Food Allergy


What Is It?

A food allergy is a reaction by the body’s immune system to a protein in a food. The food protein that causes an allergic reaction is called an allergen. When a person with a food allergy eats the food containing the allergen, a chain of chemical reactions can be triggered, causing inflammation and irritation in the skin, digestive and respiratory systems. Most allergic reactions occur within 30 minutes of consuming the offending food. Often, the reaction occurs within five to 10 minutes, but it can occur as long as four to six hours after ingestion. In extreme cases, the result can be anaphylaxis, a potentially life-threatening condition marked by faintness, rapid pulse, difficulty breathing and other severe symptoms requiring emergency medical treatment.

Many children who are thought to have a food allergy actually have food intolerance. Food intolerance involves a physical reaction to the food itself or to an additive in the food. Food intolerance does not involve the immune system. Lactose intolerance is the most common type of food intolerance. It occurs in people who do not have enough lactase, the enzyme needed to digest lactose, the sugar in milk. After eating or drinking dairy products, people with lactose intolerance can experience bloating, gas, abdominal pain or diarrhea. Food additives, including flavor enhancers, such as MSG, or preservatives, such as sulfites, also can trigger a food-intolerance reaction.

Food allergy is not as common as generally believed. Although many people think they have food allergies, only about 1 percent of adults do. Approximately 8 percent of preschool children and 2 percent of older children have food allergies. Controlled studies with newborns have found that about 2 percent of infants have either a milk allergy or milk intolerance. Children of parents who have a history of food allergies have the highest risk of developing food allergies.

The majority of food allergies are caused by a few foods. In young children, the foods most likely to cause allergies are cow’s milk, eggs, wheat and peanuts. In older children and adults, peanut and seafood allergies are most common. Other foods that commonly cause allergic reactions include soy products and tree nuts, such as almonds, pecans and Brazil nuts.


Common symptoms of food allergies include:

  • Tingling, swelling and itching of the lips, mouth and throat that can begin within seconds of eating the food
  • Nausea and vomiting
  • Diarrhea
  • Itching and a skin rash (usually hives)

Sometimes, a severe allergic reaction to food can cause breathing difficulties when the upper airways become swollen and obstructed. This is not the same as asthma, which involves inflammation and constriction of the lower airways.

In rare cases, food allergies can cause anaphylaxis, a severe reaction that requires immediate medical attention.


A family history can help to determine predisposition to food allergy, and various tests are used to determine the food(s) causing the reaction.

Skin tests
The allergy skin-prick test is the most common screening test because it is inexpensive and easy to perform. It involves pricking the skin with a solution of the suspected food. A positive test will produce a small hivelike reaction, but a positive result does not always indicate a true allergy.

RAST blood tests
These 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. Like skin-prick tests, RAST tests are prone to false positives, meaning a positive response can occur even when the person is not actually allergic to the food.

Elimination and challenge
The most certain way of diagnosing 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 commonly used form of this test involves recording what is eaten and monitoring 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.


The following measures can help lower the risk that food allergies will develop in infants and young children in families with a history of allergies:

  • Don’t introduce solid foods until 6 months of age.
  • Breast-feed babies until they are at least 1 year old. However, you also will have to avoid highly allergenic foods since allergens can pass through breast milk. If breast-feeding isn’t possible, discuss the choice of formula with your doctor.
  • Introduce new foods in small portions and one at a time. Monitor for reactions for several days before introducing the next food. Do not introduce foods that commonly cause allergies, such as peanuts, egg whites or fish until the second year of life.
  • Wait to give a child cow’s milk until age 1 and peanuts until age 3.

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 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.
  • 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.
  • Speak to your child’s school. Food in the cafeteria can be a hazard.

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.


In minor cases, over-the-counter or prescription antihistamines or nasal sprays can be used to relieve the symptoms of an allergic reaction. Severe reactions require prompt medical attention. Highly sensitive people should carry epinephrine for self-injection to treat a severe food allergy reaction until medical help can be reached. Ask your doctor about this potentially life-saving medication.

When To Call A Professional

If you or your child develops allergic symptoms after eating, call your health care professional immediately. If breathing difficulties occur, call an ambulance or go to an emergency room for treatment.


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.

Johns Hopkins patient information

Last revised:

Diseases and Conditions Center

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All ArmMed Media material is provided for information only and is neither advice nor a substitute for proper medical care. Consult a qualified healthcare professional who understands your particular history for individual concerns.