hives (urticaria), rash consisting of blotches or localized swellings (wheals) of the skin, caused by an allergic reaction (see allergy). The swelling is caused by distention of the skin capillaries and escape of serum and white cells into the skin and tissues. Hives are usually extremely itchy, and they may occur in a small area or cover virtually the entire body. The allergic reaction is commonly to a food or a drug, although injections of serum, insect bites, inhalants (pollen), and physical factors (cold, light, heat) may also be causative. Usually crops of hives come and go, remaining at one site for several hours and then reappearing at another; commonly an acute attack subsides spontaneously in a week or two. However, chronic cases of hives may last for long periods of time. Antihistamines and cortisone are considered helpful in relieving the itching and reducing the swelling.
or urticaria

Allergic skin reaction in which slightly raised, flat-topped, very itchy swellings appear suddenly. The acute form, probably most often caused by food allergies, subsides in 6–24 hours, but the chronic form, believed to be due to emotional and mental stress, lasts much longer. Acute hives may also be triggered by drugs, especially penicillin, inhaled allergens or toxins, or diseases. Treatment involves identifying and avoiding the allergen; epinephrine and antihistamines may help the acute skin symptoms.

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