Progesterone-induced urticaria--need it be autoimmune?

A patient with persistent urticaria related to the premenstrual phase of the menstrual cycle is presented. Although systemic administration of progesterone provoked the eruption, we were unable to confirm that there was an immunological reaction to endogenous progesterone or oestrogen. Mechanisms whereby progesterone can augment subclinical types I and IV hypersensitivity reactions are discussed.