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.
GreenFacts, based on DermNet NZ
Idiopathic photodermatoses - Immune system - Ultraviolet radiation