Hapten-Ficoll conjugates induce T-cell-dependent IgM anti-hapten responses and T cells mediating hapten-specific delayed-type hypersensitivity.

It was generally believed until recently that the B-cell IgM response induced by type-II "thymus-independent" antigens, that are not mitogenic for B cells, was T cell independent. Neither has it been possible in the past to demonstrate that these antigens activate specific T cells. This has led to the general belief that type-II thymus-independent antigens have the anomalous property of not being able to activate such cells. Recent evidence has shown that the IgM response to hapten-Ficoll conjugates, the prototype of type-II thymus-independent antigens, is T cell dependent in at least some circumstances. Here evidence that these antigens activate T cells mediating hapten-specific delayed-type hypersensitivity (DTH) is presented. Furthermore, the conditions under which they do so are similar to those that allow thymus-dependent antigens to activate DTH-mediating T cells.