 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.