IgM antibodies induce the production of antibodies of the same specificity.

Injection in normal mice of IgM antibodies against sheep erythrocytes or dextran, in the form of an immune serum depleted og IgG, induces direct plaque-forming cells of the same specificity as the injected antibodies. The response is 10--70 times higher than the background plaque-forming cell titer of untreated control mice. Nanogram amounts of IgM induce a detectable response, and a ceiling is reached with a few hundred nanograms of monoclonal IgM. The inducing agent is not residual antigen: (i) treatment of the injected material and the recipients with dextranase abolishes the immunogenicity of dextran, but not the response to anti-dextran IgM; (ii) monoclonal IgM specific for sheep erythrocytes or trinitrophenyl likewise induces plaque-forming cells of the respective specificity, but variant hybridoma products (in which the light chain is that of the myeloma parent) are inactive. In normal mice, IgM-induced antibody responses were observed with antibodies to both thymus-dependent and thymus-independent antigens, but such could not be induced in athymic (nude) mice. Because the mechanism underlying this phenomenon would operate also in a normal immune response and, presumably, in the normal dynamic state of the immune system of unstimulated animals, a network regulation among the elements of the immune system itself is implied.