Related concepts:

Chronic Urticaria, Urticaria, Welts, Wheals

What is hives?

Hives are a sign that the whole body is experiencing a hypersensitivity reaction. Hives occur when the body's capillaries and tiny veins get leaky. Fluid escaping from some of these blood vessels becomes trapped in parts of the skin and lining membranes of the body, causing localized swelling.

This leakiness can be caused by classic allergic reactions in which histamine triggers an inflammatory response. It can also be caused by a number of other regulatory systems in the body in response to different types of triggers.

The most common allergic triggers are drugs, especially antibiotics; foods, especially fish, shellfish, nuts, peanuts, eggs, berries, and food additives; infections (which we'll discuss below); insect bites or stings; inhalants such as animal danders, pollens, and molds; and contact allergens like plant substances, skin creams, cat scratches, moth scales, or animal saliva. In college I had a pet tarantula for a while and some of my friends got hives from petting her hairy abdomen. :-)

There are also a number of distinct physical causes of hives (known as urticaria in doctor-speak):

• Cold urticaria – the most common of the physical causes. These hives are triggered by exposure to cold water or air. This would be a good excuse to avoid swimming in cold pools (something my kids love for me to do).
  
  
• Dermatographia -- hives that appear where the skin is firmly stroked. (Named because you can write on someone's skin by raising welts where your finger traced). This occurs in about 5% of people (and many more if you repeatedly stroke hard enough).
  
  
• Pressure urticaria -- hives that appear under tightly fitting clothing or jewelry. Unlike dermatographia, which occurs in seconds, this can appear many hours later, obscuring the cause.
  
  
• Cholinergic urticaria -- hives that occur in response to heat, exercise, or emotional stress. This usually doesn't begin before adolescence.
  
  
• Aquagenic urticaria -- hives that are triggered by contact with sweat or with water. In these people, exercise itself is not a trigger, and they can drink water without a problem.
  
  
• Solar urticaria -- a rare disorder in which sun exposure results in hives. Sunscreen can help!

I mentioned earlier that infections could trigger hives. We know that some parasites can be responsible for hives (including giardia and pinworms, which are common in day care settings). We also know that bacterial infections, most notably Strep, can be the culprit. Viruses of many types can trigger hives as well.

Over 70 percent of the time, no specific cause is found when children have hives. They were hypersensitive to something, but no one discovers what, and the hives go away. Doctors think that most of these unidentified cases may be from viruses, which would make them the most common cause of hives.