CD1a is a non-polymorphic major histocompatibitity complex class I-related cell surface glycoprotein (45 to 55 kDa) and is expressed in association with β-microglobulin. In normal tissues the anti-CD1a antibody reacts with cortical thymocytes, Langerhans cells, interdigitating dendritic cells, and rare antigen-presenting cells of the lymph node. Anti-CD1a antibody also reacts with Langerhans cell histiocytosis, and a subset of pre-T lymphoblastic lymphoma/leukemia (cortical T LBL/L). When combined with S100 and CD68, CD1a has been reported to be very useful in distinguishing Rosai-Dorfman disease from other histiocytic diseases.