DR P. MARAZZI / SCIENCE PHOTO LIBRARY DR P. MARAZZI / SCIENCE PHOTO LIBRARY
Dermatographism. Close-up of dermatographism causing lines of urticaria on the skin of a 17- year-old man's back. Urticaria or nettle rash is a allergic reaction of the skin characterized by the eruption of itchy weals. Normally it is only associated with external irritants such as nettles, insect bites or certain foods or drugs. In dermatographism patients, however, contact with virtually any object can cause the skin to react. To demonstrate this, an instrument was used to "draw" on the skin of this patient. The reaction occurred within five minutes. Dermatographism has no known cause. Treatment is with antihistamines.
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