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