From the publishers of The New England Journal of Medicine

      Doctors refer to allergies as "hypersensitivity reactions" because they result from overly reactive immune systems. Normally, the immune system defends the body against genuinely harmful bacteria, viruses and other infectious agents. But with allergies, the immune system responds to innocuous invaders. The damage and suffering come predominantly from the immune response itself, rather than from the substances that provoke it.
      The microscopic components of bacteria and viruses that cause an immune response during a genuine infection are called antigens. The similarly small bits of food, plants, animals and other normally harmless substances that cause an allergic response are a special class of antigens called allergens. Allergens are inhaled, eaten or injected, or they come in contact with the skin or mucous membranes.
      Common airborne allergens include pollen from trees, grasses and weeds; mold spores; animal dander; and dust mite and cockroach droppings. Nuts, fruits, shellfish and eggs contain the most common food allergens. Venom from yellow jackets, hornets, bees, wasps and fire ants can also initiate strong allergic responses. Common allergens that cause skin reactions include plants such as poison ivy; various cosmetics, fragrances and sunscreens; and metals such as nickel and cobalt. Drugs, latex and dyes used in certain X-ray tests may also cause allergic responses.

"Priming the Pump"

Most allergic reactions require at least one prior exposure to an allergen to "prime" -- or sensitize -- the immune system. This initial exposure usually goes unnoticed, but after subsequent exposure, three types of allergic reaction may occur: immediate, late phase or delayed.
      Immediate hypersensitivity reactions usually happen within minutes of exposure to an allergen. If the skin is affected, blood vessels dilate and fluid seeps into the tissues to cause redness and swelling. In the eyes and nose, increased fluid and mucus secretions cause tearing and a runny nose. And smooth muscles contract in the lung's bronchial tubes, impeding airflow.
      Systemic anaphylaxis is a potentially life-threatening, immediate hypersensitivity reaction that results from widespread, rather than local, tissue reactions. Blood pressure drops and breathing becomes difficult because of constriction throughout the airways. Immediate treatment with injected epinephrine is necessary to restore blood pressure, strengthen the heartbeat and open the airways.
      Severe allergic reactions can induce late-phase inflammation and symptoms that persist for hours to days after the allergens are removed. Over time, late-phase inflammation can cause cumulative damage, such as progressive lung disease.
      At the other end of the spectrum is the delayed hypersensitivity reaction, which occurs after sensitization to certain drugs or chemicals. Poison ivy is a good example. During an initial exposure, allergens in the plant's oil sensitize the immune system, which in turn activates special white blood cells called T cells. Poison-ivy-specific T cells "remember" the allergen. Subsequent contact with poison ivy activates the T cells, which multiply and travel to the contacted area of skin. The T cells release inflammatory substances and attract other types of immune system cells that cause the red, itchy, bumpy and swollen rash that we call poison ivy. Delayed reactions often take several days to cause symptoms.

Allergy Avoidance

There is no cure for allergy, but you can prevent or diminish the severity of allergic reactions. Allergy testing is a logical first step. Allergists usually scratch or inject tiny amounts of common allergens into individual spots on the skin. Those that become red and raised indicate the likely presence of antibodies sensitive to that substance.
      Once you know what you are allergic to, you may be able to avoid or minimize exposure. First, make certain that all your health-care providers, including your dentist, know about any past reactions to drugs, diagnostic tests or treatments.
      Food allergies pose a special problem because avoiding the allergens requires that you know exactly what's in all the food you eat. Severe allergies to animals may make it necessary to remove certain pets from the home. Molds, pollen and dust mites are more difficult to avoid because most are microscopic and ubiquitous. You can minimize exposure to these allergens with dehumidifiers, air filters, air conditioners, frequent cleaning and by limiting the amount of fabric and carpeting in your home.

Treating Allergies

Many effective prescription and over-the-counter allergy medications help minimize allergy symptoms. The main-stays include antihistamines, decongestants, mast-cell stabilizers, anticholinergics, bronchodilators and corticosteroids.
      A new class of drugs (anti-leukotrienes) reduces inflammation in the lungs by blocking leukotriene release by mast cells. Another new type of drug under development attaches directly to antibodies and prevents them from clinging to the mast cells and basophils.
      Some allergy-prone people are candidates for immunotherapy (allergy shots), which reduces the immune system's sensitivity to certain allergens. After several years, immunotherapy may be stopped for some patients who've had lasting improvements.