Enhances all basic T-cell responses to a foreign antigen, namely proliferation, secretion of lymphokines, up-regulation of molecules that mediate cell-cell interaction, and effective help for antibody secretion by B-cells. Essential both for efficient interaction between T and B-cells and for normal antibody responses to T-cell dependent antigens. Does not up-regulate the production of interleukin-2, but superinduces the synthesis of interleukin-10. Prevents the apoptosis of pre-activated T-cells. Plays a critical role in CD40-mediated class switching of immunoglobin isotypes (By similarity). Homodimer; disulfide-linked. Strongly expressed in the spleen and lung. Lower expression seen in liver, kidney and testis. Expression on T-cells is drastically induced by phorbol myristate acetate (PMA) and Ca-ionophore or the engagement of CD3 and CD28. N-glycosylated.