The health benefits of natural sunlight have been noted since the rise of civilization, even without the knowledge of its mechanisms of action. Currently, phototherapy remains an effective and widely used treatment for a variety of skin diseases. Ultraviolet radiation, from either the sun or artificial light sources, has a profound immunomodulatory effect that is responsible for its beneficial clinical outcomes. Ultraviolet radiation mostly induces the innate while suppressing the adaptive immune system, leading to both local and systemic effects. It is antigen specific, acts on both effector and regulatory T cells, alters antigen-presenting cell function, and induces the secretion of cytokines and soluble mediators. This review provides an overview of the immunologic mechanisms by which ultraviolet radiation is responsible for the therapeutic effects of phototherapy.
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