CD80 is a member of the Ig superfamily and serves as the ligand for two T cell molecules, CD28 and CTLA4. Interactions between CD28 and CD80 on activated B cells result in enhanced T cell activation. CD80 is rapidly induced on the surface of in vitro activated B cells, Epstein Barr Virus (EBV) transformed B cell lines, Burkitts lymphoma cell lines, freshly isolated follicular B lymphoma cells, T cells, and monocytes. It is also expressed at high levels in dendritic cells. It reacts weakly with a small proportion of non activated normal B cells and with HTLV1 infected T cells. CD80 does not react with peripheral monocytes, resting and activated normal T cells, T cell lines and T cell clones, nor with myelomonocytic cell lines.