Slowly sedimenting serum components reacting with anti-IgM sera.

Slowly sedimenting components with a sedimentation velocity of about 7S and reacting with specific antisera against IgM were found in six pathological human sera. Four of these came from patients with different forms of reactive macroglobulinaemia of infectious origin. Similar components were produced from purified IgM preparations by bacterial action, and to a lesser extent by spontaneous splitting at room temperature. The conditions for such a splitting were, however, absent for most of the sera investigated in this study. The possibility that slowly sedimenting IgM-like components are artefacts, is discussed.