DR P. MARAZZI / SCIENCE PHOTO LIBRARY DR P. MARAZZI / SCIENCE PHOTO LIBRARY
Nettle rash. Urticaria is an allergic skin condition in which a rash of itchy weals develops on the skin. In this case the reaction was triggered by stings from the nettle plant, Urtica dioica. Nettles have stinging hairs, the tips of which break off when the plant is brushed against the skin. The sting injects a mixture of acetylcholine and histamine, causing the swellings seen here. Urticaria can also be caused by several other factors, but often the cause is not known. Calamine lotion can soothe the itching and antihistamine drugs may be given.
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