Hives is the common term for urticaria, a unique type of skin rash characterized by itchy, raised, welts, which may be few and isolated or large and numerous, covering large areas of the body. Welts frequently form at sites of scratching or pressure such as around the waist or bra strap. Individual welts last only a few hours and then fade, only to be replaced by new lesions. Other itchy rashes are frequently mistaken for hives. If individual bumps last more than 24 hours then the rash is not typical hives.
We know what causes hives but we do not always know why. Hives are the result of chemicals such as histamine released from a white blood cell in the skin cell called the mast cell. Everyone has histamine-filled mast cells in the skin but as long as the histamine stays in the cell, it does not cause problems. When the histamine is released, which occurs commonly in an allergic reaction, the result is redness, itching and swelling. An allergic reaction to pollen typically causes itching, swelling and dripping of the nose as histamine is released in the nasal tissue from mast cells coming into contact with the pollen. A reaction in the airways may cause an asthma attack. In food allergy, the allergen, usually a protein, is absorbed into the blood stream and is distributed around the body. Mast cells in the skin react to the food protein and release histamine. Histamine released in the upper layers of skin cause hives while histamine released in the deeper layers may cause itching and swelling without obvious rash, a condition called angioedema. A severe allergic reaction will cause sudden, widespread release of histamine . Such a dramatic release of histamine into the tissues of the body may result in a rapid drop in blood pressure as well swelling of the tissues around the throat and airways compromising breathing. This rare but serious allergic reaction is called anaphylaxis.
Allergy is common and most true allergic reactions involve release of histamine from mast cells and, while allergic reactions to foods, insect stings, medications, contactants, and on occasion airborne allergen, frequently cause brief outbreaks of urticarial rash, hives that last more than 4-6 weeks are rarely caused by simple allergy. In fact, in more than 90% of cases of hives lasting more than 6 weeks, an allergic cause for the rash cannot be found. In these cases, the diagnosis is Chronic Idiopathic Urticaria.
Because the rash, itching an swelling of hives is for the most part caused by histamine, antihistamines are the primary treatment. Corticosteroids are often used for a short time get a bad case of hives under control although the side effects associated with long term use of oral steroids makes there daily use unworkable. The dose of antihistamines required to treat chronic urticaria can be more the four times the dose commonly used to treat nasal allergies.
Hives are uncomfortable, at times scary, and almost always frustrating. The desperate search to understand and eliminate the cause for the horrible rash and itching is usually unfruitful. However, with proper treatment, it is manageable and usually (but not always) resolves in time.