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Food Allergy

Author: Professor Cassim Motala, South Africa
Web Editor: Professor Richard Lockey, USA

What is food allergy?

Adverse reactions to food can be sub-divided into non-toxic or toxic.

The term food allergy is used when an adverse reaction is due to an immunologic mechanism. Allergic reactions to food may be caused by antibodies in the blood, or cells in the immune system. The IgE-antibody is responsible for immediate allergic reactions occurring after eating foods. Allergic reactions involving immune cells (T-cells) tend to be delayed reactions, where symptoms first occur 4 to 28 hours after eating the food.

Non-allergic (non-immune) adverse reactions are termed food intolerance (e.g., pharmacological reactions caused by chemical components of foods, or intolerance to the milk protein lactose, caused by a deficiency of lactase, the enzyme which helps to digest the milk protein lactose).

Pharmacological reactions to foods are due to chemicals naturally present in the foods, eg, theobromine in chocolate or tyramine in aged cheeses. Adverse reactions to food additives such as sodium and potassium sulfites, metabisufites, monosodium glutamate and gaseous sulphur dioxides are also examples of pharmacological reactions. These additives are used in foods and drinks to prevent discoloration and as preservatives in a variety of medicines. Sulfites are converted to gases in the acid environment of the stomach from whence they are inhaled and may produce non-allergic irritant reactions or asthma in susceptible people.

Toxic reactions to food can occur in up to 100% of the population provided a sufficient amount of the food is ingested; they are due to toxins (poisons), e.g., to histamine in scombroid fish or salmonella toxins in foods.

How does food allergy develop?

The gastrointestinal tract (gut) starts at the mouth. Food moves down from the mouth via the oesophagus into the stomach and from there is processed through the small and large intestines, where it is eventually eliminated with defecation. The intestinal tract processes food to extract the nutrients and normally blocks allergens from entering the body. Many of the immunological and mechanical barriers involved in this process are immature at birth, leaving the infant at risk for allergens to enter the blood stream. Large amounts of food allergens penetrate the gut barriers in children and adults, but the body's defense, the immune system, develops a tolerance to the allergens. Thus, no symptoms occur. A failure to develop tolerance or a breakdown in tolerance results in allergen sensitization (excessive production of food-specific IgE antibodies). When food allergens subsequently penetrate the gut lining and reach the food-specific IgE antibodies bound to mast cells, a number of chemicals, including histamine, are released, leading to a variety of symptoms affecting the gut, skin, nose, lungs and/or the heart and circulation. The skin, nose and lungs are most often affected by food-induced allergic reactions involving specific IgE antibodies to a food. Disorders of the gut are mostly due to non-allergic reactions.

How common is food allergy and whom does it affect?

Food allergy is uncommon in the general population. It occurs more frequently in children (up to 8%) than adults (2%). Food allergy is more common in children with other allergic diseases; about 35% of children with severe eczema experience food allergy involving IgE antibodies, and 6% of children with asthma experience food-induced wheezing.

Food sensitivity can be so severe that a systemic reaction involving the whole body, anaphylaxis, can occur from breathing airborne particles of the food allergen to which the subject is allergic, for example inhaling the odors of cooked or raw fish.

Which foods commonly cause allergy and anaphylaxis?

Infants/Young children
Older Children And Adults
Milk (cow/goat)  
Chicken egg  
Tree nuts (walnut, hazel/filbert, cashew, pistachio, Brazil , pine nut, almond)  
Shellfish (shrimp, crab, lobster, oyster, scallops)  
Seeds (cotton, sesame, psyllium, mustard)  

Similarities between different types of allergens

A severe pollen allergy can be associated with a condition called the oral allergy syndrome (see below) or anaphylaxis when eating certain foods derived from plants. Such reactions are due to common proteins that are found both in pollens and in plants and fruits. The oral allergy syndrome has been experienced after eating shellfish by people who are sensitive to house dust mites.

Typical associations between allergies to inhalants and foods include:

Allergens in the Air

Allergens in Foods

Birch pollen Apple, raw potatoes, carrots, celery, hazelnut, pear, peach, plum, cherry
Mugwort pollen Celery, apple, peanut, kiwi fruit, carrot, parsley, spices (fennel, coriander, aniseed, cumin)
Ragweed pollen Melons, (watermelon, cantaloupe, honeydew, etc.), bananas
Latex Avocado, kiwi fruit, chestnut, papaya, banana
Chironomidae Crustaceans (shellfish)

What are the symptoms and signs of food allergy?

Skin Symptoms

Hives (acute urticaria) and angioedema (swelling of the eyes, lips, tongue) frequently occur in people with food allergy. The onset of symptoms may be rapid, within minutes after eating the offending food. Foods most often responsible are raw meats, fish, vegetables and fruits.

About one-third of infants and young children with atopic eczema have IgE-mediated food allergy. Egg allergy is the most common food hypersensitivity in children with eczema. Appropriate diagnosis of food allergy and elimination of the offending allergen lead to significant clearing or improvement of eczema in many children with eczema and food allergy. Food allergens are believed to act as trigger factors for acute worsening of eczema in these patients, and are not the primary cause.

Gut Symptoms

Symptoms caused by immediate sensitivity in the gut usually develop within minutes to 2 hours of eating the offending food. Symptoms include nausea, cramping, vomiting and diarrhoea.

The oral allergy syndrome (OAS) is a form of contact urticaria from ingesting a food (usually fresh fruit) confined to the lips, mouth and throat, which most commonly affects patients who are allergic to pollens. Symptoms include itching of the lips, tongue, roof of the mouth and throat, with or without facial swelling, and/or tingling of the lips, tongue, roof of the mouth and throat.

Infantile colic affects babies in the first 2-4 weeks of life. Symptoms include crying, bloated tummy, excessive belching, and the baby curls up its legs in distress. Symptoms may persist through the third or fourth month of life. Food allergy involving IgE antibodies may be a factor in some infants with colic.

Allergic eosinophilic oesophagitis, gastritis or gastroenteritis: The exact cause of these disorders remains unknown but reactions involving both IgE-antibodies and T-cells have been implicated. Patients with these conditions have symptoms of nausea and vomiting after eating, abdominal pain, diarrhoea, and weight loss (in adults) and failure to thrive (in young infants). Food induced allergy involving IgE antibodies may be a trigger for symptoms in some patients.

Nose and Lung Reactions

Allergic rhinoconjunctivitis and asthma have been reported following food challenge testing. However, nose and lung symptoms, in the absence of skin or gut symptoms are unusual.


This is the most serious allergic reaction to a food. In addition to gut symptoms, individuals may experience urticaria, angioedema, asthma, rhinitis, conjunctivitis, low blood pressure, shock and irregular heart beat.

Certain food-allergic individuals may experience anaphylaxis only after exercise. Food-associated, exercise-induced anaphylaxis usually occurs following exercise performed 2-4 hours after eating the offending food. The offending food, or exercise alone, will not cause the reaction. Those most at risk to experience food-induced anaphylaxis include people with asthma and those who have had previous allergic reactions to the offending food.

Non-IgE Mediated Food Allergic Disorders (some are probablyT-cell mediated)

Dietary protein enterocolitis syndrome
Affected group: Early infancy.
Symptoms: Irritability, protracted vomiting 1 to 3 hours after feeding, bloody diarrhoea, (may result in dehydration); anaemia, bloated abdomen, failure to thrive.
Implicated food: Cow's milk, soy.
Affected group: Older infants and children.
Symptoms: As above.
Implicated foods: Egg, wheat, rice, oat, peanut, nut, chicken, turkey, and fish.
Affected group: Adults.
Symptoms: Severe nausea, abdominal cramps, protracted vomiting.
Implicated foods: Crustaceans (e.g. shrimp, crab, and lobster).

Dietary protein proctitis
Affected group: Early infancy
Symptoms: Blood-streaked stools in otherwise healthy infants. Blood loss is usually modest but can be severe enough to cause anaemia.
Implicated food: Cow's milk or soy protein-based formulas.

Dietary protein enteropathy
Affected group: Early infancy.
Symptoms: Protracted diarrhoea, vomiting, bloated abdomen, failure to thrive.
Implicated foods: Cow's milk sensitivity is the most frequent cause of this syndrome.
Affected group: Older children.
Symptoms: As above.
Implicated foods: Soy, egg, wheat, rice, chicken, fish

Coeliac disease
Affected group: All ages.
Symptoms: Chronic diarrhoea, oily/fatty stools, bloated abdomen, excessive belching, weight loss or failure to thrive.
Implicated foods: Patients with coeliac disease are sensitive to gliadin, the alcohol-soluble portion of gluten found in wheat, oat, rye and barley.


Dermatitis herpetiformis
Affected group: All ages.
Symptoms: Chronic, intensely itchy, blistering rash affecting both sides of the body, affecting the front of the arms, back of the legs, and buttocks, sometimes mistaken for atopic dermatitis.
Implicated foods: Gluten.


Food induced pulmonary haemosiderosis (Heiner's syndrome)
Affected group: Infants and young children.
Symptoms: Very rare syndrome characterized by recurrent episodes of bleeding into the lungs, blood loss from the gut, iron deficiency anaemia, failure to thrive.
Implicated foods: Cow's milk; reactions to egg and pork have also been reported.

How Is Food Allergy Diagnosed?

Double-blind, placebo controlled food challenge (DBPCFC), where neither the patient nor the doctor knows whether the patient is eating the food to which he/she is thought to be allergic, or a harmless substitute, is the preferred test for diagnosing food allergies. DBPCFC should be performed in specialist centres under close supervision in case an allergic reaction occurs.

Skin prick testing (SPT) and radio-allergosorbent testing for IgE antibodies (RAST) are the most common tests for food allergies involving the IgE antibody. A skin prick test weal of 3 mm or more is regarded as positive. A positive SPT, even of 3 mm or more may not be relevant, as a patient may have IgE antibodies to the food, but not be allergic to the food. A SPT weal of 7mm-8mm, to cow's milk, egg or peanut is a better indication that the food will cause allergic symptoms. A negative SPT is strong evidence that the food may be safely eaten.

How Can Food Allergy be Prevented and Treated?

The World Allergy Organization/World Health Organization evidence-based document, Prevention of Allergy and Allergic Asthma, recommends exclusive breast-feeding until 4-6 months and no special diet for the breast-feeding mother. The report recommends that infants who have cow's milk allergy should avoid cow's milk proteins, and if a supplement is needed, a hypoallergenic formula (extensively hydrolysed or partially hydrolysed), should preferably be given to improve symptom control.

Food Allergen Avoidance

The foods to which an individual has been proven to be allergic should be avoided. Where avoidance of the food could result in an inadequate diet, it may be necessary to add supplements to the diet, eg, calcium for those avoiding dairy products. This should be supervised by a dietician to ensure that the diet is nutritionally adequate. Dieticians can also provide suitable recipes and education about food labelling.

How to avoid hidden sources of offending foods

Processed foods may contain many hidden proteins, eg, milk, egg and soy proteins may be added to increase protein content or enhance flavour. Peanuts and nut products are added to thicken and flavour sauces. It is important to learn to identify hidden food components in processed foods. Commonly used ‘hidden' proteins are casein, whey and lactose, derived from milk, and albumin from egg. The name arachis is frequently used to describe peanut products, both in foods and in cosmetics.

In some countries major supermarket chains provide lists of allergen-free products suitable for individuals with specific food allergies. Some manufacturers include advice when processed foods have been produced in food preparation areas where nut or milk proteins may be present and could contaminate other foods.

Individuals with food allergies should alert restaurant personnel about their food allergy and ask whether certain ingredients are contained in menu dishes, or whether there is possible contamination of foods due to shared food preparation areas or equipment. Allergen avoidance cards for travellers are available in a number of translations ( e.g., Allergy UK and Food Allergy and Anaphylaxis Network websites).

Individuals allergic to egg should not be given influenza vaccines without prior consultation with their physician. Egg allergy is not usually a reason to withhold measles and MMR vaccines.

Medication for Symptoms not Controlled by Dietary Measures Alone

A number of medications have been prescribed for the treatment of food allergy. These include H1 and H2 antihistamines, ketotifen, corticosteroids and prostaglandin synthetase inhibitors. These drugs may modify allergic symptoms but, in general, are not very effective and sometimes have unacceptable side effects.

Patients receiving immunotherapy for pollen allergy may lose their oral allergy syndrome.

Anti-IgE therapy is licensed for use in some countries, and studies are under way to determine whether it has a role in the management of serious food allergies.

Can food allergy be “outgrown”?

Individuals may lose their allergies over time. Younger children appear more likely to outgrow food allergy involving IgE antibodies. Older children and adults will lose their symptoms if the allergen can be identified and is completely eliminated from the diet, although SPT and RAST results often remain positive. Patients allergic to nuts, peanut, fish and shellfish rarely lose their allergies. Children with milk, soya and wheat allergy can be tested every 1-2 years, and children with egg allergy can be tested every 2-3 years to determine if their allergy still exists.

Food-induced enterocolitis and allergic eosinophilic gastroenteritis in older children and adults appears to persist for prolonged periods. Coeliac disease is a life-long sensitivity and gluten must be avoided for life.

Source Documents

Motala, C: New perspectives in the diagnosis of food allergy. Current Allergy and Clinical Immunology, September/October 2002, Vol 15, No. 3: 96-100

Motala C. Food Allergy - Current issues. Current Allergy and Clinical Immunology 2000;13(4):8-12.

Sampson H A: Food Allergy. Journal of Allergy and Clinical Immunology, 2003; Vol 111, No 2, S540-S547

Sampson H: Food Allergy. In Allergy and Allergic Diseases, Ed. AB Kay, Blackwell Scientific, 1997

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