The effect of chronic ultraviolet radiation on the human immune system.

A single or a limited number of UVR exposures is recognized to suppress cell-mediated immunity in human subjects. The complex pathway leading from the absorption of photons by chromophores in the skin to the generation of T regulatory cells has been, at least partially, elucidated. However, the effect of repeated UV exposures on immune responses and associated mediators is not well studied, particularly to assess whether they lead, first, to the development of photoprotection so that these immune changes are reduced or no longer occur, and, secondly, to the development of photoprotection against the normal downregulation of immunity induced by a high UV dose. For almost all the parameters evaluated in this review--epidermal DNA damage/erythema, urocanic acid, Langerhans and dendritic cells, natural killer cells, macrophages, mast cells, contact and delayed hypersensitivity responses--none, aside from epidermal DNA damage/erythema and macrophage phagocytic activity, show convincing evidence of photoadaptation or, where appropriate, photoprotection. It is concluded that repeatedly irradiating individuals with UVR is likely to continue to result in downregulation of immunity.