Dr. Greene, my 16-month-old daughter is suffering from hives, as are three other toddlers in her day care group. Our doctor says it is an allergy, while the doctor of another child says she also has an allergy, but the doctor of the third child says that he is allergic to a protein (or somesuch) in a cold virus. Parenthetically, the children seem to break out usually after they go outside in the fairly cool autumn air. All of us (parents and day care provider) are concerned about this allergy, because we all wonder what we should/could be doing to address the problem. My question: Are you hearing of such allergies breaking out in other areas of the country? Have you heard of an allergy to a virus? What do you think we should do. Currently, all of us are treating with Benadryl.
Thanks so much.

Over the last few weeks, I've received a "rash" of hives questions from across the United States. What might this cluster mean?

Hives are an outward sign of a whole-body hypersensitivity reaction. About 20% of people will experience hives at some point in their lives. Wheals or welts come and go on the skin or mucous membranes, and may itch intensely. Each typically has a whitish, raised plaque of skin surrounded by a reddish halo. The welts may be tiny or large, and may be all over the body or confined to one area. The individual welts often disappear quickly, and are usually gone within 48 hours, although new ones may continue to appear for quite some time.

Hives occur when the body's capillaries and tiny veins get leaky. Fluid escaping from some of these blood vessels gets trapped in parts of the skin and lining membranes of the body, causing the localized swelling in the hives. 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, and food additives), infections (which we'll discuss below), insect bites or stings, inhalants (animal danders, pollens, and molds), and contact allergens (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):

• 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 tight 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!
• Cold urticaria -- Hives that 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).

I mentioned earlier that infections can trigger hives.