Itches and sneezes and rash, oh my

The basic job of your immune system is to identify things that are a part of your body, and to rid your body of things that don't belong. This process allows your immune system to detect and fight off dangerous microorganisms. But not all invaders are equally dangerous, and some are even benign. People suffering from allergies are actually mounting an immune response to a material that, while it doesn't belong in the body, is normally harmless.

Cells of the immune system produce antibodies in response to foreign substances. Allergic reactions involve a special class of antibodies (IgE). All antibodies, including IgE, react with a specific chemical structure on a foreign invader, or antigen, in almost a lock and key mechanism. This is an extremely specific reaction; antibodies to ragweed won't react with, for example, peanut antigens, and peanut antibodies won't react with ragweed.

Antigens that cause allergic reactions are called allergens. Common allergens are molds, pollens, pet dander, dust mites, nuts, tomatoes, shellfish, strawberries, sulfa and penicillin drugs, latex, aspirin, insect venom, eggs, peanuts, wheat, soy, fish, shellfish and cow's milk.

What happens during a reaction
The antibodies released during an allergic reaction bind to the offending allergen, attracting inflammatory cells such as mast cells to the site of the invasion. IgE antibodies cause other immune system cells to release histamine and other chemical mediators, which cause itching and swelling of surrounding tissues. Mucosal membranes such as those found in the eyes, nose, gastrointestinal tract, skin and lungs contain large numbers of cells that produce histamine and other mediators. When these cells are activated, allergic symptoms such as sneezing, watery eyes, and digestive upset occur.

Urticaria (hives) is a condition that occurs during some allergic reactions. The itchy, red bumps characteristic of hives appear on the surface of the skin. The bumps vary in size, and can appear just about anywhere on the body. Not all cases of hives are the result of allergic reactions. Allergic hives are often associated with foods (often tomatoes, berries, nuts, shellfish) and medications (particularly penicillin derivatives).

The histamine and other mediators released during an allergic reaction affect immune cells found throughout the body. Anaphylaxis, a rare but potentially deadly condition, occurs when the allergic reaction cascades uncontrollably. In severe cases, the patient's airway becomes constricted, and the cardiovascular system may fail. Symptoms of anaphylaxis include extreme difficulty in breathing or swallowing, sudden weakness or dizziness. The reaction frequently occurs within seconds or minutes after contact with the offending allergen, but can be delayed as much as 24 hours. Some of the more common anaphylactic agents include bee venom, penicillin derivatives, latex, and nuts. People who experience generalized reactions such as widespread hives or respiratory distress after exposure to these agents should consult a physician to determine whether they may be at risk for a more serious anaphylactic reaction if they are exposed the allergen again.

Food allergy symptoms range from rashes around the mouth and gastrointestinal upset to anaphylaxis. Severe reactions are quite rare, and are most frequently caused by milk, egg, wheat, peanut, soy, tree nuts, fish, and shellfish. If you know that you are allergic to a certain food, careful reading of the list of ingredients found on packaged food is essential for eliminating the allergen from your diet. The Food Allergy Network is a non-profit organization that provides a great deal of useful information about living with food allergies.

Diagnosing allergies
Allergies can be difficult to diagnose. The symptoms are diverse, and it's not always easy to identify the triggering allergen. In the doctor's office, allergens are often identified by skin testing. To perform skin testing, a tiny amount of suspected allergen is injected just below the skin's surface. If antibodies to the allergen are present in the body, the site becomes red and bumpy. Another way to identify allergens is the RAST test, which screens the blood for IgE antibodies. This procedure isn't as sensitive as skin testing.

The most effective way to prevent allergic reactions is to avoid exposure to the things that cause them. This is far easier if the allergen is something easily identified and not present in our usual environment. A person who is allergic to strawberries can more easily live an allergy-free life than can someone who is allergic to dust mites.

There are measures people who are allergic to unavoidable substances can take to minimize contact with these allergens. Dust masks can help prevent inhaling airborne allergens such as pollen and mold. Masks worn while mowing the lawn or raking leaves can make these chores bearable. Delegating these jobs to someone who isn't allergic is an even better avoidance strategy.

Molds and dust mites in the home are difficult to eliminate. Removing carpets, venetian blinds, and fluffy bedding such as feather pillows will help clear the air in the home. Getting rid of unused clothing in the closets will help cut down on mold and dust mites. Seasonal bedding should be stored in airtight, sealed containers when not in use. If it's not possible to dust-proof the whole house, try to focus on the bedroom and other rooms that you visit most often.

When drugs are indicated
If an allergen can't be avoided, drugs can help alleviate most allergy symptoms. Decongestants and over-the-counter antihistamines are commonly used to treat mild allergy symptoms. Traditional antihistamines such as Benedryl give symptomatic relief, but can cause severe drowsiness. Newer prescription antihistamines such as loratadine (Claritin) and fexofenadine (Allegra) are considered safe for most patients, and in most cases they control symptoms without causing drowsiness. Patients with severe allergy symptoms may be prescribed intranasal glucocorticoids in addition to antihistamines.

Always be alert to possible drug interactions, even if you only take over-the-counter medications. Your doctor or pharmacy can tell you if your different medications are safe to take in combination.

Desensitization (immunotherapy) was developed in the early part of the 20th century, and is still practiced today. This treatment uses repeated subcutaneous injections of tiny but increasing amounts of an allergen to desensitize a person to that allergen. Since there is a small risk of developing a severe allergic reaction to the injected antigen, imunotherapy is performed under a doctor's close supervision. Injections are usually needed every 2 to 6 weeks for three or more years. Unlike drugs, which treat the symptoms of allergy, immunotherapy targets the specific antigens that cause the allergic reaction. The treatment works on immune cells all over the body, so allergic symptoms throughout the respiratory tract, skin, and mucosal membranes are all alleviated. Immunotherapy works best with the allergens that cause hay fever, and improvement is usually seen after about six months of treatment. Although there is wide variation in the effectiveness of immunotherapy in individual patients, relief from allergy symptoms is often long-term, and may be permanent.

A recent article in the New England Journal of Medicine (SR Durham et al., 341:468-475, 1999) describes a small study of patients who had already responded well to grass pollen immunotherapy for 3-4 years. One group was given a placebo for three additional years, while the other group continued immunotherapy. Symptomatic relief and immune cell changes were similar in both groups, providing evidence that the cellular changes caused by immunotherapy last long after treatment has ceased. Much still remains to be discovered about the immune mechanisms that occur during immunotherapy.

The US National Institutes of Health has responded to the public's growing interest in alternative and non-traditional medicine by funding research in these areas. One NIH-funded facility, the UC Davis Center for Complementary & Alternative Medicine Research in Asthma, studies non-traditional treatments for asthma and related diseases. A study in progress is testing the effectiveness of a homeopathic grass juice mixture taken orally on hay fever symptoms.

Although allergies can't really be cured in the usual sense, effective treatments exist for many allergy sufferers. If you think you have developed an allergy, a trip to your healthcare provider may start you on the road to relief.