B7-1 and B7-2, together with their receptors CD28 and CTLA­4, constitute one of the dominant co-stimulatory pathways that regulate T­ and B­cell responses. Although both CTLA­4 and CD28 can bind to the same ligands, CTLA­4 binds to B7­1 and B7­2 with a 20 ­ 100 fold higher affinity than CD28 and is involved in the down­regulation of the immune response. B-lymphocyte activation antigen B7-1 (referred to as B7) also known as cluster of Differentiation 80 (CD80), is a member of cell surface immunoglobulin superfamily and is expressed on activated B cells, activated T cells, macrophages and dendritic cells. It is the ligand for two different proteins on the T cell surface: CD28 (for autoregulation and intercellular association) and CTLA-4 (for attenuation of regulation and cellular disassociation). CD80 works in tandem with CD86 to prime T cells. CD80 plays a role in induction of innate immune responses by activating NF-κB-signaling pathway in macrophages. CD80 is thus regarded as promising therapeutic targets for autoimmune diseases and various carcinomas.