US NATIONAL LIBRARY OF MEDICINE / SCIENCE PHOTO LIBRARY US NATIONAL LIBRARY OF MEDICINE / SCIENCE PHOTO LIBRARY
Urticaria. Historical medical illustration of cases of urticaria factitia. In dermatographism contact with virtually any object can cause the skin to form itchy red weals where contact has occurred. Urticaria pigmentosa is a rare disease caused by excessive numbers of mast cells (a type of white blood cell) in the skin. Red or brown spots are characteristic and weals appear on the skin when irritated. From Atlas of Venereal and Skin Diseases by Prince Albert Morrow, published in 1889.
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