The symptoms of a food allergy almost always develop a few seconds or minutes after eating the food.

Some people may develop a severe allergic reaction (anaphylaxis), which can be life threatening.

The most common type of allergic reaction to food is known as an IgE-mediated food allergy.

Symptoms include:

• tingling or itching in the mouth
• a raised, itchy red rash (hives) – in some cases, the skin can turn red and itchy, but without a raised rash
• swelling of the face, mouth (angioedema), throat or other areas of the body
• difficulty swallowing
• wheezing or shortness of breath
• feeling dizzy and lightheaded
• feeling sick (nausea) or vomiting
• abdominal pain or diarrhoea
• hay fever-like symptoms, such as sneezing or itchy eyes (allergic conjunctivitis)

Anaphylaxis

The symptoms of a severe allergic reaction (anaphylaxis) can be sudden and get worse very quickly.

Initial symptoms of anaphylaxis are often the same as those listed above and can lead to:

• swollen tongue
• breathing difficulties
• tight chest
• trouble swallowing or speaking
• feeling dizzy or faint
• collapse

Anaphylaxis is a medical emergency. Without quick treatment, it can be life threatening. If you think you or someone you know is experiencing anaphylaxis, dial 999 and ask for an ambulance as soon as possible.

Non-IgE-mediated food allergy

Another type of allergic reaction is a non-IgE-mediated food allergy. The symptoms of this type of allergy can take much longer to develop – sometimes up to several days.

Some symptoms of a non IgE-mediated food allergy may be what you would expect to see in an allergic reaction, such as:

• redness and itchiness of the skin – although not a raised, itchy red rash (hives)
• the skin becomes itchy, red, dry and cracked (atopic eczema)

Other symptoms can be much less obvious and are sometimes thought of as being caused by something other than an allergy. They include:

• vomiting with or without diarrhoea
• abdominal cramps
• constipation
• in babies: excessive and inconsolable crying, even though the baby is well fed and doesn't need a nappy change (colic).

Mixed reaction

Some children can have a mixed reaction where they experience both IgE symptoms, such as swelling, and non-IgE symptoms, such as constipation.

This can happen to children who have a milk allergy.

Exercise-induced food allergy

In some cases, a food allergy can be triggered after eating a certain food and then exercising. This can lead to anaphylaxis in severe cases, sometimes known as food-dependent exercise-induced anaphylaxis.

Drinking alcohol or taking an non-steroidal anti-inflammatory drug (NSAID) such as aspirin or ibuprofen may also trigger an allergy in people with this syndrome.

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

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

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.

Non-IgE-mediated food allergy

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.

Foods

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

• eggs
• 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
• 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
• 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
• 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's at risk?

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.

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 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.

Food additives

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.

Sulphites

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.

Sulphur dioxide is produced naturally when wine and beer are made, and is sometimes added to wine. 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 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.

Benzoates

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.

If you think you or your child has a food allergy, make an appointment with your GP.

They will ask you some questions about the pattern of your child's symptoms, such as:

• How long did it take for the symptoms to start after exposure to the food?
• How long did the symptoms last?
• How severe were the symptoms?
• Is this the first time these symptoms have occurred? If not, how often have they occurred?
• What food was involved and how much of it did your child eat?

They'll also want to know about your child's medical history, such as:

• Do they have any other allergies or allergic conditions?
• Is there a history of allergies in the family?
• Was (or is) your child breastfed or bottle-fed?

Your GP may also assess your child's weight and size to make sure they're growing at the expected rate.

Referral to an allergy clinic

If your GP suspects a food allergy, you may be referred to an allergy clinic for testing.

The tests needed can vary, depending on the type of allergy:

• if the symptoms developed quickly (an IgE-mediated food allergy) – you'll probably be given a skin-prick test or a blood test
• if the symptoms developed more slowly (non-IgE-mediated food allergy) – you'll probably be put on a food elimination diet

Skin-prick testing

During a skin-prick test, drops of standardised extracts of foods are placed on the arm. The skin is then pierced with a small lancet, which allows the allergen to come into contact with the cells of your immune system.

Credit:

GAVIN KINGCOME/SCIENCE PHOTO LIBRARY

Occasionally, your doctor may perform the test using a sample of the food thought to cause a reaction.

Itching, redness and swelling usually indicates a positive reaction. This test is usually painless.

A skin-prick test does have a small theoretical chance of causing anaphylaxis, but testing will be carried out where there are facilities to deal with this – usually an allergy clinic, hospital, or larger GP surgery.

Blood test

An alternative to a skin-prick test is a blood test, which measures the amount of allergic antibodies in the blood.

Food elimination diet

In a food elimination diet, the food thought to have caused the allergic reaction is withdrawn from your diet for 2 to 6 weeks. The food is then reintroduced.

If the symptoms go away when the food is withdrawn but return once the food is introduced again, this normally suggests a food allergy or intolerance.

Before starting the diet, you should be given advice from a dietitian on issues such as:

• the food and drinks you need to avoid
• how you should interpret food labels
• if any alternative sources of nutrition are needed
• how long the diet should last

Don't attempt a food elimination diet by yourself without discussing it with a qualified health professional.

Alternative tests

There are several shop-bought tests available that claim to detect allergies, but should be avoided.

They include:

• vega testing – claims to detect allergies by measuring changes in your electromagnetic field
• kinesiology testing – claims to detect food allergies by studying your muscle responses
• hair analysis – claims to detect food allergies by taking a sample of your hair and running a series of tests on it
• alternative blood tests (leukocytotoxic tests) – claim to detect food allergies by checking for the "swelling of white blood cells"

Many alternative testing kits are expensive, the scientific principles they are allegedly based on are unproven, and independent reviews have found them to be unreliable. They should therefore be avoided.

Questions to ask

If your child is diagnosed with a food allergy, or you're an adult who has just been diagnosed with a food allergy, you may want to ask questions such as:

• What type of allergy is it?
• What are the chances of having a severe allergic reaction?
• Will the allergy have an impact on other areas of my or my child's health, such as diet, nutrition and vaccination? Some vaccines contain traces of egg protein.
• Is my child likely to grow out of their allergy and, if so, when?

There are 2 main types of medication that can be used to relieve the symptoms of an allergic reaction to foods

• antihistamines – used to treat mild to moderate allergic reaction
• adrenaline – used to treat severe allergic reactions (anaphylaxis)

Antihistamines

Antihistamines work by blocking the effects of histamine, which is responsible for many of the symptoms of an allergic reaction.

Many antihistamines are available from your pharmacist without prescription – stock up in case of an emergency. Non-drowsy antihistamines are preferred.

Some antihistamines, such as alimemazine and promethazine, aren't suitable for children under the age of 2.

If you have a younger child with a food allergy, ask your GP about what types of antihistamines may be suitable.

Avoid drinking alcohol after taking an antihistamine as this can make you feel drowsy.

Adrenaline

Adrenaline works by narrowing the blood vessels to counteract the effects of low blood pressure and opening up the airways to help ease breathing difficulties.

You'll be given an auto-injector of adrenaline to use in case of emergencies if you or your child is at risk of anaphylaxis or had a previous episode of anaphylaxis.

Read the manufacturer's instructions that come with the auto-injector carefully and train your child how to use it when they are old enough.

Using an auto-injector

If you suspect that somebody is experiencing a severe allergic reaction, call 999 and ask for an ambulance. Tell the operator that you think the person has anaphylaxis.

Older children and adults will probably have been trained to inject themselves. You may need to inject younger children or older children and adults who are too sick to inject themselves.

There are 3 types of auto-injectors:

• EpiPen
• Jext
• Emerade

They all work in much the same way. If anaphylaxis is suspected, you should remove the safety cap from the injector and press firmly against the thigh, holding it at a right angle, without using the thumb at the end.

A "click" indicates the auto-injector has been activated, and it should be held in place for 10 seconds. Ensure you're familiar with the device and know the correct end to place against the thigh.

The injections can be given through clothing. This will send a needle into your thigh and deliver a dose of adrenaline.

If the person is unconscious, check their airways are open and clear, and check their breathing. Then put them in the recovery position. Putting someone who is unconscious in the recovery position ensures they don't choke if they were to vomit.

Place the person on their side, making sure they're supported by one leg and one arm. Open the airway by tilting the head and lifting the chin.

If the person's breathing or heart stops, cardiopulmonary resuscitation (CPR) should be performed.

Owning an auto-injector

As a precaution, the following advice should be taken: 

• Carry the auto-injector at all times or encourage your child to do so if they're old enough. You may be prescribed 2 injectors – check with your GP or the doctor in charge of your care. You may also be given an emergency card or bracelet with full details of your child's allergy and the contact details of their doctor to alert others. They should wear this at all times.
• Extreme temperatures can make adrenaline less effective. Don't leave an auto-injector in places like your fridge or the glove compartment of your car.
• Check the expiry date regularly.  An out-of-date injector will only offer limited protection.
• The manufacturers offer a reminder service, where you can be contacted near the date of expiry. Check the information leaflet that comes with the medication for more information.
• If your child has an auto-injector, they'll need to change over to an adult dose once they reach a weight of 30 kilos (4.7 stone). Depending on the shape and size of your child's body, this could be anywhere between the ages of 5 and 11 years old.
• Don't delay injecting if you think you or your child may be experiencing the start of anaphylaxis, even if the initial symptoms are mild. It's better to use adrenaline early and find out it was a false alarm than delay treatment until you're sure your child is experiencing severe anaphylaxis.

The advice here is primarily written for parents of a child with a food allergy. However, most of it is also relevant if you're an adult with a food allergy.

Your child's diet

There's currently no cure for food allergies, although many children will grow out of certain ones, such as allergies to milk and eggs.

The most effective way you can prevent symptoms is to remove the offending food – known as an allergen – from their diet.

However, it's important to check with your GP or the doctor in charge of your child's care first before eliminating certain foods.

Removing eggs or peanuts from a child's diet is not going to have much of an impact on their nutrition. Both of these foods are a good source of protein, but this can be replaced by alternative sources.

A milk allergy can have more of an impact, as milk is a good source of calcium, but there are many other ways you can incorporate calcium into your child's diet, such as with green leafy vegetables. Many foods and drinks are also fortified with extra calcium.

See your GP if you're concerned that your child's food allergy is affecting their growth and development.

Reading labels

It's very important to always check the ingredients list on any pre-packed food or drinks your child has.

Under EU law, any pre-packed food or drink sold in the UK must clearly state on the label if it contains any of these 14 ingredients that can cause an allergy:

• celery
• cereals that contain gluten – including wheat, rye, barley and oats
• crustaceans – including prawns, crabs and lobsters
• eggs
• fish
• lupin (common garden plants) – seeds from some varieties are sometimes used to make flour
• milk
• molluscs – including mussels and oysters
• mustard
• tree nuts – such as almonds, hazelnuts, walnuts, brazil nuts, cashews, pecans, pistachios and macadamia nuts
• peanuts
• sesame seeds
• soybeans
• sulphur dioxide and sulphites (preservatives used in some foods and drinks) – at levels above 10mg per kg or per litre

Also look out for voluntary "may contain" labels, such as "may contain traces of peanut". Manufacturers sometimes put this label on their products to warn consumers that they may have become accidentally contaminated with another food product during the production process.

Be careful with foods labelled as vegan. There is no legal definition of vegan, which means that foods labelled vegan are not always completely free of animal products. If you have food allergies to milk, eggs, fish, crustaceans or molluscs, read the ingredients list carefully on vegan food products. And look out for labels with warnings such as "may contain".

Read more detailed information about allergen labelling on the Food Standards Agency website.

Some non-food products contain allergy-causing food:

• some soaps and shampoos contain soy, egg and tree nut oil
• some pet foods contain milk and peanuts
• some glues and adhesive labels used on envelopes and stamps contain traces of wheat

Again, read the labels of any non-food products your child may come into close physical contact with.

Unpackaged food

Currently, unpackaged food does not need to be labelled in the same way as packaged food. The law requires food businesses to tell customers if their food products contain any of the 14 allergens, but this can be done in different ways.

A business could provide full allergen information on a menu, or they could have a sign advising customers to ask a member of staff for this information. This can make it more difficult to know what ingredients are in a particular food.

Examples of unpackaged food include food sold in:

• bakeries – including in-store bakeries in supermarkets
• delis
• buffets
• salad bars
• "ready-to-eat" sandwich shops
• takeaways
• cafes, canteens and restaurants
• market stalls

If you or your child have a severe food allergy, you need to be careful when you eat out.

The following advice should help:

• let the staff know – when booking a table at a restaurant, make sure the staff know about any allergies. Ask for a firm guarantee that the specific food will not be in any of the dishes served. The Food Standards Agency (FSA) offers chef cards that provide information about allergies, which you can give to restaurant staff. As well as informing the chef and kitchen staff involved in cooking your food, let waiters and waitresses know so they understand the importance of avoiding cross-contamination when serving you.
• read the menu carefully and check for hidden ingredients – some food types contain other foods that can trigger allergies, which restaurant staff may have overlooked. Some desserts contain nuts (such as a cheesecake base) and some sauces contain wheat and peanuts.
• prepare for the worst – it's a good idea to prepare for any eventuality. Always take anti-allergy medicine with you when eating out, particularly an adrenalin auto-injector. Read more about using an auto-injector on our page about treating food allergies.
• use what's known as a taste test in older children – before your child begins to eat, ask them to take a tiny portion of the food and rub it against their lips to see if they experience a tingling or burning sensation. If they do, it suggests that the food will cause them to have an allergic reaction. However, the taste test does not work for all foods, so it should not be used as a substitute for the above advice.

Further advice

Here's some more advice for parents: 

• notify your child's school about their allergy – depending on how severe their allergy is, it may be necessary to give the staff at their school an emergency action plan in case of accidental exposure. Arrange for the school nurse or another staff member to hold a supply of adrenalin. Food allergy bracelets, which explain how other people can help your child in an emergency, are also available. Both the Anaphylaxis Campaign and Allergy UK provide advice on working with schools if your child has an allergy.
• let other parents know – young children may easily forget about their food allergy and accept food they should not have when visiting other children. Telling the parents of your child's friends about their allergy should help prevent this.
• educate your child – once your child is old enough to understand their allergy, it's important to give them clear, simple instructions about what foods to avoid and what they should do if they accidentally eat them.

Can food allergies be prevented?

It used to be thought that avoiding eating peanuts during pregnancy and when breastfeeding could help reduce the risk, but this theory has now been questioned.

There's some evidence that introducing peanuts early in life may reduce the risk of peanut allergy, but this may not apply to all children and requires confirmation from further studies.

It's important to follow the standard recommendations for pregnancy and breastfeeding, whether or not you have a family history of food allergies.

Further information

• Foods to avoid in pregnancy
• Breastfeeding and diet
• Food allergies in babies and young children