Immunoglobulin

Ig; one of a class of structurally related proteins, each consisting of two pairs of polypeptide chains, one pair of light (L) [low molecular weight] chains (κ or λ), and one pair of heavy (H) chains (γ,α,μ,δ or ε), all four linked together by disulfide bonds

Description
On the basis of the structural and antigenic properties of the H chains, Igs are classified (in order of relative amounts present in normal human serum) as IgG (7 S in size, 80%), IgA (10 to 15%), IgM (19 S, a pentamer of the basic unit, 5 to 10%), IgD (less than 0.1%), and IgE (less than 0.01%). Each class of H chain can associate with either κ or λ L chains. Subclasses of Igs, based on differences in the H chains, are referred to as IgG1, etc.

When split by papain, IgG yields three pieces: the Fc piece, consisting of the C-terminal portion of the H chains, with no antibody activity but capable of fixing complement, and crystallizable; and two identical Fab pieces, carrying the antigen-binding sites and each consisting of an L chain bound to the remainder of an H chain. Antibodies are Igs, and all Igs probably function as antibodies. However, Ig refers not only to the usual antibodies, but also to a great number of pathological proteins classified as myeloma proteins, which appear in multiple myeloma along with Bence Jones proteins, myeloma globulins, and Ig fragments. From the amino acid sequences of Bence Jones proteins, it is known that all L chains are divided into a region of variable sequence (VL) and one of constant sequence (CL), each comprising about half the length of the L chain. The constant regions of all human L chains of the same type (κ or λ) are identical except for a single amino acid substitution, under genetic control. H chains are similarly divided, although the VH region, while similar in length to the VL region, is only one-third or one-fourth the length of the CH region. Binding sites are a combination of VL and VH protein regions. The large number of possible combinations of L and H chains make up the comprehensive arsenal of antibodies of each individual.