All CD1 molecules, except CD1e, are cell surface glycoproteins that are structurally related to the MHC molecules, however, in distinction, CD1 proteins are essentially non polymorphic. CD1 has considerable structural homology with both MHC class I and class II molecules, and CD1 molecules are involved in T cell activation. In contrast to MHC, however, CD1 molecules appear to present predominantly non peptide molecules originating from lipids and glycolipids.