Hives (also known as Urticaria) are very itchy (pruritus) red, swollen areas on any part of the skin. These areas come and go quickly, usually within just hours. About 10-20% of people will have hives at some point in their lives. Hives can be acute or chronic (lasting over 6 weeks). The cause of hives is never detected in 90% of all cases. Sometimes keeping a journal of eating, activities, and travel can help to identify agents causing hives. Hives can be caused by foods - with dried fruits, nuts, fish, chocolate, tomatoes, berries and milk being the most common ones. Hives can occur within an hour or two of eating. Other causes include medications (especially aspirin), inhaling fumes, and infections. Occasionally, a person can have hives as a result of environmental exposures, as in the case of heat, cold, stress or exercise. Sometimes people can get a hive-like reaction from rubbing or stroking the skin; this is known as 'dermatographism'. The biological cause of hives inside the skin results from the release of histamine from specialized cells in the skin. The chemical histamine causes swelling, itching and redness - the hallmark features of urticaria (hives). The mainstay of treatment involves the use of anti-histamine medications. Treatment is often based on controlling the symptoms and eliminating the cause. Once again, 90% of the time the cause of urticaria (hives) is unknown.
For additional helpful information on this topic Dr. Crutchfield recommends these helpful links:
American Academy of Dermatology: Urticaria (Hives)