Antibody Structure
An antibody molecule consists of two identical light chains (220 amino acids each) and two identical heavy chains (about 440-450 amino acids each) held together by disulfide bridges; this constitutes the monomeric form of an antibody.

Enzyme papain cleaves each monomeric form into two fragments that bind to the antigen (designated as FAB; fragment with antigen binding) and one fragment, which does not bind to antigen but forms crystals (hence called Fc, crystal forming fragment).

Each antibody molecule has two antigen-binding sites or domains, each domain being constituted by the variable regions of one light and one heavy chain. The constant regions of the two heavy chains of an antibody molecule form its effector function domain, which determines its interaction with the other components of the immune system.

The light chains are of two types: kappa (K) and lambda (A.); the type of a light chain is determined by its constant region. An antibody molecule contains only either two kappa or two lambda light chains. Separate genes encode the kappa (located in human chromosome (2) and lambda (chromosome 22) light chains, and the heavy chain (chromosome 14).