This is indicated by a swell-like elevation in the gamma zone (Figure 2.4b). The most common causes of polyclonal hyperglobulinemia detected by electrophoresis are severe infection, chronic liver disease, rheumatoid arthritis, systemic lupus erythromatosus and connective tissue diseases. The immunoglobulin rise in a selective manner to aid clinical diagnosis, for example,there will be rise in IgM production in infant, due to first antigenic challenges that an infant is exposed to after birth, but when there is intrauterine infection, the IgM of cord blood measured before birth is usually high, thus helping to diagnose toxoplasmosis, rubella, cytomegalovirus, herpes Figure 2.4: Serum protein electrophoresis patterns of hypergammaglobulinemia (emedicine.medscape.com) (A) Monoclonal pattern of serum protein electrophoresis. (B) Polyclonal pattern of serum protein electrophoresis simplex and syphilis. Also, different immunoglobulin responds in a different way during diseased state. In liver disease, high IgM is foundin primary biliary cirrhosis while high IgG is found in chronic hepatitis. IgA is high in cirrhosis regardless of the cause. There is an increase in IgE level in asthmatic and allergic conditions (Baron et al., 1989).2.3 IMMUNOGLOBULINSThe immunoglobulins belong to the immunoglobulin super-family (IgSF) (Torres et al., 2008). They consist of two heavy (H) and two light (L) chains (Figure 2.5). The L chain can consist of either a ? or a ? chain. Each component chain contains one NH2-terminal “variable” (V) IgSF domain and one or more COOH-terminal “constant” (C) IgSF domains, each of which consists of two sandwiched ? pleated sheets ‘pinned’ together by a disulphide bridge between two conserved cysteine residues (Williams & Barclay, 1988). The variable domain binds antigens while the constant domains specify effector functions such as activation of complement or binding to Fc receptors. The variable domains are created by means of a complex series of gene rearrangement events, and can then be subjected to somatic hypermutation after exposure to antigen to allow affinity maturation. Each variable domain can be split into three regions of sequence variability termed complementarity—determining regions (CDRs) and four regions of relatively constant sequence termed the framework regions. The three CDRs of the heavy chain are paired with the three CDRs of the light chain to form the antigen-binding site. The constant domains of the heavy chain can be switched to allow altered effector function while maintaining antigen specificity. There are five main classes of heavy chain constant domains. Each class defines the IgM, IgG, IgA, IgD and IgE isotypes. IgG can be split into four subclasses, IgG1,Figure 2.5: The basic structure of Immunoglobulin (Schroeder & Cavacini, 2010). IgG2, IgG3 and IgG4, each having its own biologic properties. IgA can also be split into IgA1 and IgA2 (Schroeder & Cavacini, 2010).The aim of these immunoglobulins is to neutralize foreign antigen through their lytic action which is accomplished by the involvement of combine effect with phagocytic cells and complement system.2.3.1 Immunoglobulin MImmunoglobulin M (IgM) is the first immunoglobulin expressed during B-cell development. Naive B cells express monomeric IgM on their surface and associate with CD79a and CD79b, polypeptide chains that participate in IgM cell signaling. On maturation and antigenic stimulation, multimeric (usually pentameric and rarely hexameric) IgM, in which single IgM units link to each other by disulfide bonds in the CH4 region, is secreted. The pentamer also contains a polypeptide chain, the J-chain, which is bound to two of the monomers by means of a disulfide bond. The J-chain facilitates secretion at mucosal surfaces. Generally, although monomeric IgM molecules have low affinity becauseof their immaturity, high avidity can be attained by means of multimeric interactions between the pentameric secreted antibody, and the antigen, especially if that antigen contains multiple repeating epitopes itself. IgM functions by opsonizing (coating) antigen for destruction and fixing complement. The pentameric nature of the antibody renders it very efficient in this process (Schroeder & Cavacini, 2010).IgM antibodies are associated with a primary immune response and are frequently used to diagnose acute exposure to an immunogen or pathogen. Given that IgM is expressed early in B-cell development, the m heavy chain associates with VH and VL regions that have not undergone much somatic mutation in response to antigen. As a result, IgM antibodies tend to be more polyreactive than other isotypes, which allows IgM-bearing B cells to respond quickly to a variety of antigens. These relatively low-affinity IgM antibodies are also called natural antibodies. Some of these natural antibodies not only participate as a first line of defense but also play a role in immunoregulation (Boes, 2000).
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