It acts by binding free IgE at the same site that IgE would bind to its high-affinity receptor (FcεRI) on mast cells, thereby reducing free IgE in the serum

Immunosuppression
Doctors will sometimes prescribe immunosuppressive drugs such as prednisolone and ciclosporin if the patient has an intense form of solar urticaria. However, the side effects of these medicines can be severe which is why they are reserved for the most extreme of cases. Plasmapheresis
In more extreme cases, plasmapheresis can be considered. This technique is used to remove the blood plasma or fluid in the red blood cells and then return the cells to the body. It "removes a circulating factor from the blood that may be involved in causing the urticaria," but is still being tested and is not always effective. When the treatment is a success, the patients photosensitivity is decreased to the degree that they can undergo PUVA which can result in the relief of the urticarial outbreaks for an extended period of time. The major setback to this treatment is that the side effects can be severe and may include anaphylactoid reactions. Epidemiology
In the United States, only about 4% of patients with photosensitive disorders are reported to have been diagnosed with solar urticaria. Internationally, the number is slightly larger at 5.3%.
Patients who have experienced solar urticarial symptoms from a young age could mistakenly be thought to have erythropoietic protoporphyria. However, the main symptom for this disease is pain and patients with have been found to have abnormal levels of protoporphyrin in their blood while these levels are normal in SU patients. Finally, cholinergic urticaria, or urticaria induced by heat, can occasionally appear to be solar urticaria because the heat from the sun will cause a person with the disease to have a reaction. Management
Antihistamines
Histamines are proteins associated with many allergic reactions. When the UV radiation or light comes in contact with a person with solar urticaria, histamine is released from mast cells. When this occurs, the permeability of vessels near the area of histamine release is increased. This allows blood fluid to enter the vessels and cause inflammation. Antihistamines suppress the activity of the histamine.Diphenhydramine, a first-generation H1 receptor antagonist or medicine that combats the H1 receptor that is associated with many allergic reactions, has been found to be the most potent antihistamine for this particular disease. Patients prescribed 50 milligrams four times per day have been able to sustain normal exposure to the sun without developing a reaction. Patients with less potent forms of solar urticaria such as fixed solar urticaria can be treated with the medication fexofenadine, which may also be used prophylactically to prevent recurrence.