Hives, or Urticaria, appears on the body as raised welts with red coloration either directly on the hive or on its periphery. These lesions can be quite small, about the size of a mosquito bite, or very large, measuring several inches in diameter. Hives will typically come and go, moving from one place on the body to another within minutes or hours. One thing for sure: they are almost always intensely itchy!
Hives are caused by release of histamine and other chemicals from special immune cells called mast cells and basophils. When mast cells and basophils are stimulated, they release of histamine and other chemicals produces dilation of blood vessels, leakage of fluid, and irritation of nerves. If these events occur only in the superficial layer of the skin, they cause the redness, swelling, and itching of hives. But, when these chemicals are released in large quantities throughout the body, they can also cause anaphylaxis, a serious systemic allergic reaction which can be life threatening.
A related skin manifestation is angioedema. Angioedema is a term used to describe localized swellings which share many characteristics with hives. Angioedema generally presents as one or two areas of localized swelling, while hives usually presents with multiple welts. Angioedema is caused by the release of the same mast cell and basophil chemicals as hives, but involves the deeper tissues, resulting in a large area of swelling.
Allergic causes of hives can include foods, food additives, medications, and insect venoms. However, mast cells and basophils can be provoked by non-allergic causes. For example, the most common cause of hives in childhood is probably viral infections.
Another important non-allergic cause of hives is autoimmunity, a condition in which the immune system mistakes its own tissues for an invader and attacks that invader. In cases in which hives last for longer than six weeks, an autoimmune attack directed at one’s mast cells and basophils is likely to be the cause. For reasons that are poorly understood, there is sometimes a concomitant autoimmunity against the thyroid gland as well. In most cases, however, no cause can be determined, despite laboratory and allergy testing. Fortunately, most hives and angioedema is self limited and tends to burn itself out over time.
It is important to remember that not all cases of hives or angioedema are caused by or exacerbated by allergies. There are many other well described non-allergic (non-IgE mediated) immunologic mechanisms to explain these skin eruptions. Non-allergic causes of angioedema are important to consider, including medications (especially blood pressure drugs called angiotensin-converting enzyme inhibitors, or ACE inhibitors) and inherited genetic diseases. Urticaria and angioedema are rarely associated with serious systemic diseases. An allergist can be very helpful in distinguishing these causes. A careful history, physical exam, and laboratory and/or allergy testing can help discern the possible causes of a these conditions.
For recurrent or chronic urticaria or angioedema, skin testing may be helpful in discovering triggers of the rash. In most cases, the allergist will look for highly allergenic foods such as milk, eggs, soy, wheat, peanuts or nuts, although many other foods may be involved. Patients can also be helpful in tracking down the cause by keeping a food diary. This detective work sometimes is very helpful in establishing an important cause and effect relationship.
In certain cases, blood testing (RAST or ImmunoCAP) may be helpful. This form of testing may be preferable if one’s skin is too involved with the rash to perform skin tests, or if the patient is unable to discontinue antihistamine usage five days prior to skin testing. Other blood tests looking for more unusual, non-allergic causes of urticaria or angioedema may also be indicated for patients who have had symptoms lasting at least six weeks.
Critical in the treatment for allergic hives and angioedema is avoidance of known triggers. For mild and intermittent outbreaks, over-the-counter or prescription antihistamines, either individually or in various combinations, usually work well to prevent release of chemical mediators or to block the effects of the histamine once released by mast cells and basophils. For more severe cases, your allergy specialist may use multiple antihistamines, and, when necessary, medications such as oral or injectable steroids for brief durations. Other unusual causes of recurrent swelling or hives may require special medications.