CD8 is a transmembrane glycoprotein composed of two disulfide-linked chains.It can be present as a homodimer of CD8 or as a heterodimer of CD8 and CD8 (PMID: 3264320;8253791).CD8 is found on most thymocytes.The majority of class I-restricted T cells express mostly the CD8 heterodimer while CD8 homodimers alone have been found on some gut intraepithelial T cells on some T cell receptor (TCR) T cells and on NK cells (PMID: 2111591;1831127;8420975).CD8 acts as a co-receptor that binds to MHC class-I and participates in cytotoxic T cell activation (PMID: 8499079).During T cell development CD8 is required for positive selection of CD4-/CD8+ T cells (PMID: 1968084).