A food allergy is caused when your immune system mistakenly treats harmless proteins found in certain foods as a threat to your health. It then releases a number of chemicals which triggers an allergic reaction.
The immune system
The immune system protects the body by producing specialised proteins called antibodies.
Antibodies identify potential threats to your body, such as bacteria and viruses. They then signal to your immune system to release chemicals to kill the threat and prevent the spread of infection.
In the most common type of food allergy a type of antibody known as immunoglobulin E (IgE) mistakenly targets a certain protein found in food as a threat to your body. IgE releases several chemicals, the most important being histamine.
Histamine causes most of the typical symptoms that occur during an allergic reaction. For example, histamine:
- causes small blood vessels to expand and the surrounding skin to swell
- affects the nerves in the skin, which can cause the skin to feel itchy
- increases the amount of mucus produced in your nose lining, causing local itching and burning
In most food allergies, the release of histamine is limited to certain parts of the body, such as your mouth, throat or skin.
In anaphylaxis, the immune system goes into overdrive and releases massive amounts of histamine into your blood. This causes the wide range of symptoms associated with anaphylaxis.
Non IgE-mediated food allergy
There is another type of food allergy known as an non-IgE-mediated food allergy.
In this type of allergic reaction the immune response is largely confined to the digestive system and skin, which can cause symptoms such as heartburn, indigestion and eczema. In babies, an non-IgE-mediated food allergy can also cause diarrhoea and reflux (stomach acid leaks up into the throat).
In children, the foods that most commonly cause an allergic reaction are:
- milk: if a child has an allergy to cow's milk, they will also be likely to be allergic to all types of milk, such as goat’s milk, as well as infants' and follow-on formula milk
In adults, the foods that most commonly cause an allergic reaction are:
- some types of fruit such as apples, pears,
- kiwi fruit and peaches
- some types of vegetables such as potatoes, carrots, celery and parsnip
- crustaceans (shellfish), such as crab, lobster and prawns
- tree nuts, such as walnuts, brazil nuts, almonds and pistachios
However, potentially any type of food can cause an allergy.
Allergic reactions have been reported in association with:
- celery or celeriac: this can sometimes cause anaphylactic shock
- gluten: a type of protein found in cereals
- sesame seeds
- fruit and vegetables: usually only cause symptoms affecting the mouth, lips and throat (oral allergy syndrome)
- pine nuts (a type of seed)
- meat: some people are allergic to just one meat, while others are allergic to a range of meats. A common symptom is skin irritation.
Sulphur dioxide (E220) and other sulphites (E221, E222, E223, E224, E226, E227 and E228) are used as preservatives in a wide range of foods, especially soft drinks, sausages, burgers and dried fruit and vegetables.
Sulphur dioxide is produced naturally when wine and beer are made. It is often added to wine to stop it from continuing to ferment in the bottle. Usually, most of the 'head space' in a bottle of wine (the part of the bottle not filled with wine) is sulphur dioxide.
Anyone who has asthma may react to inhaling sulphur dioxide. A few people with asthma have had an attack after drinking acidic drinks containing sulphites, but this is not thought to be very common.
Food labelling rules require pre-packed food sold in the UK, and the rest of the European Union, to show clearly on the label if it contains sulphur dioxide or sulphites at levels above 10mg per kg or per litre.
Benzoic acid (E210) and other benzoates (E211, E212, E213, E214, E215, E218 and E219) are used as food preservatives to prevent yeasts and moulds from growing, most commonly in soft drinks. They occur naturally in fruit and honey.
Benzoates could make the symptoms of asthma and eczema worse in children who already have these conditions.
Who is at risk?
Exactly what causes the immune system to mistake harmless proteins as a threat is unclear. However, a number of risk factors for food allergies have been identified, which are outlined below.
If you have a parent, brother or sister with an allergic disease, such as asthma, eczema or a food allergy, you are at a higher risk of developing a food allergy. However, you may not develop the same food allergy as your family members.
Other allergic conditions
Children who are born with other allergic conditions, such as asthma or atopic dermatitis (eczema, an allergic skin condition) are more likely to develop a food allergy.
The rise in food allergy cases
Another puzzling aspect of food allergies is that the number of cases has risen sharply over the past two decades. For example, the number of children admitted to hospital for food-related anaphylaxis has risen by seven-fold since 1990.
One theory is that the rise in cases is due to the changes in a typical child’s diet that has occurred over the last 30 to 40 years.
Another theory is that children are increasingly growing up in "germ-free" environments. This means that their immune system may not receive sufficient early exposure to the germs it needs to develop properly. This is known as the hygiene hypothesis.