CD8 is a cell surface glycoprotein found on most cytotoxic T lymphocytes that mediates efficient cell-cell interactions within the immune system. CD8 acts as a co-receptor， and the T-cell receptor on the T lymphocyte recognize antigen displayed by an antigen presenting cell （APC） in the context of class I MHC molecules. The functional CD8 is either a homodimer composed of two alpha chains， or a heterodimer composed of one alpha and one beta chain. Both alpha and beta chains share significant homology to immunoglobulin variable light chains.
