(Ig), globular proteins present in the blood serum of vertebrates and man.

Immunoglobulins form a group of chemically similar carbohydrate-containing compounds. All immunoglobulins appear to be antibodies to certain antigens. There are five classes of human immunoglobulins: G, M, A, D, and E (see Table 1). The immunoglobulins of the G class (IgG) have received the most study. Their molecules are constructed of two identical light (molecular weight, 22,000) and two identical heavy (molecular weight, 55,000–70,000) polypeptide chains connected by disulfide bonds (see Figure 1). When split by proteolytic enzymes (for example, papain), the immunoglobulin molecule breaks down into three parts: two identical fragments, designated Fab, that remain capable of being bound to an antigen, and a fragment designated Fc that helps the immunoglobulins to cross biological membranes. All three fragments are connected by short, flexible sections in the middle of the heavy chain. Flexibility permits the immunoglobulin molecules to combine optimally with antigens possessing different spatial structure. The parts of the molecules responsible for combining with antigens (the active center) are formed by the N-terminal segments (carrying the amino group NH₂ at the end) of the heavy and light chains. The sequence of amino acids in these segments is specific to each IgG, and it scarcely varies in other parts of the chains. Immunoglobulins are classified according to the differences in structure of the heavy chains.

Figure 1. Diagram of a molecule of immunoglobulin G, showing two heavy and two light polypeptide chains connected by disulfide bonds. The light chain consists of two structural units (loops) formed by intrachain disulfide bonds; the heavy chain consists of four of these units. The thick lines denote the N-terminal segments of the chains. The arrow indicates the section sensitive to proteolysis, a process that results in the breakdown of the molecule into two Fab fragments, which retain antibody activity, and a third, Fc, fragment.

Most antibodies are of the IgG class (for example, the gamma globulins used for therapeutic purposes consist chiefly of IgG). The IgM are evolutionarily the oldest immunoglobulins and they are synthesized during the initial stages of the immune response. Their molecules are made up of five monomeric subunits, each

of which resembles the IgG molecule. The IgA are characteristically capable of penetrating various secretions (saliva, colostrum, intestinal juice), where they are found in polymeric form. The antibodies that participate in allergic reactions belong to the recently discovered IgE.

Immunoglobulins are synthesized by the lymphoid cells. After certain injuries to these cells, a large quantity of so-called myelomic immunoglobulins accumulates in blood and urine. Unlike the immunoglobulins of healthy persons, they are uniform in composition.