The B7 family of proteins are structurally related, cell membrane-associated molecules that regulate immune responses by delivering costimulatory or coinhibitory signals via their ligands. So far, eight family members have been identified, including B7.1, B7.2, B7-H1, B7-H2, B7-H4, B7-H6, PD-L1, and PD-L2. These proteins are mainly expressed by activated antigen presenting cells (APCs) and when paired with either CD28 or CTLA-4 on a T lymphocyte, they induce an increase or decrease in the downstream signaling activity of the MHC-TCR interaction between APCs and T cells. B7.1 has been shown to bind both CTLA-4 and CD28; however, it has a 20-fold higher affinity to CTLA-4 than to CD28. Despite their structural similarities, the individual contribution of B7.1 and B7.2 to the development of pathogenic T cells in autoimmune diseases and protective T cells in infectious diseases is markedly distinct. Besides regulating T cell immune responses, B7 also plays a role in anti-tumor immunity. This is confirmed by demostrating that cytotoxic T cells were able to eradicate tumor cells transfected with B7.1 and B7.2. After activation, follicular lymphoma was shown to upregulate B7.1 and other costimulatory and adhesion molecules, therefore increasing APC activity and amplifying primed T cell responses. Homozygous null B7.1/B7.2 mice fail to generate antigen specific IgG1 and IgG2a responses, suggesting that B7 proteins are important in Ig immune responses. Mouse and human B7.1 share 44% amino acid identity.