THE exceptional immunogenicity of major histocompatibility complex (MHC) antigens is well known but poorly understood. As there is evidence that MHC products are intimately involved in the presentation of antigen to T cells1,2 it seems reasonable to expect that antigenic variants of these ‘presenting structures’ should be very immunogenic for T cells. However, because MHC antigens on non-nucleated cells and in subcellular forms are weak T-cell immunogens and may even function as tolerogens3, it has been postulated that an additional signal from living antigenic cells is needed to stimulate the immune response4. The response of B cells to MHC antigens is far less well understood but again an additional signal from antigenic cells seems to be needed in some primary responses, as antigenic cell fragments are not immunogenic5. We report here that in studies of primary antibody responses to H–2-incompatible cells, immunogenicity is almost completely restricted to a small, novel subpopulation of cells found in the spleen and bone marrow. Furthermore, as the response we have studied is independent of T cells, these stimulating cells seem to be unique (perhaps specialised?) in their ability to present surface-borne antigen and the additional signal to responding B cells.
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