A food allergy is when the body's immune system reacts unusually to specific foods. Although allergic reactions are often mild, they can be very serious.

Symptoms of a food allergy can affect different areas of the body at the same time. Some common symptoms include:

• an itchy sensation inside the mouth, throat or ears
• a raised itchy red rash (urticaria, or "hives")
• swelling of the face, around the eyes, lips, tongue and roof of the mouth (angioedema)
• vomiting

Read more about the symptoms of food allergies.

In the most serious cases, a person has a severe allergic reaction (anaphylaxis), which can be life threatening.

Call 999 if you think someone has the symptoms of anaphylaxis, such as:

• breathing difficulties
• trouble swallowing or speaking
• feeling dizzy or faint

Ask for an ambulance and tell the operator you think the person is having a severe allergic reaction.

Food allergies happen when the immune system – the body's defence against infection – mistakenly treats proteins found in food as a threat.

As a result, a number of chemicals are released. It's these chemicals that cause the symptoms of an allergic reaction.

Almost any food can cause an allergic reaction, but there are certain foods that are responsible for most food allergies.

Foods that most commonly cause an allergic reaction are:

• milk
• eggs
• peanuts
• tree nuts
• fish
• shellfish
• some fruit and vegetables

Most children that have a food allergy will have experienced eczema during infancy. The worse the child's eczema and the earlier it started, the more likely they are to have a food allergy.

It's still unknown why people develop allergies to food, although they often have other allergic conditions, such as asthma, hay fever and eczema.

Read more information about the causes and risk factors for food allergies.

Food allergies are divided into 3 types, depending on symptoms and when they occur.

• IgE-mediated food allergy – the most common type, triggered by the immune system producing an antibody called immunoglobulin E (IgE). Symptoms occur a few seconds or minutes after eating. There's a greater risk of anaphylaxis with this type of allergy.
• non-IgE-mediated food allergy – these allergic reactions aren't caused by immunoglobulin E, but by other cells in the immune system. This type of allergy is often difficult to diagnose as symptoms take much longer to develop (up to several hours).
• mixed IgE and non-IgE-mediated food allergies – some people may experience symptoms from both types.

Read more information about the symptoms of a food allergy.

Oral allergy syndrome (pollen-food syndrome)

Some people experience itchiness in their mouth and throat, sometimes with mild swelling, immediately after eating fresh fruit or vegetables. This is known as oral allergy syndrome.

Oral allergy syndrome is caused by allergy antibodies mistaking certain proteins in fresh fruits, nuts or vegetables for pollen.

Oral allergy syndrome generally doesn't cause severe symptoms, and it's possible to deactivate the allergens by thoroughly cooking any fruit and vegetables.

The Allergy UK website has more information.

The best way to prevent an allergic reaction is to identify the food that causes the allergy and avoid it.

Research is currently looking at ways to desensitise some food allergens, such as peanuts and milk, but this is not an established treatment in the NHS.

Read more about identifying foods that cause allergies (allergens).

Avoid making any radical changes, such as cutting out dairy products, to your or your child's diet without first talking to your GP. For some foods, such as milk, you may need to speak to a dietitian before making any changes.

Antihistamines can help relieve the symptoms of a mild or moderate allergic reaction. A higher dose of antihistamine is often needed to control acute allergic symptoms.

Adrenaline is an effective treatment for more severe allergic symptoms, such as anaphylaxis.

People with a food allergy are often given a device known as an auto-injector pen, which contains doses of adrenaline that can be used in emergencies.

Read more about the treatment of food allergies.

If you think you or your child may have a food allergy, it's very important to ask for a professional diagnosis from your GP. They can then refer you to an allergy clinic if appropriate.

Many parents mistakenly assume their child has a food allergy when their symptoms are actually caused by a completely different condition.

Commercial allergy testing kits are available, but using them isn't recommended. Many kits are based on unsound scientific principles. Even if they are reliable, you should have the results looked at by a health professional.

Read more about diagnosing food allergies.

Most food allergies affect younger children under the age of 3.

Most children who have food allergies to milk, eggs, soya and wheat in early life will grow out of it by the time they start school.

Peanut and tree nut allergies are usually more long lasting.

Food allergies that develop during adulthood, or persist into adulthood, are likely to be lifelong allergies.

For reasons that are unclear, rates of food allergies have risen sharply in the last 20 years.

However, deaths from anaphylaxis-related food reactions are now rare.

A food intolerance isn't the same as a food allergy.

People with food intolerance may have symptoms such as diarrhoea, bloating and stomach cramps. This may be caused by difficulties digesting certain substances, such as lactose. However, no allergic reaction takes place.

Important differences between a food allergy and a food intolerance include:

• the symptoms of a food intolerance usually occur several hours after eating the food
• you need to eat a larger amount of food to trigger an intolerance than an allergy
• a food intolerance is never life threatening, unlike an allergy

Read more about food intolerance.