Hives (urticaria) are an allergic reaction of the skin that may last a few minutes or can persist for a few days. They appear as raised, red, itchy bumps (wheals) of different shapes and sizes, with defined red margins and pale centers.
Hives may appear and then disappear at random and seem to move from place to place on the skin. Hives may range in size from less than 0.25 in. (0.6 cm) to 3 in. (7.6 cm) across or larger. Patches of hives may combine to form raised, reddened skin over large areas of the body.
Hives may appear as a reaction to a medication, food, or infection. A single area of swelling often occurs after an insect bite at the site of the bite. Other possible causes include contact with plants, things you breathe in (inhalants), stress, makeup, and exposure to heat, cold, or sunlight. Often a cause cannot be found.
Hives are often minor, but they can also be the first sign of a life-threatening allergic reaction (anaphylaxis) that requires emergency care.
WebMD Medical Reference from Healthwise