 The first is a hypersensitivity caused by a reaction to photoallergens located only in people with SU; while the second is caused by photoallergens that can be found in both people with SU and people without it.A subgroup of solar urticaria, fixed solar urticaria, has also been identified.It is a rare, less intense form of the disease with wheals (swollen areas of the skin) that affect certain, fixed areas of the body.