is present on B cells and its main function apparently is the control of B-cell activation. B-cells create IgM antibodies as a first line of defense. Their large size gives them excellent binding avidity, and can pick up trace amounts of infection to mark for recognition by phagocytes. is primarily found in serum and due to its size, it cannot diffuse well, and is found in the interstitium only in very low quantities.
Interestingly, it is also the primary antibody against A and B antigens on red blood cells and responsible for the blood-clotting reaction during blood-transfusion from wrong donors.
Structure of IgM
() is constructed of five or six units (i.e. mostly as pentamers but also hexamers occur) which are each comprised of two heavy-chains (mu-chains) and two light chains, bound together by disulfide bonds and a so-called J-chain. A J-chain is usually, but not always, found in pentameric but not in the hexameric form. The J chain, which is added just before secretion of pentamer helps in the polymerization of the monomers.
It is not yet clear if pentamers without the J-chain fullfill different functions. Each monomer has two antigen binding sites, hence a pentameric has 10 binding domains for different antigens; however, not all of these sites can be occupied at the same time due to limitations in space.
is by far largest antibody in the human circulatory system. In its pentamer form has a molecular mass of approximately 900 kDa.