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Food Allergy

Harvard Health Publications

A food allergy is a reaction by the body's immune system to something in a food, usually a protein; the body mistakenly acts like it is a germ or some other invader, and does its best to defend itself.

While any food could possibly cause an allergy, certain foods are much more likely to do so. In children, the foods that most commonly cause allergic reactions are:

  • Peanuts

  • Eggs

  • Milk

  • Soy

  • Wheat

  • Shellfish

  • Tree nuts

Most allergic reactions occur within 30 minutes of consuming the problem food. Often, the reaction occurs within 5 to 10 minutes, but it can occur as long as 4 to 6 hours after ingestion.

Food allergy is different from food intolerance. In food intolerance, there is a physical reaction to a food, but that reaction isn't allergic. Lactose intolerance is a common example. People who suffer from this have trouble digesting one of the sugars in milk and can have stomachaches or diarrhea when they drink milk or eat dairy products.

While the symptoms of food intolerance can be uncomfortable, this condition is not dangerous. There are also other conditions, such as celiac disease, which can cause reactions to foods (people with celiac disease have trouble with anything containing wheat) that are not allergic.

Another form of allergy is called the oral allergic syndrome. People who have this get itching of the lips, mouth, and throat (and sometimes swollen lips) after eating certain fruits or vegetables. This is rarely dangerous. Food allergies are becoming more common, especially among children. Experts estimate that up to 8 percent of children suffer from food allergy. In adults, that number is 1 percent to 2 percent. While the exact cause of food allergy is unknown, it does tend to run in families.


It's not always easy to know when someone has a food allergy, because there are many different symptoms. They include:

  • Hives (a raised, pink, itchy rash)

  • Scratchy throat

  • Stomachache

  • Nausea or vomiting

  • Diarrhea

  • Swelling, especially around the mouth and face

  • Itchy eyes

  • Nasal congestion/runny nose

  • Other rashes, including eczema

  • Wheezing, or just trouble breathing

  • Difficulty with swallowing

  • Rapid heart rate

  • Lightheadedness

  • In babies and small children, poor growth or blood in the stool

In severe cases, something called anaphylaxis can happen. This whole-body allergic reaction combines many of the symptoms mentioned above and can lead to death if not treated immediately.


Many times, the diagnosis of food allergy can be made by the history. For example, if a child has hives or swelling of the face after eating something with peanuts, they are likely allergic to peanuts. But because symptoms can be so varied, the diagnosis is sometimes harder to make. A detailed diary of food intake and symptoms and information about food allergies in the family can be helpful. But your doctor may need to do testing to be sure. There are two commonly used tests:

Skin testsThe allergy skin-prick test is the most common screening test because it is inexpensive, easy to do, and generally reliable. It involves pricking the skin with a solution of the suspected food. A positive test will produce a small hive-like reaction.

The downside of this test is that it is uncomfortable – and for children with eczema or other skin conditions, the results may be difficult to interpret. And in children with severe allergies, even the small amount of the food injected into the skin can cause significant reactions.

Another problem with skin tests is that in order for them to be truly reliable, the patient can't take any antihistamines for about two weeks before the test. For children who suffer from bad hay fever or other allergies, two weeks without antihistamines may be impossible.

RAST blood testsRadioallergosorbent (RAST) laboratory tests measure the amount of food-specific IgE in the blood. IgE antibodies are made by the body in response to allergens. Once you have developed these IgE antibodies, they are constantly circulating in your blood. Therefore, this blood test can be done at any time. The greater the amount of IgE, the higher the probability the person has an allergy to that particular food.

RAST tests have the advantage of being less uncomfortable (one needle stick for the blood test instead of a bunch of pricks for the skin test) and can be done without stopping antihistamines. The downside of these tests (besides the expense) is that they can have both false positive and false negative results.

Elimination and challenge Another way to diagnose food allergies is with a test called a double-blind, placebo-controlled (DBPC) food challenge. In this test, capsules containing the suspected food and others containing sugar are given to a person, and the reaction is observed. Because there can be a risk of a serious, life-threatening reaction, this test usually is done in a clinic or hospital.

A more common way to do this test is for the person to record what he or she eats and monitors any reactions, which typically occur within two hours of ingestion. First, the suspected foods are eliminated from the diet for one to two weeks. Then, the foods are added back into the diet slowly under medical supervision. It's helpful to keep a food diary of everything consumed and the amounts. This approach would only be taken if the reaction was mild and didn't cause any trouble with breathing.


The following measures can help lower the risk that food allergies will develop in infants and young children:

  • Don't introduce solid foods until 4 to 6 months of age.

  • If possible, provide nutrition exclusively with breast milk for the first 4 months. Then continue to breastfeed as you add new foods to the diet until they are at least 1 year old.

  • Introduce new foods in small portions and one at a time. Monitor for reactions for several days before introducing the next food.

  • Wait to give a child cow's milk until age 1 (this also helps avoid iron deficiency anemia).

For infants that have a family history of food allergies:

  • If breastfeeding isn't possible or is not providing sufficient calories, discuss the choice of formula with your child’s doctor.

  • Do not introduce foods that commonly cause allergies, such as nuts, wheat, egg whites and fish, until you have spoken with your child’s doctor.

For older children who have developed allergies, the best way to avoid an allergic reaction is to avoid the food or foods that cause a reaction. Here are some steps you can take:

  • Educate yourself so that you can recognize other forms or minor traces of the allergens in the foods your child eats. Read food labels (new labeling laws have made this easier) and ask questions when you eat in restaurants. If your child has a milk allergy, for example, avoid foods with ingredients that include casein, caseinate, whey or milk solids. And some foods, while they don't contain peanuts, are manufactured on machines that also manufacture foods with peanuts, making them dangerous to allergic people.

  • Educate your child about foods that must be avoided and why.

  • Inform all adults who have contact with your child about the allergy and what to do in an emergency.

  • Learn cardiopulmonary resuscitation (CPR).

  • If your child has a history of severe reactions to certain foods, you and your child should carry epinephrine at all times and use it at the first sign of an allergic reaction. Epinephrine is usually given with an EpiPen; ask your doctor for several of them (so that you can leave one everywhere your child goes often, to be safe), and make sure that everyone who cares for your child knows how to use it.

  • Make sure your child's school or daycare is aware of the allergy. Even small amounts of peanut butter in a classroom or on a lunch table can be dangerous, for example. Many schools have peanut-free classrooms and lunch tables.

Expected Duration

While most children outgrow food allergies, some carry them into adulthood. Teenagers need to be aware of food allergies they had as children and not assume they have outgrown them. In particular, allergies to peanuts, tree nuts, fish and shellfish usually are not outgrown.


For minor allergic reactions, over-the-counter or prescription antihistamines or nasal sprays can be used to relieve the symptoms. Talk to your doctor about the best medication for your child.

Severe reactions require prompt medical attention. Epinephrine (usually via the EpiPen mentioned above) can be lifesaving. If the EpiPen is used, the child still needs medical attention, as symptoms can return once the medication wears off. All those caring for the child should be instructed to either call 911 or take the child directly to an emergency room (never take the child yourself if there is any trouble breathing or the child appears ill – always call 911).

When To Call a Professional

If you or your child develops allergic symptoms after eating, call a health care professional immediately. If breathing difficulties occur, call 911.


The symptoms of food allergy can range from mild to life threatening. If a food or ingredient is easy to avoid, the allergy may interfere very little with daily life. Some foods and ingredients, however, are widespread, and avoiding them will involve careful monitoring. Some food allergies disappear as a child ages.

Additional Info

The Food Allergy & Anaphylaxis Network11781 Lee HighwaySuite 160 Fairfax, VA 22033-3309Toll-Free: 1-800-929-4040 Fax: 703-691-2713

National Institute of Allergy and Infectious Diseases (NIAID) Office of Communications & Public Liaison6610 Rockledge Drive, MSC6612Bethesda, MD 20892-6612Phone: 301-496-5717

American Academy of Allergy, Asthma & Immunology (AAAAI), Phone: 414-272-6071Toll-Free: 1-800-822-2762

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