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. O10 detects cortical thymocytes, Langerhans cells in epidermis, dendritic cells of dermis and Langerhans cells of mucosa of tonsil. It may also detect small focal groups of lymphocytes outside the germinal centers of tonsil indicating a cross reaction with CD1b. This antibody is useful in the characterization of leukemias and lymphomas.