CD40 (protein)

CD40 is a costimulatory protein found on antigen presenting cells. CD40 binds to CD154 (CD40L) on T cells to activate them and produce a variety of downstream effects.

Specific effects on cells
In the macrophage, the primary signal for activation is IFN-γ from Th1 type CD4 T cells. The secondary signal is CD40L on the T cell which binds CD40 on the macrophage cell surface. As a result, the macrophage expresses more CD40 and TNF receptors on its surface which helps increase the level of activation. The activated macrophage can then destroy phagocytosed bacteria and produce more cytokines.

The B cell can present antigens to helper T cells. If the T cell recognizes the peptide presented by the B cell, the T cell synthesizes CD40L. The CD40L binds to the B cell's CD40 receptor which causes resting B cell activation. The T cell also produces IL-4 which directly binds to B cell receptors. As a result of this interaction, the B cell can undergo division, antibody isotype switching, and differentiation to plasma cells. The end result is a B cell which is able to mass produce specific antibodies against an antigenic target.