Photoallergy.

Many--topically applied or systemically administered--substances may trigger photoallergic skin reactions following exposure to ultraviolet (UV) radiation. While most cases represent photoallergic contact dermatitis due to topically applied agents, systemically triggered photoallergy is considerably less common. The photopatch test--a UV-exposed variant of the conventional patch test--is the mainstay in the diagnosis of photoallergic or phototoxic reactions.