A food allergy is caused when your immune system mistakenly treats harmless proteins found in certain foods as a threat. It releases a number of chemicals, which then 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, 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

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 massive amounts of histamine and many other chemicals 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 a non-IgE-mediated food allergy, caused by different cells in the immune system. This is much harder to diagnose as there is 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 (stomach acid leaks up into the throat).

Foods

In children, the foods that most commonly cause an allergic reaction are:

• eggs
• milk – if a child has an allergy to cow's milk, they are probably allergic to all types of milk, as well as infants' and follow-on formula
• soya
• wheat
• peanuts

In adults, the foods that most commonly cause an allergic reaction are:

• peanuts
• tree nuts – such as walnuts, brazil nuts, almonds and pistachios
• fish
• crustaceans (shellfish) – such as crab, lobster and prawns

However, any type of food can potentially 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
• mustard
• 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

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.

Family history

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 rise in food allergy cases

The number of people with food allergies has risen sharply over the past few decades, although the reason is unclear. For example, the number of children admitted to hospital for food-related anaphylaxis since 1990 has risen seven-fold.

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 that their immune systems may not receive sufficient early exposure to the germs needed to develop properly. This is known as the hygiene hypothesis.

Food additives

It is 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.

Sulphites

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 fruits 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 or allergic rhinitis 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.

Benzoates

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.