A food allergy is an exaggerated immune response triggered by eggs, peanuts, milk, or some other specific food.

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.

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.

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
• Milk
• Peanuts
• Shellfish (shrimp, crab, lobster, snails, clams)
• Soy
• Tree nuts
• Wheat

A food allergy frequently starts in childhood, but it can begin at any age. Fortunately, many children will outgrow their allergy to milk, egg, wheat, and soy by the time they are 5 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:

• Fish
• Peanuts
• Shellfish
• Tree nuts

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

An oral allergy syndrome may occur 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.

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:

• Corn products
• Cow's milk and dairy products (See: Lactose intolerance)
• Wheat and other gluten-containing grains (See: Celiac disease)

Symptoms usually begin immediately, seldom more than 2 hours after eating. Rarely, the symptoms may begin hours after eating the offending food.

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

Other symptoms that may occur include:

• Abdominal pain
• Angioedema (swelling, especially of the eyelids, face, lips, and tongue)
• Diarrhea
• Difficulty swallowing
• Itching of the mouth, throat, eyes, skin, or any area
• Light-headedness or fainting
• Nasal congestion
• Nausea
• Runny nose
• Shortness of breath
• Stomach cramps
• Vomiting

Symptoms of oral allergy syndrome:

• Itchy lips, tongue, and throat
• Swollen lips (sometimes)

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

Elimination diets involve avoiding the suspected food until the symptoms disappear, then reintroduced to see if allergic reaction develops

Provocation (challenge) testing involves exposing a person to a suspected allergen under controlled circumstances. This may be done in the diet or by breathing in the suspected allergen. This type of test may provoke severe allergic reactions. Challenge testing should only be done by a doctor.

Never try to deliberately cause a reaction or reintroduce a food on your own. These tests should only be performed under the guidance of a health care provider -- especially if your initial reaction was severe.

See: Allergy testing

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 on only 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 likely go away in a brief time. Antihistamines may relieve the discomfort. Soothing skin creams may provide some relief.

Consult your doctor if you think you have had an allergic reaction to food, even if it is only a local reaction.

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

Food Allergy and Anaphylaxis Network -- www.foodallergy.org

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.

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

A life-threatening complication is anaphylaxis, a severe, whole-body allergic reaction that can result in death. While people with oral allergy syndrome rarely have an anaphylactic reaction, they should ask their doctor whether they need to carry injectable epinephrine.

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

• 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.

Breastfeeding may help prevent allergies. Otherwise, there is no known prevention of food allergies except to delay introducing allergy-causing foods to infants until the gastrointestinal tract has had a chance to mature. The timing for this varies from food to food and from baby to baby.

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

American Gastroenterological Association medical position statement: guidelines for the evaluation of food allergies. Gastroenterology. 2001 Mar;120(4):1023-5.

American College of Allergy, Asthma, & Immunology. Food allergy: a practice parameter. Ann Allergy Asthma Immunol. 2006 Mar;96(3 Suppl 2):S1-68.

Adkinson NF Jr. Middleton’s Allergy: Principles and Practice. 6th ed. Philadelphia, Pa: Mosby; 2003.