Psoralen photoallergy caused by plant contact.

A case of acquired photocontact allergy to furocumarins in plants is reported. Photopatch testing was performed with four psoralens [8-methoxypsoralen (8-MOP), 5-methoxypsoralens (5-MOP), trimethylpsoralen (TMP) and imperatorin (IMP)[. The use of serial dilutions of the test compounds made it possible to differentiate between photoallergic and phototoxic reactions. 8-MOP gave a positive eczematous test reaction down to a concentration of 0.0001%. The reactions to 5-MOP and imp also were positive, while that to TMP awas negative. Histopathological examination of a biopsy specimen from a positive test site showed changes consistent with photoallergic contact dermatitis. The multiple reactions could be explained on the basis of multiple sensitization but cross reactions cannot be ruled out.