Understanding Food Allergies

If you or a family member suffer from food allergies, you know what an impact it has on your everyday life. Food allergies occur when the immune system responds aggressively to eating certain foods.

The incidence of food allergies has been increasing over the past 3 decades. Currently, around 6% to 8% of children are affected by food allergies. Many children outgrow their allergies and the number of adults with a diagnosed food allergy is lower.

There are actually four types of food allergies, which are related to the type of immune cells involved:

  • Immunoglobulin E-related (IgE)
  • Non-immunoglobulin E-related
  • Immunoglobulin A-related (IgA)
  • Immunoglobulin G-related (IgG)

Immunoglobulin E is a type of antibody reaction. In the case of a food allergy, the persons body has IgE antibodies that identifies allergens in food as a threat. After exposure to a particular antigen, immunoglobulin is primed to cause an excessive immune response. Food allergies related to immunoglobulin E normally occur immediately after eating the food. They are responsible for the well-known life-threatening allergic reactions to foods like anaphylaxis as well as less severe reactions like rash and gastric upset.

Non-immunoglobulin E allergies involve the immune system, but immunoglobulin E is not part of the reaction. These allergies normally occur several hours after eating the allergenic food, and up to 24 hours. It can be more difficult to diagnose non-immunoglobulin E-related food allergies because there is no blood test. The delayed allergic reaction also makes it difficult to pinpoint the food that has caused the allergy. Examples of non-immunoglobulin allergies include celiac disease and cow’s milk protein-induced anemia.

Like IgE, IgA and IgG are antibody type reactions. However, unlike IgE reactions which are immediate and considered a “true allergy” IgA and IgG reactions can take hours to days to appear. Often symptoms will be related to inflammation, skin rash, joint pain, stomach upset, fatigue, brain fog.

What happens to you when you have a food allergy?

A food allergy occurs because the immune system reacts to a small piece of protein from the allergenic food. The allergic reaction experienced after exposure to certain foods depends on the type of allergy that you have. Immunoglobulin E-related allergies are more severe and include an itchy raised rash, swelling of the face and throat, shortness of breath, vomiting, diarrhea, dizziness and hay fever-like symptoms.

A severe allergic reaction, called anaphylaxis, can sometime occur in people with immunoglobulin E-related food allergy. This reaction has a rapid onset and is potentially life-threatening. Anaphylaxis occurs due to an overreaction of the immune system that floods the body with inflammatory molecules. This causes a drop in blood pressure and swelling in the air passages. Unless treated quickly, people with anaphylaxis can lose consciousness and stop breathing. People who may experience anaphylactic shock should carry an “epi-pen” with them at all times. This is a special pen that allows the immediate administration of adrenaline to the person in anaphylactic shock. The person affected should then go to the emergency department at a hospital for further monitoring.

In contrast, non-immunoglobulin E-related food allergies have milder symptoms that are not life-threatening. The skin can become red and itchy and eczema can occur. Gastrointestinal symptoms include vomiting, abdominal cramps and diarrhea. People can have both immunoglobulin E and non-immunoglobulin E-related food allergies.

Why are food allergies a problem?

Food allergies affect daily life in many ways. For people with multiple or severe allergies, it can be difficult to have a “normal” lifestyle as many common foods cannot be eaten. Foods not containing the allergen must be carefully chosen. Eating at a restaurant can be problematic because cross-contamination can occur even if people with allergies do not order food containing certain allergens, and it’s possible that foods contain allergens unexpectedly. When young children have food allergies, their parents must be vigilant that their child does not eat any food that causes their allergies and requires the involvement of friends, family and early-life educators.

People with allergies need to become experts at reading nutrition labels on foods. Food manufacturers must report common allergens on their labels, including if there is a chance for cross-contamination. Manufacturers use the same production line for several products, and trace amounts that are missed by standard cleaning may be enough to trigger an allergic reaction. People with less common allergies need to be aware of what foods may contain the allergens that they are sensitive for. For example, people with an apple allergy need to avoid pectin in jam as it can be derived from apple.

People with severe allergic reactions to certain foods, found most commonly with fish, shellfish and peanut allergies, must be prepared for unintended consumption of the allergen. Even a tiny amount can cause a reaction that could lead to an emergency hospital visit. They need to carry an “epi-pen” with them when they leave the house, and it is useful to instruct people with whom they regularly interact how to use it in an emergency situation.

Types of food allergies

Food allergies can occur from any type of food. However, some types of allergies are much more common than others. In an international comparison, the most common food allergies found were cow’s milk, egg, peanut, tree nuts, fish, shellfish, wheat and soy. These are sometimes referred to at the “big 8” food allergies. Because some food allergies decrease with age, children are more likely to have certain allergies compared to adults. Allergy to eggs and cow’s milk is more common in young children, while peanut, tree nut, fish and shellfish allergy is more common in adults.

Other foods that can cause allergies include fruits and vegetables such as mustard, peaches, kiwi fruit, apple, carrots and celery. These food allergies are often a result of “cross-reactivity”, which means that people become sensitized to a certain fruit or vegetable via the pollen of a related plant. For example, people who get hay fever from the silver birch in the spring may pick up an allergy to apples, hazelnuts or carrot. This is because there is a protein in birch pollen that closely resembles the main allergen in many fruits and vegetables. The pollen from grass, mugwort and ragweed also causes cross-reactivity for various fruits and vegetables. This type of food allergy has symptoms that are largely restricted to the mouth, lips and throat. Cooked fruit and vegetables are less likely to cause an allergic reaction, presumably because cooking breaks down the protein that people react to. Anaphylaxis is rare.

Type of pollen Foods with cross-reactivity
Silver birch Apple, hazelnut, cherry, parsnip, pear, carrot
Grass Orange, tomato, watermelon, certain spices
Mugwort Apple, celery, melon, chamomile tea
Ragweed Banana, honey, sunflower seeds

*Adapted from Dr Adrian Morris https://www.allergy-clinic.co.uk/allergies/airway-allergy/oral-allergy-syndrome/

How to test for food allergies

Immunoglobulin E-related food allergies can be diagnosed by a blood test or skin test. Levels of immunoglobulin E can be measured in the blood. If levels are higher than normal, it can be a sign that the subject has an allergy. The blood test cannot identify what allergies are present.

The skin prick test is used to work out what type of allergy someone has. A nurse draws marks on the skin and applies a drop of antigen to each one. With a small needle (lancet), the skin is punctured slightly. If a red, raised bump develops at the test site within a few minutes, it usually means that the subject is allergic to that allergen.

Another option to test for allergy is to perform a challenge test. Since immunoglobulin E allergy develops quickly and the adverse reaction can be treated with medical supervision, people with suspected allergy can consume the food under the watchful eye of a doctor, who monitors for signs of allergy.

There are fewer options for people with suspected non-immunoglobulin-related food allergies. The skin prick and blood tests will not show a reaction, and the lag between exposure to the allergen and the reaction can take several hours. Often, the best technique is to eliminate all potential allergenic foods from the diet for several weeks. Gradually, foods can be introduced back and the effect monitored. In some cases, such as celiac disease, looking at the cells affected via a biopsy can be used to diagnose the allergy.

Preventing food allergies

Unfortunately, the best way to avoid an allergic reaction is to avoid eating the food that causes it. This requires vigilance, especially for people likely to have a life-threatening reaction. However, researchers are trying to find out how to reduce the number of people who get a food allergy.

Considerable research has looked into whether food allergies can be prevented by controlling the timing of when the most common allergenic foods are introduced to very young children. As there appears to be a genetic component to allergies, this could be important for the children of allergy sufferers. Advice to parents of children prone to develop an allergy was to delay the introduction of the food until the child reached one year of age. Expectant mothers were also recommended to avoid the potential allergenic foods. However, after this advice was shown to not be effective, the recommendation was dropped. There are no specific recommendations now in terms of the timing of the introduction of allergenic foods. All foods may be carefully introduced when infants are 4-6 months old.

Allergies vs intolerances

While also causing a drop in quality of life, food intolerances are much less serious than food allergies. Since some symptoms overlap, they are often considered as a similar disease even though they have a different cause. With food intolerances, an overreaction of the immune system is not involved. Rather, the intolerance occurs when the body is unable to digest a certain food, leading to gastrointestinal symptoms. Accordingly, food intolerances cannot cause life-threatening disease.

One of the most common food intolerances is lactose, a type of naturally-occurring sugar in dairy products. We can use the energy from lactose because our small intestine produces the enzyme lactase, which breaks down lactose into simple sugars. Virtually all infants can break down the lactose in milk, however some adults lost the ability. The small intestine no longer produces lactase.   This means that the lactose in milk and dairy reaches the large intestine, where it is consumed by the gut bacteria there, causing the production of gas and short chain fatty acids. Particularly gas formation causes uncomfortable symptoms such as bloating, abdominal pain and flatulence.

While people who experience lactose intolerance will probably want to avoid consuming large quantities of dairy products, especially milk, they may be able to tolerate small quantities. Dairy foods like yoghurt and cheese contain much less lactose than milk and therefore they may be eaten in small quantities without triggering symptoms.   

Lactose intolerance can be diagnosed based on the results of several different tests. Hydrogen breath testing uses the principle that microbes in the large intestine produce more hydrogen, which we breathe out. After establishing a baseline reading, subjects are given a drink containing lactose. If the amount of hydrogen in the breath increases after the lactose drink, lactose intolerance is suspected. A blood challenge test is also given to diagnose lactose intolerance. For this test, a blood sample is taken, then subjects are given a lactose drink. If blood sugar rises, then lactose intolerance is not suspected because the lactose is able to be taken up by the body and metabolized. However, if blood sugar remains flat or hardly increases, lactose intolerance is likely.


  • Food allergies are common and the incidence is increasing
  • The most common allergies are the “big 8” – cow’s milk, egg, wheat, peanuts, tree nuts, fish, shellfish and soy
  • People with allergies have to be vigilant about what they eat
  • Severe effects of allergy include anaphylactic shock, which requires immediate medical care
  • Other effects include gastrointestinal complaints and a red rash

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