Immunoglobulins
Immunoglobulins comprise a class of structurally related proteins of immunoglobulin superfamily with two types of paired polypeptide chains namely: light (L) of low molecular mass, and heavy (H) of high molecular mass. All 4 chains are bound by disulfide bonds. There are two types of the light chain constant regions designated as к and X. There are five main forms of the heavy chain constant regions designated as follows: u, y, 8, а, and e. Each form is associated with 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 5 classes: IgG (up to 85%), IgA (up to 15%), IgM (up to 10%), IgD (less 0.3%), IgE (less 0.003%) where 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 in excreted liquids (lachrymal fluid, saliva and mucus) are dimers. Numerous possible combinations of L- and H-chains compose a diversity of AB in each person.
IgM synthesis is induced by primary Ag penetration in organism. The synthesis reaches maximum on 4-5th day and the titer decreases afterwards. IgM is constantly being synthesized in response to certain AG, e.g. bacterial filamental AG. Most AB against gram-negative bacteria belong to IgM class. Presence of IgM against a certain pathogenic Ag is indicative of an acute infectious process. IgM molecule is a pentamer: 5 subunits are joined by J-chain (J-joining) as a result IgM molecule acquires 10 AG-binding sites. IgM molecules opsonize, agglutinate, precipitate, lyse Ag containing structures, and activate complement system by classical pathway (one IgM molecule is capable of complement-mediated bacterial lysis).