Immunoglobulins
Immunoglobulins comprise a class of structurally related proteins of the immunoglobulin superfamily with two types of paired polypeptide chains: light (L) of low molecular mass, and heavy (H) of high molecular mass. All four chains are bound by disulfide bonds. There are two types of the light chain constant regions designated as к and л. There are five main forms of the heavy chain constant regions designated as: м, г, д, б, and е. Each form is associated with a separate Ig class. Based on structural and antigenic features of their H-chains, Ig are divided (listed in order of their percentage in blood) into five classes: IgG (up to 85%), IgA (up to 15%), IgM (up to 10%), IgD (less 0.3%), IgE (less 0.003%) where a capital letter designates the immunoglobulin class. IgG, IgD and IgE molecules are present as monomers, IgM - as pentamers; IgA molecules in blood are monomers, and those in excreted liquids (lachrymal fluid, saliva and mucus) are dimers. Numerous possible combinations of L- and H-chains compose a diversity of antibodies in each person.
IgM synthesis is induced by primary antigen penetration in the organism. The synthesis reaches maximum on day 4-5, and the titer decreases afterwards. IgM is constantly being synthesized in response to certain antigens, e.g. bacterial filamental antigens. Most antibodies against gram-negative bacteria belong to the IgM class. Presence of IgM against a certain pathogenic antigen is indicative of an acute infectious process. IgM molecule is a pentamer: five subunits are linked together by disulfide bonds and J-chain (J-joining) as a result IgM molecule has 10 antigen-binding sites. IgM molecules opsonize, agglutinate, precipitate, lyse antigen-containing structures, and activate the complement system by classical pathway (one IgM molecule is capable of complement-mediated bacterial lysis).