How do narrow-band UVB and PUVA phototherapy work?

The precise mechanisms of nb-UVB and PUVA phototherapy are not known.  The effects of ultraviolet light are to decrease the rate of cell turnover (particularly in psoriasis patients) and to decrease the activity of the immune system in the skin, reduce inflammation and resulting damage to the skin.  Narrow-band UVB is one of the safest effective treatments for widespread inflammatory skin diseases.  PUVA phototherapy is one of the most effective therapies for chronic, dyshidrotic eczema and psoriasis of the hands and feet.  2-3 months of PUVA therapy may induce a prolonged period of disease remission.  It is important not to apply sunscreen, moisturizer or any prescription topical medication on areas to be treated prior to treatment, as it will block the UV light.