CD8, also known as Lyt-2, Ly-2, or T8, consists of disulfide-linked a and b chains that form the a(CD8a)/b(CD8b) heterodimer and a/a homodimer. CD8a is a 34kD protein that belongs to the immunoglobulin family. The CD8 a/b heterodimer is expressed on the surface of most thymocytes and a subset of mature TCR a/b T cells. CD8 expression on mature T cells is non-overlapping with CD4. The CD8 a/a homodimer is expressed on a subset of g/d TCR-bearing T cells, NK cells, intestinal intraepithelial lymphocytes, and lymphoid dendritic cells. CD8 is an antigen co-receptor on T cells that interacts with MHC class I on antigen-presenting cells or epithelial cells. CD8 promotes T cell activation through its association with the TCR complex and protein tyrosine kinase lck.