Structure of Antibodies

Each antibody molecule consists of two identical short polypeptides, called light chains, and two identical long polypeptides, called heavy chains . The four chains in an antibody molecule are held together by disulfide(—S—S—) bonds, forming a Y-shaped molecule . Comparing the amino acid sequences of different antibody molecules shows that the specificity of antibodies for antigens resides in the two arms of the Y, which have a variable amino acid sequence. The amino acid sequence of the polypeptides in the stem of the Y is constant within a given class of immunoglobulins. Most of the sequence

variation between antibodies of different specificity is found in the variable region of each arm. Here, a cleft forms that acts as the binding site for the antigen. Both arms always have exactly the same cleft and so bind to the same antigen. Antibodies with the same variable segments have identical clefts and therefore recognize the same antigen, but they may differ in the stem portions of the antibody molecule. The stem is formed by the so-called “constant” regions of the heavy chains. In mammals there are five different classes of heavy chain that form fiveclasses of immunoglobulins: IgM, IgG, IgA, IgD, and IgE. IgE antibodies bind to mast cells.

The heavy-chain stems of the IgE antibody molecules insert into receptors on the mast cell plasma membrane, in effect creating B receptors on the mast cell surface. When these cells encounter the specific antigen recognized by the arms of the antibody, they initiate the inflammatory response by releasing histamine. The resulting vasodilation and increased capillary permeability enable lymphocytes, macrophages, and complement proteins to more easily reach the site where the mast cell encountered the antigen.  IgA antibodies are present in secretions such as milk, mucus, and saliva. In milk, these antibodies are thought to

provide immune protection to nursing infants, whose own immune systems are not yet fully developed