Urocanic acid (UCA) is a metabolite of the amino acid histidine. It represents an important chromatophore in
epidermis, which can absorb ultraviolet rays in UVB and UVA region and sequentially convert it from trans- to
cis-isomer. Cis-isomer is not further degraded; it accumulates in the skin and is excreted with sweat and in shedding
keratin scales.UCA has several important functions, the regulation of the homeostasis of the acidic cutaneous surface,
the terminal differentiation of epidermal cells and namely the immunomodulatory role. As and immunomodulator
UCA can suppress contact allergic reaction and the delayed hypersensitivity of the organism. It can affect reactions
mediated by Th-lymphocytes, cytokine system, Langerhans cells, and by some neuropeptides. UCA is related to the
development of non-pigmented skin tumors (basaliomas) and indirectly also to pigmented tumors. Cis-UCA can
inhibit both the local and systemic resistance to infectious agents. In the immunomodulation some adductive
compounds with another important cutaneous chromatophore DNA can participate.
Urocanic acid, immunomodulation, photoimmunosupression, ultraviolet light, UVB, UVA, photobiology.