Immunoglobulin M: fixation of human complement by the Fc fragment.

The Fc fragment [(Fc)(5)micro with a molecular weight of 342,000] of human immunoglobulin M from patients with Waldenström's macroglobulinemia has a complement fixing ability approximately 19 times greater on a molar basis than that of the parent immunoglobulin M. This fragment, which occurs naturally as a pentamer fixes human but not guinea pig complement, and this activity remains unchanged even after the fragment is reduced to the monomeric form.