Pentameric structure ?: (A) IgM, (B) IgG, (C) IgA, (D) IgD
Answer is A. Ans. is 'a' i.e., Ig M IgM is a pentameric immunoglobulin with five, four - peptide subunits. o The subunits are joined together by J chain. IgM IgM is the earliest immunoglobulin class to be synthesized by the fetus, begining at 20 weeks of age. As it is not transpoed across the placenta, the presence of IgM in the fetus or newborn indicates intrauterine infection and its detection is useful in the diagnosis of congenital infections such as syphilis, rubella and toxoplasmosis. It normally circulates as pentamer. It is the oldest immunoglobulin class. It is the predominant antibody in primary antibody response. It is sho lived, disappearing earlier than IgG.. Hence, its demonstration in serum indicates recent infection. Treatment with 2-mercaptoethanol selectively destroys IgM without affecting IgG. This is a simple method for the differential estimation of IgG and IgM. The isohemagglutinins (blood group anti A, anti B) are usually IgM Being largely confined to the intravascular space, IgM is believed to be responsible for protection against blood invasion by microorganisms. Monomeric IgM is the major antibody receptor on the surface of B lymphocytes for antigen recognition. IgM is an impoant component of immune complexes in autoimmune diseases e.g. IgM antibodies against IgG in Rheumatoid ahritis.