Photoallergic urticaria induced by visible light. Additional cases and further studies.

Four patients with solar urticaria induced by visible light developed an urticarial wheal at the site of injection of their own serum, which previously had been exposed to light in vitro. The in vitro-irradiated patients' sera did not produce the urticarial response in normal control subjects. Direct passive transfer studies with the patients' sera yeilded positive results, while reverse passive transfer studies showed negative results. Based on the experimental data, the patients' conditions might be induced by an allergic mechanism in which a circulating photoallergen appears to be an etiologic factor. One of the patients demonstrated an urticarial reaction at the site of injection of in vitro-irradiated normal serum as well as his own serum. The urticaria formation was blocked by intradermal injection of epinephrine but not by local injection of antihistamines. The repeated exposures to light had a beneficial effect on wheal formation in all cases.