IMMUNOGLOBULIN M: A POLYMERIC MOLECULE
Serum IgM is basically constituted by five subunits (monomeric subunits, IgMs), each of which is constituted by two light chains ( k or l ) and two heavy chains (μ). The heavy chains are larger than those of IgG by about 20,000 daltons, corresponding to an extra do-main on the constant region (Cμ4). A third polypeptide chain, the J chain, can be revealed by adequate methodology in IgM molecules. This is a small polypeptide chain of 15,000 daltons, also found in polymeric IgA molecules. One single J chain is found in any poly-meric IgM or IgA molecule, irrespective of how many monomeric subunits are involved in the polymerization. It has been postulated that this chain plays some role in the polymer-ization process.