Receptor involved in the costimulatory signal essential for T-lymphocyte proliferation and interleukin-2 production, by binding CD28 or CTLA-4. May play a critical role in the early events of T-cell activation and costimulation of naive T-cells, such as deciding between immunity and anergy that is made by T-cells within 24 hours after activation. Also involved in the regulation of B cells function, plays a role in regulating the level of IgG(1) produced. Upon CD40 engagement, activates NF-kappa-B signaling pathway via phospholipase C and protein kinase C activation (By similarity). Homodimer. Interacts with MARCH8 (By similarity). Interacts (via cytoplasmic domain) with PHB1 and PHB2; the interactions increases after priming with CD40 (By similarity). Polyubiquitinated; which is promoted by MARCH8 and results in endocytosis and lysosomal degradation.