Hives, also called urticaria, are circumscribed swellings on the skin that often are itchy. Often they are pink or red, but they don't have to be. Hives happen when the cells in the skin called mast cells release histamine, a chemical that causes tiny blood vessels (capillaries) to leak fluid. When this leaking fluid accumulates in the skin, it forms the swellings that we recognize as hives.
Hives can be triggered by physical factors such as heat, cold, exercise, sunlight, stress, sustained pressure on a skin area (such as from a belt or shoulder strap), a sudden increase in body temperature (from a fever or a hot bath or shower) or from an irritating chemical, cosmetic or soap applied to the skin. Hives also can be one symptom of a whole-body (systemic) allergic reaction to something that was:
Inhaled — Pollens, animal dander, molds
Injected — Insect stings or bites, especially bee stings, or injected medication
Ingested — Foods (tree nuts; fish and shellfish; dairy products; legumes, especially peanuts), food additives, medications such as penicillin or aspirin
Hives probably affect about 20% of people in the United States at some time in life, with the greatest number of episodes occurring in people aged 20 to 30. In rare cases, allergic reactions that trigger hives set off a chain reaction throughout the body, resulting in a life-threatening condition called anaphylaxis. Sometimes, hives last for six weeks or more, a condition called chronic (or idiopathic) urticaria. Often, no cause is found for this chronic condition, and it usually goes away on its own after several weeks.