A surface glycoprotein that characterizes cytotoxic T cells. It recognizes MHC class I proteins, which are produced by virtually all cells and form complexes with processed antigen derived from viruses or other pathogens that have invaded the cell. CD8 binds to any antigen-MHC I complex on the surface of an infected host cell and acts as a coreceptor, helping to activate the T cell and prompting it to kill the infected host cell. The CD8 molecule comprises two distinct chains, α‎ and β‎, linked by a disulphide bond.