IgM in serum is composed of a pentamer of the basic 4 chain structure held together by inter H chain disulfide bonds. (Each basic unit has the same amino acid sequence as the others in the pentamer.) Thus, it has the ability to bind to 10 antigen at the same time. In addition, its heavy chain () is larger than the chain because it has four constant domains. Its molecular weight is about 65,000 daltons. Thus, the molecular weight of the monomeric form of IgM is about 180,000 daltons.

Polymerization is initiated by and the resulting pentamer is further stabilized by another molecule, the joining chain or J chain. J chain is a peptide of about 15,000 daltons which is rich in cysteines that are available to form disulfide bonds. IgM is so large it is normally confined to the intravascular space. One of the benefits of the inflammatory response is it lets IgM escape into the tissue spaces at the site where it may be needed to bind potential pathogens.