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

Allergy to 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 related to your body making a type of allergy-producing substance called immunoglobulin E (IgE) antibodies to a particular food.

Although 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

Food additives -- such as dyes, thickeners, and preservatives – may rarely cause an allergic or intolerance 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, within 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.

Other symptoms that may occur include:

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

Symptoms of oral allergy syndrome:

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

In severe reactions, you may have low blood pressure and blocked airways.

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

With elimination diets, you avoid the suspected food until your symptoms disappear. Then the foods are reintroduced to see if you develop an allergic reaction.

In provocation (challenge) testing, you are exposed 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 first reaction was severe.

See: Allergy testing

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

Avoiding the offending foods may be easy if the food is uncommon or easily identified. However, you may need to severely restrict your diet, carefully read all package ingredients, and ask detailed questions when eating away from home.

Anaphylaxis is a severe, whole-body allergic reaction that is life-threatening. Although people with oral allergy syndrome rarely have an anaphylactic reaction, they should ask their doctor whether they need to carry injectable epinephrine.

Food allergies can trigger or worsen asthma, eczema, or other disorders.

• Call your local emergency number, such as 911, if you have any serious or whole-body reactions (particularly wheezing or difficulty breathing) after eating a food.
• 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 way to prevent food allergies except to delay introducing allergy-causing foods to infants until their 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.

Lack G. Clinical practice. Food allergy. N Engl J Med. 2008;359:1252-1260.