Pentameric IgM structure

IgM exists as a monomer when located on the surface of B cells and as multimers in association with an additional polypeptide chain, the J chain, when secreted. The monomers are crosslinked by disulfide bonds to each other and to the J chain. IgM can also form hexamers that lack a J chain. 1 = light chain (variable region); 2 = light chain (constant region); 3 = heavy chain (variable region); 4, 5, 6, and 7 = heavy chain (constant regions 1, 2, 3, and 4 [CH1 - CH4]).