Costimulatory molecule that belongs to the immunoglobulin superfamily that plays an important role in T-lymphocyte activation (PubMed:12196291, PubMed:7694363). Acts as the primary auxiliary signal augmenting the MHC/TCR signal in naive T-cells by acting as a ligand for the CD28 receptor which is constitutively expressed on the cell surface of T-cells (PubMed:12196291, PubMed:7694363). 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). Also acts as an inhibitor of T-cell activation by acting as a ligand for CTLA4, a decoy receptor, thereby blocking CD28-mediated T-cell priming (PubMed:11279501). {ECO:0000250|UniProtKB:P42082, ECO:0000269|PubMed:11279501, ECO:0000269|PubMed:12196291, ECO:0000269|PubMed:7527824, ECO:0000269|PubMed:7694363}. [Isoform 2]: 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. . This is the function of Ensembl gene identifier ENSG00000114013 (CD86, CD86 molecule).