CD8a is a 32 kD glycoprotein also known as T8, Lyt2, Ly-2, and CD8α. CD8a is a member of the immunoglobulin superfamily expressed on most thymocytes, subset of mature T cells, most NK cells, macrophages, and some activated CD4+ T cells (not resting). CD8a forms heterodimers with the CD8β chain (CD8b) on the surface of most thymocytes, while mature peripheral T lymphocytes express almost exclusively the CD8 αβ heterodimer. Intestinal intraepithelial lymphocytes express CD8a without CD8b. CD8 is an antigen co-receptor on T cells that interacts with MHC class I on antigen-presenting cells or epithelial cells. CD8 participates in T cell activation through its association with the T cell receptor complex and protein tyrosine kinase lck (p56lck).