T helper cells.

B-cell proliferation and differentiation is controlled by T helper cells. Recent studies have determined that the expression of a novel, 39 kD, T-cell membrane protein is responsible for inducing T-cell-dependent B-cell activation. The receptor for this protein on the resting B cell is CD40. Once activated, B cells are induced to grow and differentiate by the elaboration of interleukin-4 and interleukin-5 from activated T cells. Together, T cell-B cell contact and soluble factors provide all the signals required for B-cell growth and differentiation.