At least five CD1 genes (CD1a, b, c, d, and e) are identified. CD1 is expressed on cortical thymocytes, Langerhans cells, and dendritic cells. It is absent on mature peripheral blood T cells but intracytoplasmic expression is detected on activated T lymphocytes. CD1 proteins have been demonstrated to restrict T-cell response to non-peptide lipid and glycolipid antigens and play a role in non-classical antigen presentation.