Longwave ultraviolet radiation (UVA, 320-400 nm)-induced tan protects human skin against further UVA injury.

The protective effect of a UVA (320-400 nm) induced tan against cutaneous injury by further UVA-irradiation was studied by evaluating the histopathologic changes in tanned and untanned normal human buttock skin 24 h after exposure to 2 and 4 minimal erythema doses of UVA. In each subject there were fewer polymorphonuclear leukocytes and less endothelial cell prominence and vessel wall necrosis in the UVA tanned skin than in the untanned UVA-irradiated skin. In the tanned control and tanned UVA-irradiated skin there was a prominent mononuclear cell inflammatory infiltrate that was much greater than in untanned skin. In immunoperoxidase stained tissue sections, the mononuclear cells were predominantly T cells, and in all of the specimens the number of phenotypic helper/inducer cells exceeded the phenotypic cytotoxic/suppressor cells. This demonstrates that a UVA tan provides photoprotection against acute UVA exposure. In addition, tanning, with or without further UVA-irradiation, was associated with a mononuclear cell inflammatory infiltrate.