Causes
Hives appear as a reaction to an external irritant. The chemical that causes most of the itching and inflammation is histamine, the same chemical that causes the symptoms of hay fever and asthma.
In a minority of people with hives, a clear allergy link can be proven. They develop rashes after eating certain foods, taking certain medications, or from insect stings. Foods that can be associated with hives include:
- eggs
- fish and shellfish
- milk
- nuts
Coffee, alcohol, and tobacco are rarely identified as triggers of acute attacks of hives, but people who suffer from chronic hives often report that these substances make their symptoms worse.
Almost any medication can provoke hives. These are some of the most common culprits:
- anesthetics
- angiotensin converting enzyme inhibitors (ACE inhibitors)
- antiepileptics
- bromide
- antimalarials
- aspirin* and other nonsteroidal anti-inflammatory drugs (NSAIDs)
- diuretics (water pills)
- iodide
- penicillin
- vancomycin
If you get a rash from medications or food, it may not be the primary ingredient that's causing the problem. It could be a preservative or other additive. For example, if several different foods and medications seem to give you hives, you may be reacting to tartrazine, a coloring agent frequently used in pills and food.
Cosmetics, soaps, perfumes, and lotions can also cause reactions. Often, this will only occur when you switch to a new type. This may make it easier to identify the cause of your hives. Other common irritants are nickel in jewellery and latex in gloves or condoms.
Allergic urticaria is the name given to hives caused by food, insect bites, medications, or makeup. Other types are caused by physical irritation, such as sunlight, cold, or rubbing of the skin. While we don't call these types allergy-induced, the underlying process is much the same.
Hives brought on by sunlight (photosensitive urticaria) may not seem like an allergic condition at first sight, but the evidence suggests it is. People have become sensitive to light after receiving the blood product immunoglobulin from others with this rare condition. The same is true of hives caused by cold temperatures. It seems likely that the blood of people with these conditions carry abnormal immune cells. These cells attack when confronted with chemicals that are normally released in the skin by cold, sunlight, or some other external circumstance.
Dozens of infections can cause hives, including throat, stomach and genital or urinary (genitourinary) tract infections; fungal infections; mononucleosis; and hepatitis. The common cold often causes hives in children. Hives aren't directly caused by the infectious organism, as in chickenpox or cellulitis; rather, they're caused by "friendly fire" from the human body's own defenses.
Other triggers frequently associated with hives include:
- friction - about 5% of the population is prone to develop skin rashes after quite gentle rubbing or scratching of the skin; this condition is called dermatographism
- swimming, which can cause fairly severe reactions, probably more due to the cold than to the water
- exercise, stress, and hot showers, which cause a form of hives called cholinergic urticaria
- pressure from belts, straps, and elastic, causing pressure urticaria
- pregnancy - hives often appear late in pregnancy
Chronic or recurring hives might mean you're being repeatedly exposed to a trigger, or it might be a sign of underlying disease. Lupus, rheumatoid arthritis, and thyroid disorders are diseases likely to cause hives to appear.