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 (PubMed:7527824). 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). Interferes with the formation of CD86 clusters, and thus acts as a negative regulator of T-cell activation. (Microbial infection) Acts as a receptor for adenovirus subgroup B. Homodimer. Interacts with MARCH8. Interacts (via cytoplasmic domain) with PHB1 and PHB2; the interactions increases after priming with CD40 (By similarity). (Microbial infection) Interacts with adenovirus subgroup b fiber protein. Expressed by activated B-lymphocytes and monocytes. Polyubiquitinated; which is promoted by MARCH8 and results in endocytosis and lysosomal degradation. CD86 entry