CD8 is a heterodimeric membrane glycoprotein on MHC class I-restricted T lymphocytes that cooperates with the alpha beta CD3 TCR in the recognition of MHC class I molecules presenting antigenic peptides. Cell surface CD8 molecules exist as either alpha/alpha homodimers or alpha/beta heterodimers, and most CD8+ T cells that undergo differentiation in the thymus express alpha/beta heterodimers. The CD8 b chain appears to be important for the thymic maturation of CD8+ T cells, because the number of mature CD8 single-positive T cells is decreased in CD8b gene-disrupted mice and most CD8+ T cells among gut intraepithelial lymphocytes in thymectomized mice express CD8 alpha/alpha homodimers.