Solar urticaria: studies on mechanisms of tolerance

The mechanisms by which tolerance is induced in solar urticaria were investigated in two patients whose eruptions were induced by wavelengths in the range 320-455 nm in one patient and 400-495 nm in another. Tolerance to radiation was induced by repeated exposures of the skin to the eliciting wavelengths of light. Weal and flare responses to intradermal injections of histamine and the histamine-releasing agent (codeine) were unaltered in the tolerant skin when compared with adjacent normal skin. Intradermal injection of in vitro irradiated serum or plasma from the patients induced an urticarial reaction in the unexposed skin but not in tolerant skin and repeated injections induced tolerance to the eliciting radiation. The results suggest that tolerance is not due to exhaustion of the photoallergen in the skin, or to an increase of the mast-cell degranulation threshold caused by exposure to ultraviolet radiation, or mast-cell mediator depletion, or histamine tachyphylaxis. It is likely that binding sites of IgE on mast cells remain occupied by the photoallergen during the state of tolerance, and that histamine release from mast cells is blocked.