A food allergy is caused by your immune system handling harmless proteins in certain foods as a threat. It releases a number of chemicals, which trigger an allergic reaction.
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 signal your immune system to release chemicals to kill the threat and prevent the spread of infection.
In the most common type of food allergy, an antibody known as immunoglobulin E (IgE) mistakenly targets a certain protein found in food as a threat. IgE can cause several chemicals to be released, 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 become red and swell up
- affects nerves in the skin, causing itchiness
- increases the amount of mucus produced in your nose lining, which causes itching and a burning sensation
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 large amounts of histamine and many other chemicals into your blood. This causes the wide range of symptoms associated with anaphylaxis.
There's another type of food allergy known as a non-IgE-mediated food allergy, caused by different cells in the immune system.
This is much harder to diagnose as there's no test to accurately confirm non-IgE-mediated food allergy.
This type of reaction is largely confined to the skin and digestive system, causing symptoms such as heartburn, indigestion and eczema.
In babies, a non-IgE-mediated food allergy can also cause diarrhoea and reflux, where 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 cows' milk, they're probably allergic to all types of milk, as well as infants' and follow-on formula
In adults, the foods that most commonly cause an allergic reaction are:
- tree nuts – such as walnuts, brazil nuts, almonds and pistachios
- shellfish – such as crab, lobster and prawns
However, any type of food can potentially cause an allergy. Some people have allergic reactions to:
- celery or celeriac – this can sometimes cause anaphylactic shock
- gluten – a type of protein found in cereals
- sesame seeds
- fruit and vegetables – these 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 type of meat, while others are allergic to a range of meats; a common symptom is skin irritation
Exactly what causes the immune system to mistake harmless proteins as a threat is unclear but some things are thought to increase your risk of a food allergy.
If you have a parent, brother or sister with an allergic condition – such as asthma, eczema or a food allergy – you have a slightly 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 have atopic dermatitis (eczema) in early life are more likely to develop a food allergy.
The number of people with food allergies has risen sharply over the past few decades and, although the reason is unclear, other allergic conditions such as atopic dermatitis have also increased.
One theory behind the rise is that a typical child's diet has changed considerably over the last 30 to 40 years.
Another theory is that children are increasingly growing up in "germ-free" environments. This means their immune systems may not receive sufficient early exposure to the germs needed to develop properly. This is known as the hygiene hypothesis.
It's rare for someone to have an allergic reaction to food additives. However, certain additives may cause a flare-up of symptoms in people with pre-existing conditions.
Sulphur dioxide (E220) and other sulphites (from numbers E221 to E228) are used as preservatives in a wide range of foods, especially soft drinks, sausages, burgers, and dried fruits and vegetables.
A few people with asthma have had an attack after drinking acidic drinks containing sulphites, but this isn't 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 to E215, E218 and E219) are used as food preservatives to prevent yeasts and moulds 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.