CD40 is a surface receptor within the TNF receptor family expressed on В lymphocytes, dendritic cells, activated monocytes, endothelial cells, and epithelial cells. The ligand for CD40, CD40L, is expressed on activated T cells, and CD40L+ T cells provide the classic 4T cell help4 by inducing via CD40 В cell proliferation and differentiation. The CD40L/CD40 signaling pathway also plays an important role in regulating T cell activation by antigen-presenting cells. Engaging CD40 on antigen-presenting cells such as dendritic cells induces them to express more costimulatory receptors such as CD80 and CD86, to produce cytokines such as IL-12, and to effectively stimulate CDS effector T cells. Thus, the CD40 receptor plays a key role in the development of T cell-dependent responses to foreign antigens and pathogens.