Experiments carried out in mice about 30 years ago indicated that the exposure to ultraviolet radiation (UVR) could down-regulate cell-mediated immunity Since then, considerable progress has been made to elucidate the complex cascade that starts with the absorption of photons at or just below the body surface and ends with the generation of T cells that can suppress immune responses. More recently it has been recognized that these cells belong to a subset of T cells called T-regulatory cells, which are CD4+CD25+ and express the transcription factor Foxp3. T-regulatory cells are involved in the maintenance of peripheral tolerance and have been proposed to be particularly important in avoiding autoimmunity.
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