trans-Urocanic acid is a major epidermal chromophore for the immunosuppressive effects of UV radiation that is produced in mammalian stratum corneum by the action of histidine ammonialyase on L-histidine. It contributes to the acidification of the surface of the stratum corneum, and its function has been proposed as that of an endogenous sunscreen, endowing a low-level of protection (SPF ~ 1.5) against UV-induced DNA damage and excessive keratinocyte apoptosis. Upon UVB exposure, trans-urocanic acid is converted to the cis isomer, which is known to activate regulatory T cells.