ALLERGY

Allergy, exaggerated and sometimes harmful reactions to external substances, called allergens. Allergy may result from exposure to such common allergens as plant pollens from grasses, trees, or ragweed; animal danders, which are tiny scales shed from the skin and hair of cats and other furred animals; arachnids and insects, such as house dust mites, bees, and wasps; and drugs, such as penicillin.

The most common food allergies are caused by crustacean shellfish, eggs, fish, milk, peanuts, soybeans, tree nuts, and wheat. Researchers estimate that at least 24 million people in the United States suffer from allergies—about 19 percent of the population.

In an allergic reaction, the immune system mistakenly interprets a harmless substance as a harmful one. The immune system responds by producing antibodies called immunoglobulin E (IgE). These IgE antibodies are designed to help neutralize specific parasitic invaders and protect the person from future exposures.

Upon first exposure to an allergen, no allergic symptoms develop. But when the person is exposed to the same substance at a later time, the IgE antibodies against the allergen activate an allergic reaction. Allergy symptoms may include itching, sneezing, a stuffy nose, watery eyes, inflammation of the airways in the lungs and wheezing (known as asthma), and even allergic shock and death in rare situations.