The CD8 antigen is a cell surface glycoprotein found on most cytotoxic T lymphocytes that mediates efficient cell-cell interactions within the immune system. The CD8 antigen, acting as a coreceptor, and the T cell receptor on the T lymphocyte recognize antigen displayed by an antigen presenting cell in the context of class I MHC molecules. The functional coreceptor is either a homodimer composed of two  chains, or a heterodimer composed of one  and one  chain. Both  and  chains share significant homology to immunoglobulin variable light chains. CD8  chains binds to class-I MHC molecules 3 domains. CD8 identifies cytotoxic/suppressor T-cells that interact with MHC class I bearing targets. CD8 is thought to play a role in the process of T-cell mediated killing. This monoclonal antibody KT15 reacts with a non-polymorphic epitope on the mouse CD8  chain (mouse Ly2.1 and Ly2.2 cells).