In humans, the multi-gene family of CD1 encodes at least 5 isoforms: CD1A, CD1B, CD1C, CD1D, and CD1E. CD1A is a 49 kDa cell surface glycoprotein expressed in association with 2-microglobulin. It structurally resembles the major histocompatibility complex class I antigen and preferentially presents lipid antigens to T lymphocytes. CD1A is expressed by cortical thymocytes, monocyte-derived dendritic cells, and Langerhans cells. Anti-CD1A antibody can be used to define thymic T cell development and dendritic cell maturation.