Following challenge with a thymus-dependent antigen T helper cells regulate B cell growth and differentiation in several ways. Initially, the T cells physically associate with antigen presenting B cells. While in conjugate, the two cells communicate with each other through the actions of cell surface receptors whose ligands are integral membrane proteins expressed on the surface of the apposed cell. The ensuing biochemical pathways regulate the expression of genes required for B cell cycle progression. As a consequence of this interaction, the T cells are induced to synthesize and secrete soluble mediators that also affect B cell proliferation, as well as determining the ultimate fate of the activated B cell. We also suggest a role for normal, Th cells during the development, and continued expansion of certain types of B cell lymphomas.