There are five major immunoglobulin (Ig) classes namely: 1. IgG 2. IgA 3. IgD 4. IgM 5. IgE.
Class # 1. IgG:
(a) It constitutes 75% of the total serum immunoglobulin in human.
(b) During the secondary immune response, it is the major Ig to be synthesized. Hence, it plays a vital role in the defense against infection.
(c) It is the only Ig class that can cross the placenta. Hence it is responsible for the protection of the neonate during the first few months of life.
(d) It diffuses readily into extra-vascular spaces and hence provides a major defense against bacterial toxins and other blood born infectious agents.
(e) Organisms coated with IgG attract macrophages via their FC region receptors thus enhancing phagocytosis.
(f) The complement binding site on the IgG molecule appears to be on the CH-II region.
(g) Ig is unable to bind onto the mast cells but has the ability to bind guinea-pig skin— the significance remains unclear.
(h) The property of Fc portion of IgG to bind to protein-A on the surface of Staphylococcus aureus has been greatly exploited for use in diagnosis and research.
Class # 2. IgA:
(a) It is actively secreted by mucosal associated lymphoid tissue (MALT).
(b) It appears selectively in sero-mucous secretions such as saliva, tears, nasal fluids and in the secretions of the lung and also in GI tracts and in UG system.
(c) It is present in fluids as a dimer stabilized against proteolysis by combination with another protein, the secretory compound (J-chain) which is synthesized by local epithelial cells and has a single peptide chain of MW. 60 kd.
(d) The IgA chain is synthesized locally by plasma cells and dimerized intra-cellularly before secretion with the help of J-chain.
(e) It is actively endocytosed and transported within the endocytic vacuole and the mucosal surface. Cleavage of the receptor releases the IgA, still attached to the part of receptor termed the secretory piece, into sero-mucous secretion.
(f) IgA is the most abundant in body secretions. It performs the role of defending the exposed external surfaces of the body against the attack of the microorganisms.
(g) IgA activates complements by the alternative pathway.
(h) It, the prime functional units of MALT, facilitates passage through epithelial cells and protects the secretory molecule from proteolytic degestion.
Class # 3. IgD:
(a) It is present in serum in trace amounts.
(b) Because of an extended hinge region it is relatively liable to degradation by proteolytic enzymes.
(c) The main functions of IgD has not yet been determined with IgM, it is found abundantly on the surface of B-lymphocytes. It has been suggested that they may operate as antigen receptors and in the control of lymphocytic activation and suppression.
Class # 4. IgM:
(a) It is the largest Ig in science and exists as a pentamer of the basic 4-chain subunit, held together by disulfide bond.
(b) A relatively small molecule, the J-chain participates in the polymerization of IgM via SH residue near the C-terminal.
(c) The heavy-chain of IgM are designated as μ-chains.
(d) Electron microscopic study reveals that it is shaped like a star but when it is touched to a bacterium, its antigen binding sites (Fab) are bound to the bacterial surface. This changes the appearance of IgM to a crab like form, and causes cross linking of the different antigenic determinants (epitopes) on the bacterial cell-surface by polyvalent IgM molecule.
(e) IgMs appear early in response to injection and because of their size are largely confined to their blood stream.
(f) They are an important defense mechanism against bacteria.
(g) The size and valency of IgM makes it a very effective, agglutinating and cytolytic agent.
(h) Since it does not cross the placenta its presence in blood vessels indicates active foetal injection.
(i) Since IgM response is short lived, its presence may be helpful in establishing an acute injection.
Class # 5. IgE:
(a) It is the present in very low concentration in serum.
(b) IgE antibody has a very high affinity for mast cells and binding occurs via Fc portion of the Ig molecule.
(c) On contact with specific antigens called allergens, the mast cells undergo degranulation with release of vasoactive amines (Histamine). This process is responsible for skin reaction in allergy for the symptoms of Hay fever and an extrinsic asthma.
(d) IgE also have the ability to attach to human skin where they probably bound to mast cells.
(e) It is found mainly in lining of the respiratory and GI tracts where they form constituents of MALT.
(f) The main physiological role of IgE appears to be the protection of external mucosal surface.
(g) Infection agents penetrating the IgA defenses, combined with specific IgE on the mast cell surface to trigger the release of vasoactive agents and other factors, chemotactic for granulocyte.
(h) It is possible that IgE acts in the way as a defense against helminth infection which is characterised by an extra-aggregated IgE response.
