Normally, your body's immune system defends against potentially harmful substances, such as bacteria, viruses, and toxins. In some people, an immune response is triggered by a substance that is generally harmless, such as a specific food.

The cause of food allergies is not fully understood, because they can produce such a variety of symptoms. Reactions to foods may vary from mild to fatal.

While many people have a food intolerance, food allergies are less common. In a true food allergy, the immune system produces antibodies and histamine in response to the specific food.

The symptoms may be confined mainly to the stomach and intestines, or may involve many parts of the body after the food is digested or absorbed. The symptoms usually begin immediately, seldom more than 2 hours after eating. Rarely, the symptoms may begin hours after eating the offending food.

Asthma, eczema, or other disorders may be triggered or worsened by food allergies.

Any food can cause an allergic reaction, but a few foods are the main culprits. In children, the most common food allergies are to:

• eggs
• peanuts
• milk
• soy
• tree nuts
• wheat
• shellfish (shrimp, crab, lobster, snails, clams)

Fortunately, many children will outgrow their allergy to milk, egg, wheat, and soy by the time they are five years old if they avoid the offending foods when they are young. Allergies to peanuts, tree nuts, and shellfish tend to be lifelong.

In older children and adults, the most common food allergies are:

• peanuts
• tree nuts
• shellfish
• fish

It is controversial whether food additives -- such as dyes, thickeners, and preservatives -- likely cause a true allergic reaction.

Many Americans believe they have food allergies, while in reality fewer than 1% have true allergies. Most people's symptoms are caused by intolerances to foods such as:

• wheat and other gluten-containing grains (see celiac disease)
• cow's milk and dairy products (see lactose intolerance)
• corn products

A food allergy frequently starts in childhood, but it can begin at any age.

• scratchy throat
• anaphylaxis (a severe, whole-body allergic reaction that can result in death)
• abdominal pain
• diarrhea
• nausea
• vomiting
• stomach cramps
• itching of the mouth, throat, eyes, skin, or any area
• hives
• angioedema (swelling of the eyelids, face, lips, and tongue)
• light-headedness or fainting
• nasal congestion
• runny nose
• shortness of breath
• wheezing
• difficulty swallowing

If you develop symptoms shortly after eating a specific food, you may have a food allergy. Key signs include hives, hoarse voice, and wheezing. In severe reactions, you may have low blood pressure and blocked airways.

A blood test to identify elevated antibody levels (particularly IgE) can confirm that you have an allergy.

The food causing the allergy can sometimes be identified by:

• Food elimination diets -- the suspected food is eliminated from the diet until the symptoms disappear, then reintroduced to see if allergic reaction develops. This method is not foolproof but may be used to narrow the list of suspected foods.
• Food provocation diet.
• Skin tests.
• RAST test -- blood tests for antibodies to a specific allergen.

Never try deliberately causing a reaction, or reintroducing a food, on your own. These tests should only be performed under the guidance of a physician -- especially if the initial reaction was severe.

The only proven treatment for a food allergy is to avoid the food! If you suspect you or your child has a food allergy, consult an allergy specialist.

If you have symptoms confined to one area of the body (for example, a hive on the chin after eating the specific food), you may not need any treatment. The symptoms will subside in a brief time. Antihistamines may relieve the discomfort. Soothing skin creams may provide some relief.

If a person develops any type of serious or whole-body reaction (even hives) after eating the offending food, the person should inject the epinephrine, and then go to the nearest hospital or emergency facility, preferably by ambulance. Anyone diagnosed with a food allergy should carry (and know how to use) injectible epinephrine at all times.

THE ORAL ALLERGY SYNDROME

The oral allergy syndrome consists of itching lips, tongue, and throat, and sometimes swollen lips. It occurs after eating certain fresh fruits and vegetables. The allergens in these foods are similar to certain pollens. Examples are Melon/Ragweed pollen and Apple/Tree pollen. While people with oral allergy syndrome rarely have an anaphylactic reaction, they should ask their doctor whether they need to carry injectible epinephrine.

Avoiding the offending foods may be easy if the food is uncommon or easily identified. However, success may involve a severely restricted diet and often requires diligent reading of all package ingredients and detailed inquiries when eating away from home. Children may outgrow allergies to milk, egg, wheat, and soy.

• Call your local emergency number, such as 911, if you have any serious or whole-body reactions after eating a food (particularly wheezing or difficulty breathing).
• If your doctor prescribed epinephrine for severe reactions, inject it as soon as possible, even BEFORE calling 911. The sooner you inject the epinephrine, the better.
• Anyone who has had an allergic reaction to a food should be evaluated by an allergy specialist.

Once an allergy has developed, carefully avoiding the offending food usually prevents further problems.