Solar uticaria. Photoallergen in a patient's serum.

A 45-year-old man had solar urticaria that was activated by visible light. Passive transfer of the reactivity with the patient's serum to the skin of normal recipients was accomplished. Results of reverse passive transfer studies were negative. The patient developed an urticarial wheal at the site of injection of his own serum that had been previously exposed to light in vitro. The experimental data suggested that his condition was attributable to an allergic response. Systemic administration of reserpine was of some therapeutic value, and increasing exposure to natural sunlight was associated with substantial in crease in his tolerance to sunlight. Unfortunately, the possible loss of reactivity that may occur in the natural course of the disease makes substantiation of the therapeutic effects difficult.