Hives, also known as urticaria, are an outbreak of swollen, pale red bumps, patches, or welts on the skin that appear suddenly -- either as a result of allergies, or for other reasons.
Hives usually cause itching, but may also burn or sting. They can appear anywhere on the body, including the face, lips, tongue, throat, or ears. Hives vary in size (from a pencil eraser to a dinner plate), and may join together to form larger areas known as plaques. They can last for hours, or up to several days before fading.
Angioedema is similar to hives, but the swelling occurs beneath the skin instead of on the surface. Angioedema is characterized by deep swelling around the eyes and lips and sometimes of the genitals, hands, and feet. It generally lasts longer than hives, but the swelling usually goes away in less than 24 hours.
Occasionally, severe, prolonged tissue swelling can be disfiguring. Rarely, angioedema of the throat, tongue, or lungs can block the airways, causing difficulty breathing. This may become life threatening.
Hives and angioedema form when, in response to histamine, blood plasma leaks out of small blood vessels in the skin. Histamine is a chemical released from specialized cells along the skin's blood vessels.
Allergic reactions, chemicals in foods, insect stings, sunlight exposure, or medicines can all cause histamine release. Sometimes it's impossible to find out exactly why hives have formed.
There are several different types of hives and angioedema, including:
Acute urticaria and/or angioedema: Hives or swelling lasting less than six weeks. The most common causes are foods, medicines, latex, or infections. Insect bites and internal disease may also be responsible. The most common foods that cause hives are nuts, chocolate, fish, tomatoes, eggs, fresh berries, soy, wheat, and milk. Fresh foods cause hives more often than cooked foods. Certain food additives and preservatives may also be to blame. Medicines that can cause hives and angioedema include aspirin and other nonsteroidal anti-inflammatorymedications (NSAIDs, such as ibuprofen), high blood pressure medications (ACE inhibitors), or painkillers such as codeine.
Chronic urticaria and/or angioedema: Hives or swelling lasting more than six weeks. The cause of this type of hives is usually more difficult to identify than those causing acute urticaria and/or angioedema. The causes can be similar to those of acute urticaria but can also include autoimmunity, chronic infections, hormonal disorders, and malignancy.
Physical urticaria: Hives caused by direct physical stimulation of the skin -- for example, cold, heat, sun exposure, vibration, pressure, sweating, and exercise. The hives usually occur right where the skin was stimulated and rarely appear anywhere else. Most of the hives appear within one hour after exposure.
Dermatographism: Hives that form after firmly stroking or scratching the skin. These hives can also occur along with other forms of urticaria.
Hereditary angioedema: This is painful swelling of tissue. It is passed on through families.