genes in the Fab domain. These subtle changes amount to an infinite spectrum of antigen specificities.

Antibodies also vary in the constant region, resulting in one of five immunoglobulin isotypes— IgG, IgA, IgD, IgM or IgE. Through the variable and constant domains, antibodies can bind, neutralize and help eliminate pathogens and their toxins.

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