Monomeric IgM (Immunoglobulin M) is found on the surface of B cells whereas the IgM molecules secreted from plasma B cells are pentamers, i.e. five IgM molecules joined together by the J chain. IgM constitutes approximately 10% of the total Ig concentration in serum. IgM is the first antibody isotype induced in a typical immune response and hence is the isotype with the lowest affinity for antigens. Due to its structure, with 10 antigen-binding sites per IgM pentamer, it can nevertheless bind e.g. surface-bound antigens with a very high avidity.
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