Primary SU is believed to be a type I hypersensitivity (a mild to severe reaction to an antigen including anaphylaxis) in which an antigen, or substance provoking an immune response, is "induced by UV or visible radiation." Secondary SU can occur when a person comes into contact with chemicals such as tar, pitch, and dyes. People who use drugs such as benoxaprofen or patients with erythropoietic protoporphyria may also contract this secondary form. These items that cause this photosensitivity are exogenous photosensitizers because they are outside of the body and cause it to have a greater sensitivity to light.Also, there have been a few unorthodox (unusual) causes of solar urticaria. For those susceptible to visible light, white T-shirts may increase the chances of experiencing an outbreak. In one case, doctors found that the white T-shirt absorbed UVA radiation from the sun and transformed it into visible light which caused the reaction. Another patient was being treated with the antibiotic tetracycline for a separate dermatological disorder and broke out in hives when exposed to the sun, the first case to implicate tetracycline as a solar urticaria inducing agent.It is not yet known what specific agent in the body brings about the allergic reaction to the radiation. When patients with SU were injected with an irradiated autologous serum, many developed urticaria within the area of injection. When people who did not have SU were injected, they did not demonstrate similar symptoms.
The wavelength range of type III and IV spans from 400 to 500 nm, while type V can be caused by UVB radiation to visible light (280–600 nm). Type VI has only been known to occur at 400 nm.Another classification distinguishes two types. 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. Fixed solar urticaria is induced by a broad spectrum of radiative energy with wavelengths ranging from 300 to 700 nm. Differential diagnosis
Polymorphous light eruption (PMLE) is the easiest disease to mistake for solar urticaria because the locations of the lesions are similar (the V of the neck and the arms). However, patients with SU are more likely to develop lesions on the face. Also, a reaction with PMLE will take a greater amount of time to appear than with solar urticaria. Lupus erythematosus has been mistaken for SU; however, lesions from lupus erythematosus will take a longer amount of time to go away. Furthermore, when being tested for the two diseases, patients with SU have a reaction immediately while patients with lupus erythematosus will have a delayed reaction.