Solar urticaria

Solar urticaria is a stinging, itchy rash which develops within minutes of exposure to sunlight or artificial light emitting ultraviolet radiation. The cause of solar urticaria is not clearly defined but may be due to an immune reaction between a chemical created in the body (photoallergen) reacting with ultraviolet radiation to cause the allergic reaction.

Source: GreenFacts, based on DermNet NZ 

Idiopathic photodermatoses - Immune system - Ultraviolet radiation