One of a class of structurally related proteins, each consisting of two pairs of polypeptide chains, one pair of light (L) chains (<oil), and one pair of heavy (H) chains fr.a.ji.Band), usually all four linked together bydisulfide bonds.On the basis of the structural and antigenic properties of the H chains, Ig's are classified (in order of relative amountspresent in normal human serum) as IgG (7S in size, 80%), IgA (10-15%), IgM (19S, a pentamer of the basic unit,5-10%), IgD (less than 0.1%), and IgE (less than 0.01%).
All of these classes are homogeneous and susceptible toamino acid sequence analysis. Each class of H chain can associate with either*: or XL chains. Subclasses of Ig's,based on differences in the H chains, are referred to as lgG1, etc. When split by papain, IgG yields three pieces: the Fcpiece, 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 chainbound to the remainder of an H chain.
Antibodies are Ig's, and all Ig's probably function as antibodies. However, Ig refersnot 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 theamino acid sequences of Bence Jones proteins, it is known that all L chains are divided into a region of variablesequence (V.) and one of constant sequence (C ), each comprising about half the length of the L chain. The constantregions of all human L chains of the same type (k or X) are identical except for a single amino acid substitution, undergenetic controls.
H chains are similarly divided, although the VH region, while similar in length to the V region, is onlyone-third or one-fourth the length of the CH region. Binding sites are a combination of V. and V.. protein regions. The.