Photoimmunology is the study of the effects of ultraviolet radiation (UVR) on immunologic processes. Most work has examined UVR effects in animal models and in vitro systems, with limited data from humans. UVB irradiation inhibits induction of immunity systematically at high doses and locally at low doses in animals, and is associated with the appearance of transferable T suppressor lymphocytes. Photosensitized UVA and visible irradiation can have similar effects. In some mouse strains, chronic UVB exposure leads to highly immunogenic cutaneous malignancies, which are able to escape immunologic destruction in part because of protection from suppressor T lymphocytes. Immunosuppressed patients also have increased rates of skin cancer, suggesting immunologic involvement in regulation of skin cancer development. UVB or psoralen photochemotherapy (PUVA) irradiation alters circulating lymphocyte subtypes and contact hypersensitivity responses in man. UVB irradiation also induces the appearance of epidermal non-Langerhans antigen presenting cells, which may have suppressive functions. In vitro UVR exposure decreases antigen presenting cell function, lymphocyte responses to mitogens or antigens, and lymphocyte viability. UVB irradiation of keratinocytes alters interleukin 1 (IL-1) production and induces the release of immunosuppressive factors. It is clear that even modest amounts of UVR alter immunologic function and human avoidance of prolonged sun exposure would seem prudent.
