Effects of IgM on the in Vivo and in Vitro Immune Response

The effect of sheep erythrocyte specific IgM on the in vitro and in vivo immune response to SRBC is described. Kinetic data resulting from separate injection of antibody and antigen suggest that the stimulatory effect is mediated by an antigen-antibody complex formed in the peripheral circulation, and subsequently trapped in the spleen. If an optimal antigen dose is chosen, then IgM given with it does not improve the response, thus supporting the view that IgM works by increasing the spleen concentration of antigen given at suboptimal doses. In vitro studies with normal spleen cells show that high concentrations of IgM inhibit the sheep erythrocyte response. In experiments performed in vivo with the thymectomized animals, IgM does not substitute for the T cell component. However, in vitro experiments with T cell deficient spleen cells do show a small improvement of the SRBC response when low doses of IgM are given. This IgM stimulated response is much lower than the response obtained when B spleen cells are mixed with sensitized T cells.