Hives (urticaria) are an allergic reaction of the skin. 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. They may 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. (8 cm) across or larger. Patches of hives may combine to form raised, reddened skin over large areas of the body. Hives may last a few minutes or a few days.

Hives may appear as a reaction to a medication, food, or infection. A single swelling that looks like hives often occurs after an insect bite at the site of the bite. Other possible causes of hives 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 can be minor or they can be the first sign of a life-threatening allergic reaction (anaphylaxis) that requires emergency care.