B7-1 and B7-2, together with their receptors CD28 and CTLA 4, constitute one of the dominant co-stimulatory pathways that regulate T and B cell responses. Although both CTLA 4 and CD28 can bind to the same ligands, CTLA 4 binds to B7 1 and B7 2 with a 20 100 fold higher affinity than CD28 and is involved in the down regulation 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 at