Definition: 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 (? or ?), and one pair of heavy (H) chains (?, a, µ, ?, and e), usually all four linked by disulfide bonds. On the basis of the structural and antigenic properties of the H chains, immunoglobulins are classified (in order of relative amounts present 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 to amino acid sequence analysis. Each class of H chain can associate with either ? or ? L chains. Subclasses of immunoglobulin, 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 immunoglobulins, and all immunoglobulins probably function as antibodies. However, immunoglobulin refers not only to the usual antibodies, but also to a great number of pathologic proteins classified as myeloma proteins, which appear in multiple myeloma along with Bence Jones proteins, myeloma globulins, and immunoglobulin 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 (? or ?) are identical except for a single amino acid substitution, under genetic controls. H chains are similarly divided, although the VH region, although 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 “libraries” of antibodies of each individual.
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