CD8b antigen is also known as T8, Lyt2, Ly-2, and CD8β. It is a member of the immunoglobulin superfamily expressed on most thymocytes, some mature T cell subsets, and macrophages. CD8b forms heterodimers with the CD8α chain (CD8a). 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).