Studies on the mechanism of clinical tolerance in solar urticaria

Tolerance to artificial ultraviolet radiation (UVR) was induced in three patients with solar urticaria by administering graded whole body exposures to long‐wave ultraviolet radiation (UV‐A, 320–400 nm) in a phototherapy cabinet. Plasma histamine levels, mast cell ultrastructure and cutaneous responses to intradermally injected codeine and histamine were examined before and after the induction of tolerance. No evidence of serum complement activation could be demonstrated following exposure of serum samples to UVR in vitro. These studies suggest that the state of tolerance is due neither to mediator (histamine) depletion nor to a systemic effect induced by UV‐A but may be due to an increase in the mast cell degranulation threshold.