CD40, a member of the TNF receptor family, contains a cysteine-rich N-terminal domain and a Ser/Thr-rich region preceding the transmembrane domain. On B cells, signaling through CD40 induces cell growth and differentiation, mediates cell survival within the germinal center, and upregulates the expression of costimulatory and adhesion molecules, such as B7.1, B7.2, and ICAM-1. The interaction of CD40 on B cells and CD40L on activated CD4+ T cells is essential for immune functions, such as immuoglobulin class switching. Signal transduction through CD40 pathways involves interaction with proteins such as TRAFs (TRAF2, TRAF3, TRAF5, and TRAF6); Jak 3; and Tyr phosphorylation of proteins, such as Lyn, Syk, PI-3-kinase, STAT3, and STAT5. In TRAF2-deficient mice, CD40-mediated B cell proliferation and NFκB activation are defective. Ku70 and Ku80 associate with the membrane-proximal region of CD40 in human primary B cells and the engagement of CD40 leads translocation of Ku proteins to the nucleus. Thus, CD40 interacts with a variety of signal transducers which mediate its role in B cell survival, growth, differentiation, and immunoglobulin class switching.