Living with lymphocytes.

One of the most important topics in contemporary research on specific immune responses is the delineation of the mechanisms regulating the excitation, proliferation, and differentiation of lymphocytes whose specific receptors have bound antigen. Specific B cell activation may proceed through two major pathways. One of these, referred to as factor-dependent B cell activation, involves the excitation of the cell through the cross linkage of its membrane immunoglobulin (Ig) and the stepwise action on these cells of a number of stimulatory factors which leads to their proliferation and subsequent differentiation into high-rate IgM secreting cells. The properties of one of these soluble factors, B cell stimulatory factor (BSF-1; formerly designated B cell growth factor) is described in detail. The means through which it acts, together with differentiation factors, to cause the cell to enter an IgM secretory state is also discussed.