A food allergy happens when your immune system treats harmless proteins in foods as a threat. It releases chemicals that trigger an allergic reaction.
The immune system
The immune system protects the body by producing 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 releases several chemicals. The most important of these is histamine.
Histamine causes most of the 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 releases large amounts of histamine and other chemicals into your blood. This causes the wide range of symptoms associated with anaphylaxis.
Non-IgE-mediated food allergy
There's another type of food allergy known as a non-IgE-mediated food allergy. This is caused by different cells in the immune system.
This is much harder to diagnose. There's no test to accurately confirm non-IgE-mediated food allergy.
This type of reaction is confined to the skin and digestive system. It causes 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 infants' milk 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
But 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
- pine nuts
- meat – some people are allergic to just one type of meat, while others are allergic to a range of meats
What causes the immune system to mistake harmless proteins as a threat is unclear. Some things are thought to increase your risk of a food allergy.
If you have a family member with an allergic condition you have a slightly higher risk of developing a food allergy. 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. 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 enough early exposure to the germs needed to develop properly. This is called the hygiene hypothesis.
It's rare for someone to have an allergic reaction to food additives. But 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 food.
Sulphur dioxide is found in wine and beer. Anyone who has asthma or allergic rhinitis may react to inhaling sulphur dioxide.
The European Union requires that pre-packed food shows 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.
Content supplied by the NHS and adapted for Ireland by the HSE