Merck Manual

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Food Allergy


James Fernandez

, MD, PhD, Cleveland Clinic Lerner College of Medicine at Case Western Reserve University

Reviewed/Revised Oct 2022

A food allergy is an allergic reaction to a particular food.

  • Food allergies are commonly triggered by certain nuts, peanuts, shellfish, fish, milk, eggs, wheat, and soybeans.

  • Symptoms vary by age and may include rashes, wheezing, a runny nose, and, occasionally in adults, more serious symptoms.

  • Skin prick tests, blood tests, and an elimination diet may help doctors identify the food triggering the allergy.

  • The only effective treatment is to eliminate the food from the diet.

Food allergies may start during infancy. Children may outgrow a food allergy. Thus, food allergies are less common among adults. But if adults have food allergies, the allergies tend to persist throughout life.

Food allergies are sometimes blamed for such disorders as hyperactivity in children, chronic fatigue, arthritis, and depression, as well as poor athletic performance. However, these associations have not been substantiated.

Other reactions to food

Some reactions to food are not an allergic reaction.

Food intolerance differs from a food allergy because it does not involve the immune system. Instead, it involves a reaction in the digestive tract that results in digestive upset. For example, some people lack an enzyme necessary for digesting the sugar in milk (called lactose intolerance Lactose Intolerance Lactose intolerance is the inability to digest the sugar lactose because of a lack of the digestive enzyme lactase, leading to diarrhea and abdominal cramping. Lactose intolerance is caused... read more ).

Other reactions to a food may result from contamination or deterioration of the food.

Causes of Food Allergy

Food allergies typically develop because the immune system malfunctions and misinterprets a food protein as dangerous. When the immune system Overview of the Immune System The immune system is designed to defend the body against foreign or dangerous invaders. Such invaders include Microorganisms (commonly called germs, such as bacteria, viruses, and fungi) Parasites... read more is exposed to allergens, it produces a type of antibody Antibodies One of the body's lines of defense ( immune system) involves white blood cells (leukocytes) that travel through the bloodstream and into tissues, searching for and attacking microorganisms and... read more Antibodies called immunoglobulin E (IgE). (Allergens are molecules that the immune system can identify and that can stimulate a response by the immune system). The IgE antibodies trigger cells of the immune system to release substances (such as histamine, prostaglandins, and leukotrienes) that cause swelling or inflammation in the surrounding tissues. Such substances begin a cascade of reactions that continue to irritate and harm tissues. These reactions range from mild to severe.

Almost any food or food additive can cause an allergic reaction. The most common triggers vary by age group.

Infants and young children with food allergies tend to be allergic to the most common allergy triggers (allergens), such as those in the following:

  • Eggs

  • Milk

  • Wheat

  • Peanuts

  • Soybeans

To prevent such allergies from developing, many parents avoid exposing their young children to these foods. However, new evidence suggests that regularly feeding infants foods that contain peanuts may help prevent them from developing a peanut allergy. More study of this approach is needed.

For older children and adults, the most common triggers are allergens in

  • Nuts

  • Seafood, including shellfish

Being exposed to other allergens that are similar to those in foods (such as pollen) may trigger the production of antibodies to substances in food, resulting in a food allergy. This process is called sensitization. For example, children with peanut allergy may have been sensitized to peanuts when topical creams containing peanut oil were used to treat rashes. Also, many people who are allergic to latex Latex sensitivity Allergic reactions (hypersensitivity reactions) are inappropriate responses of the immune system to a normally harmless substance. Usually, allergies cause sneezing, watery and itchy eyes, a... read more Latex sensitivity are also allergic to bananas, kiwis, avocados, or a combination. Latex and these fruits contain similar allergens.

Alpha-gal syndrome

Alpha-gal syndrome is a rare, recently discovered form of allergy to red meat. Alpha-gal is a sugar that occurs in most mammals but not in fish, birds, reptiles, or people. Lone star ticks, which are present mostly in the southeastern United States, have alpha-gal in their saliva. When a lone star tick bites a person, it transmits alpha-gal to the person. Sometimes the immune system then produces antibodies to alpha-gal, resulting in an allergy to alpha-gal. Because alpha-gal is present in many red meats (such as pork, beef, lamb, and venison) and in foods produced from mammals (such as dairy products and gelatin), people with these antibodies may have an allergic reaction to these foods.

Symptoms of alpha-gal syndrome include an itchy rash, indigestion, constipation, nausea, and anaphylactic reactions Anaphylactic Reactions Anaphylactic reactions are sudden, widespread, potentially severe and life-threatening allergic reactions. Anaphylactic reactions often begin with a feeling of uneasiness, followed by tingling... read more . Unlike other food allergies, symptoms of alpha-gal syndrome frequently do not occur until 3 to 8 hours after eating.

Oral allergy syndrome

Oral allergy syndrome (pollen food allergy syndrome) occurs in people who are allergic to pollen. Certain proteins in pollen are similar to some proteins in food. Thus, people with oral allergy syndrome also have an allergic reaction to foods that contain these similar proteins (called cross-reactivity). The foods involved are often nuts and raw fruits and vegetables. People can usually eat these foods if they are cooked because heat changes the protein structure so that the food proteins no longer resemble the pollen proteins.

People who are allergic to certain pollens are often also allergic to specific foods as follows:

  • Birch pollen: Apples, almonds, carrots, celery, cherries, hazelnuts, kiwi, peaches, pears, and plums

  • Grass pollen: Celery, melons, oranges, peaches, and tomatoes

  • Ragweed pollen: Bananas, cucumbers, melons, sunflower seeds, and zucchini

Doctors can usually diagnose oral allergy syndrome when people who have pollen allergies have allergic symptoms after eating certain foods. Skin prick tests Skin testing Allergic reactions (hypersensitivity reactions) are inappropriate responses of the immune system to a normally harmless substance. Usually, allergies cause sneezing, watery and itchy eyes, a... read more Skin testing are sometimes done to confirm the diagnosis.

Often, oral allergy syndrome can be managed by not eating the problem food raw and/or thoroughly cooking the food. Doctors advise people with this allergy to carry a prefilled, self-injecting epinephrine syringe.

Symptoms of Food Allergy

Symptoms of food allergies vary depend on which food causes the allergy and how old the person is.

In infants, the first symptom of a food allergy may be a rash such as eczema (atopic dermatitis Atopic Dermatitis (Eczema) Atopic dermatitis (commonly referred to as eczema) is chronic, itchy inflammation of the upper layers of the skin that often develops in people who have hay fever or asthma and in people who... read more Atopic Dermatitis (Eczema) ) or a rash that resembles hives. The rash may be accompanied by nausea, vomiting, and diarrhea. By about age 1 year, the rash tends to develop less often, but children may start to react to inhaled allergens (such as pollen) and have asthma symptoms. They may wheeze, feel short of breath, or get a runny nose when they eat the food that triggers their allergy. By about age 10, children rarely have asthma symptoms after the food is eaten.

When food allergies persist in older children and adults, reactions tend to be more severe. In adults, food allergies cause itching in the mouth, hives, eczema, swelling (angioedema Angioedema Angioedema is swelling of areas of tissue under the skin, sometimes affecting the face and throat. Angioedema can be a reaction to a drug or other substance (trigger), a hereditary disorder... read more Angioedema ), and, occasionally, a runny nose and asthma. Food allergies sometimes cause symptoms such as light-headedness or fainting.

For some adults with a food allergy, eating a tiny amount of the food may trigger a sudden, severe reaction. A rash may cover the entire body, the throat may swell, and the airways may narrow, making breathing difficult—an anaphylactic reaction Anaphylactic Reactions Anaphylactic reactions are sudden, widespread, potentially severe and life-threatening allergic reactions. Anaphylactic reactions often begin with a feeling of uneasiness, followed by tingling... read more , which can be life threatening.

Some allergic reactions to food take hours to develop and cause symptoms such as abdominal pain, nausea, cramping, and diarrhea.

Diagnosis of Food Allergy

  • Sometimes only a doctor's evaluation

  • Sometimes skin prick tests or an allergen-specific immunoglobulin test

  • An elimination diet

Doctors suspect a food allergy based primarily on the person’s history. Usually in adults, the allergy is obvious. But diagnosing a food allergy in children may be difficult. Some food allergies may be difficult to distinguish from many other digestive problems, such as irritable bowel disease Irritable Bowel Syndrome (IBS) Irritable bowel syndrome is a disorder of the digestive tract that causes recurring abdominal pain and constipation or diarrhea. Symptoms vary but often include lower abdominal pain, bloating... read more .

If a food allergy is suspected, one of the following tests is done:

Skin prick tests Skin testing Allergic reactions (hypersensitivity reactions) are inappropriate responses of the immune system to a normally harmless substance. Usually, allergies cause sneezing, watery and itchy eyes, a... read more Skin testing with extracts from various foods may be done if a food allergy is suspected. A drop of each extract is placed on the person’s skin, which is then pricked with a needle through the drop. A skin reaction to a food tested does not necessarily mean that a person is allergic to that food, but no skin reaction means that an allergy to that food is unlikely.

Alternatively, an allergen-specific immunoglobulin (IgE) Allergen-specific serum IgE tests Allergic reactions (hypersensitivity reactions) are inappropriate responses of the immune system to a normally harmless substance. Usually, allergies cause sneezing, watery and itchy eyes, a... read more Allergen-specific serum IgE tests test may be done. The immune system produces a different type of IgE in response to each allergen. For example, the IgE that is produced after pollen is inhaled differs from the IgE that is produced when nuts are eaten. For the test, doctors withdraw a sample of blood and determine whether IgE in the person's blood binds to a specific allergen used for the test, such as one for peanuts. If binding occurs, the person has an allergy to that allergen.

If either test identifies a particular food, that food is eliminated from the diet. If eliminating the food relieves symptoms, the food is given to the person again to see whether symptoms develop after it is eaten. When possible, this step is done as part of an oral challenge test. The oral challenge test is done to confirm the diagnosis.

In an oral challenge test, the person is given another food (such as milk or applesauce) in two batches: one with the suspected food in it and one without the suspected food in it. Then the doctor observes as the person eats the food:

  • If no symptoms develop after the suspected food is eaten, the person is not allergic to the food.

  • If symptoms develop after the suspected food is eaten and not after the other food is eaten, the person is probably allergic to the suspected food.

Other ways to identify a food allergy include elimination diets:

  • A diet that eliminates only the food or foods suspected of causing the allergy

  • A diet that consists only of foods not likely to cause an allergic reaction

An elimination diet may be the only test used to diagnose a food allergy or may be used after a skin prick test or an allergen-specific IgE test.

For the first type of elimination diet, the person stops eating all foods that may be causing the symptoms for about 1 week.

The second type of elimination diet, which consists only of food not likely to cause allergic reactions, can be tried instead of the first type of diet. The second type of diet involves the following:

  • Following a diet prescribed by the doctor

  • Eating only the foods and liquids specified in the diet and only using pure products (which excludes many commercially prepared foods)

There are several possible elimination diets, which vary in the foods that are eliminated and allowed. For example, one diet may eliminate beef and lamb and allow chicken. Another may eliminate lamb and poultry and allow beef.

Following an elimination diet is not easy because many food products have ingredients that are not obvious or expected. For example, many rye breads contain some wheat flour. Eating in restaurants is not advisable because the person and the doctor need to know the ingredients of every meal eaten.

If symptoms are not relieved after 1 week, doctors may recommend a different elimination diet.

If no symptoms occur, foods are added back one at a time. Each added food is given for more than 24 hours or until symptoms appear, and thus the allergen is identified. If the person has had very severe allergic reactions to food, the doctor may ask the person to eat a small amount of a food in the office. The doctor then observes the person’s reaction to the food.

Did You Know...

  • People with severe food allergies should always carry antihistamines and an epinephrine syringe in case they have a severe reaction.

Prevention of Food Allergy

For many years, doctors have advised against feeding young infants foods that commonly trigger an allergic reaction (such as peanuts) as a way to prevent food allergies. However, new evidence suggests that regularly feeding infants foods that contain peanuts may help prevent them from developing a peanut allergy. More study of this approach is needed.

Parents should talk to their pediatrician about the best way to prevent peanut allergy in their child.

Treatment of Food Allergy

  • Antihistamines and sometimes epinephrine to treat an allergic reaction

  • An elimination diet

  • Sometimes drugs to block the allergic reaction

  • Sometimes allergen immunotherapy (desensitization)

People with severe food allergies should carry antihistamines to take immediately if a reaction starts. Antihistamines are useful for relieving hives Hives Hives are red, itchy, slightly elevated swellings. The swelling is caused by the release of chemicals (such as histamine) from mast cells in the skin, which cause fluid to leak out of small... read more Hives and swelling. People should also carry a self-injecting syringe of epinephrine to use when needed for severe reactions. Cromolyn, a prescription drug taken by mouth, can also help relieve symptoms.

People with food allergies must eliminate the foods that trigger their allergies from their diet.

Other drugs for food allergies

Omalizumab is a monoclonal antibody Monoclonal Antibodies Immunotherapy is the use of drugs that mimic or modify components of the immune system (such as tumor antigens and immune checkpoints—see also Overview of the Immune System) to fight disease... read more manufactured to block the activity of IgE (which is produced by the immune system and causes the allergic symptoms). Omalizumab is not used to treat an acute allergic reaction but is being studied as a way to limit allergic reactions, particularly during desensitization treatments. The drug is given by injection.

Other monoclonal antibodies, such as dupilumab, are being studied as treatment for peanut allergy.

Allergen immunotherapy (desensitization)

If an allergen cannot be avoided, desensitization immunotherapy is a process that tries to teach the person's immune system not to react to that allergen. The person is given progressively larger doses of the allergen. The first dose is so small that even an allergic person will not react to it. However, the small dose starts to get the person's immune system used to the allergen. Then the dose is gradually increased. Each increase is so small that the immune system still does not react. The dose is increased until the person is not reacting to the same amount of allergen that once caused symptoms.

Immunotherapy for food allergies is typically given by mouth. It is most widely used for peanut allergy in people who are 4 to 17 years old. Desensitization to various other foods is being studied.

For peanut allergy, people are given a powder of peanut allergen by mouth. On the first day, five gradually increased doses are taken in a health care facility. A doctor must observe the first day's treatment because exposure to a high dose of the allergen too soon can cause a sometimes dangerous allergic reaction. People then take a dose every day. The dose is increased every 2 weeks until the maintenance dose is reached. This process usually takes about 5 months. Each time the dose is increased, the first dose must be taken in a health care facility. People take the maintenance dose indefinitely.

People must continue to take peanut allergen powder daily to remain desensitized. Also, they still need to maintain a strict peanut-free diet, but desensitization reduces the risk of severe allergic reactions (including anaphylaxis) to peanuts consumed unknowingly.

Drugs Mentioned In This Article

Generic Name Select Brand Names
Adrenaclick, Adrenalin, Auvi-Q, Epifrin, EpiPen, Epipen Jr , Primatene Mist, SYMJEPI, Twinject
Xolair, Xolair Prefilled
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