CD1a is a member of the CD1 family of cell-surface glycoproteins that present lipid antigens to T cells. It is structurally related to MHC class I molecules, but its antigen-binding cleft is specifically designed to accommodate lipids rather than peptides. CD1a is characterized by its small antigen-binding groove and high expression on Langerhans cells in the epidermis. CD1a is involved in the adaptive immune response to many microbial lipid antigens. It is expressed on immature thymocytes and is lost upon T cell maturation. In the skin, CD1a is constitutively highly expressed on Langerhans cells, which are ideally positioned to detect breaches in the skin barrier and changes in the local extracellular milieu. Beyond Langerhans cells, CD1a is expressed at lower levels on a subset of dermal myeloid dendritic cells. In inflammatory skin diseases such as atopic dermatitis and psoriasis, the frequency of CD1a-expressing dendritic cell subsets is increased, and migratory patterns of Langerhans cells are altered.