The process that immunologists rather casually refer to as" T cell help" is a complex series of interactions in which helper T cells (Tu) regulate virtually every aspect of the antigen-dependent response of B cells. Despite many years of cumulative evidence that TH cells are required for the activation, clonal expansion, differentiation and isotype regulation of B cells which respond to most antigens, many basic questions remain as to the specific regulatory signals involved. The development of clonal TH cell lines and the recent rapid progress in cloning the genes encoding many lymphokines have made it possible to begin to answer these questions.

In this review, we summarize our efforts to define the specific TH products or lymphokines which transmit regulatory signals to B cells, and to clarify the individual events influenced by these signals. The particular focus of this work is on lymphokines that regulate the expression of dilTerent immunoglobulin (Ig) isotypes produced by a stimulated B cell population. What has emerged from this effort is the realization that isotype expression is regulated by many of the same molecules that control the activation, growth and differentiation of B cells.