T helper cell-b cell interaction: the roles of direct Th-B cell contact and cell-free mediators

The antibody responses of B cells to most protein antigens are dependent upon signals provided by T helper cells; the nature of the signals which mediate the interactions of these two cell populations is the subject of the current review. Experimental evidence will be summarized which supports the participation of both antigen-specific and nonspecific T cell signals in the triggering of B cell responses. Alternative models will be described in which highly specific T helper-dependent B cell activation can be mediated by either specific or nonspecific effector mechanisms.