Since the original discovery of T-and B-cell collaboration in the antibody response, it has been generally accepted that the helper T cell represents a single subset in the multimember family of T cells. There are, however, some controversial findings on the nature of the help with respect to its specificity, class preference, and genetic restrictions. In the antibody response to a hapten.carrier conjugate, the helper T cell is believed to recognize the carrier determinant and then to help antibody synthesis by B cells which recognize another determinant (hapten) linked to the same molecule. This type of interaction is supported by a phenomenon known as the carrier effect, in which the hapten-specific secondary antibody response is successfully elicited by the hapten coupled to the same carrier by which animals have been primarily immunized (1-4). A similar cooperative interaction between hapten-specific B cells and carrier-specific T cells is readily demonstrable in the adoptive secondary antibody response (5-8). Since in these cases hapten and carrier determinants must be present on a same single molecule, the T-B collaboration should occur upon recognition of the hapten by B cells, and recognition of the adjacent carrier determinants by T cells in a cognate form.There are certainly no denials of the presence of this type of interaction. Nevertheless, there are several examples in which the cognate interaction is not likely to occur in certain T-cell-dependent antibody responses. In fact, various antigen-nonspecific factors derived from T cells can trigger the B-cell response, in which the factors themselves do not recognize carrier determinants (9-13). Several investigators are also aware that the hapten-specific B cells can be triggered under certain circumstances in which B and T cells are independently stimulated by corresponding determinants present on two distinctly separate molecules (8,12,14,15). Hence, cognate interaction is not the only pathway for the effective T-B cell collaboration. One could ask whether the same or different helper T cells are involved in these diverse pathways of T-B cell collaboration.Such problems are now even more complicated by the growing evidence suggesting that the helper T cell may recognize not only the carrier determinants, but also the products of major histocompatibility gene complex (MHC).

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