Immunoglobulin M is one of the five major human immunoglobulin isotypes and is the first antibody to be secreted by plasmablasts in a humoral immune response after exposure to antigen. Structurally, IgM is the largest immunoglobulin and predominantly exists in pentameric form when secreted. Alternative splicing of the IgM heavy chain mRNA can generate an alternative form of the antibody, which facilitates its insertion into the plasma membrane of B cells to function in antigen recognition. IgM is the first Ig isotype to appear on the surface of developing B cells and is a major component of the B cell antigen receptor signaling complex, which drives B cell survival and proliferation upon antigen-induced ligation. Research studies have shown that defects in the assembly of the BCR account for the low levels of surface IgM expression in B-chronic lymphocytic leukemia.
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