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Immunoglobulin M(IgM),Immunology,Immunity,Immune system

Posted by Mumtaz khan Friday, 10 February 2012

Immunoglobulin M (IgM):      
    IgM accounts for 5%-10% of the total serum immunoglobulin,with an average serum concentration of 1.5 mg/ml.Monomeric IgM,with a molecular weight of 180,000, is expressed as membrane-bound antibody on B cells.IgM is secreted by plasma cells as a pentamer in which five monomer units are held together by disulfide bonds that link their domains.The five monomer subunits are arranged with their Fc regions in the center of the pentamer and the ten antigen-binding sites on the periphery of the molecule.Each pentamer contains an additional Fc-linked polypeptide called the J (joining)chain,which is disulfide-bonded to the carboxyl-terminal cysteine residue of two of the ten chains.The J chain appears to be required for polymerization of the monomers to form pentameric Igm;it is added just before secretion of the pentamer.
IgM is the first immunoglobulin class produced in a primary response to an antigen,and it is also the first immunoglobulin to be synthesised by the neonate.because of its pentameric structure with 10 antigen-binding sites,serum IgM has a higher valency than the other isotypes.An IgM molecule can bind 10 small hapten molecules;however,because of steric hindrance,only 5 or fewer molecules of larger antigens can be bound simultaneously.Because of its high valency,pentameric IgM is more efficient than other isotypes in binding antigens with many repeating epitopes such as viral particles and red blood cells(RBCs).For example,when RBCs are incubated with specific antibody,they clump together into large aggregates in a process called agglutination.It takes 100 to 1000 times more molecules of IgG than IgM to achievethe same level of agglutination.A similar phenomenon occurs with viral particles:less igM is required to neutralize viral infectivity.IgM is alsoefficient than IgG at activating complement.Complement activation requires two Fc regions in close proximity,and the pentameric structure of a single molecule of IgM fulfills this requirement.
         Because of its large size,IgM does not diffuse well and therefore is found in very low concentrations in the intercellular tissue fluids.The presence of J chain allows IgM  to bind to receptors on secretory cells,which transport it across epithelial linings to enter external secretions that bathe mucosal surfaces.Although IgA is the major isotype found in tese secretions,IgM plays an important accessory role as a secretory immunoglobulin.


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