CD8 molecule consists of two chains, termed α and β chain, which are expressed as a disulphide-linked α/β heterodimer or as an α/α homodimer on T cell subset (cytotoxic/suppressor T cells), thymocytes and NK cells. The majority of CD8+ Tcells expresses CD8 as α/β heterodimer. CD8 functions as a coreceptor in concert with TCR for binding the MHC class I/peptide complex. The HIV-2 envelope glycoprotein binds CD8 α chain (but not β chain).