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(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.
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
|Table 1. Characteristics of the different classes of inmunoglobulins in a healthy person|
|Class||Molecular weight||Carbohydrate content (in percent)||Content in serum (in mg percent)|
|IgA .........||170,000 and up||7||140-420|
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.
REFERENCESHaurowitz, F. lmmunokhimiia i biosintez anlitel. Moscow, 1969. (Translated from English.)
Nezlin, R. S. Biokhimiia antitel. Moscow, 1966.
Porter, R. “Struktura antitel.” In Molekuly i kletki, issue 4. Moscow, 1969. (Translated from English.)
Kabat, E. A. Structural Concepts in Immunology and Immunochemistry. New York, 1968.
R. S. NEZLIN