All we know is that sun allergies are immunological responses of the body. Like many allergies, the body is usually overreacting to a stimulus, many times due to having been overwhelmed toxically.
I would treat this response similarly to other allergies by simplifying my diet (eliminating meat, dairy and dramatically increasing organic vegetables and fruits), cleansing, detoxifying and making sure my liver was in the best possible working order. Finally, I would start using a good enzyme product.