Ultraviolet a augments solar-simulated ultraviolet radiation-induced local suppression of recall responses in humans.

Many studies support the role of ultraviolet B in sunlight-induced alteration of the cutaneous immune system. The role of ultraviolet A is less clear, particularly in humans. The aim of this study was to determine the effect of additional ultraviolet A on solar-simulated ultraviolet-induced suppression of recall responses to nickel in humans. Immuno suppression dose-responses were induced in volunteers by exposure to solar-simulated ultraviolet radiation for four consecutive days. The ultraviolet A radiation dose was increased daily by providing additional high-dose ultraviolet A either before, or after the solar-simulated ultraviolet radiation. These ultraviolet A doses can be readily achieved through a sunscreen. Two different ultraviolet A spectra were used; 320-400 nm and 330-400 nm. Ultraviolet A alone did not cause significant immunosuppression, but augmented solar-simulated ultraviolet radiation-induced immunosuppression. Additional ultraviolet A reduced the minimum dose of solar-simulated ultraviolet radiation that was immunosuppressive. Both ultraviolet A spectra had this effect, although photoaugmentation was less pronounced with the 330-400 nm spectrum. Ultraviolet A-induced immediate pigment darkening did not protect from solar-simulated ultraviolet radiation-induced immuno suppression.