Food allergy

Published by Bupa's Health Information Team, September 2010.

This factsheet is for people who have a food allergy, or who would like information about them.

A food allergy is when the body's immune system mistakes particular foods, for example eggs or nuts, as harmful. Most food allergy reactions are mild, but sometimes they can be very serious and even fatal.

About food allergies

In the UK, food allergies affect about one to two people in every 100. Food allergy is sometimes confused with food intolerance or food poisoning. These can make you feel ill but they aren't usually harmful in the same way that a true food allergy might be.

Foods that cause allergies include peanuts, tree nuts (Brazils, almonds, cashews, hazelnuts, walnuts and pecans), fish and shellfish, cows' milk, eggs, soya and wheat.

Food allergies in young children are more common than in adults, but many children outgrow them by school-age. It's possible to develop a food allergy as an adult, even if you never had allergies as a child.

If you have one food allergy, you may also react to other foods. For instance, if you're allergic to prawns, other shellfish may also affect you. People with hay fever or an allergy to latex may also find their allergic reaction crosses over and happens with certain foods. This is called cross-reactivity.

Food and the immune system

If you have an allergy, it means that your body's immune system reacts to an allergen - a substance that causes an allergic reaction. An allergic reaction happens when your body's immune system mistakes the allergen, for example nuts, for a harmful invader and produces antibodies. This is known as sensitisation and may not cause symptoms. However, the next time you eat the food, the antibodies are ready to react with it. This causes your body to release chemicals, which leads to a range of physical symptoms known as an allergic reaction. Allergens aren't usually harmful and most people aren't sensitive to them.

Symptoms of food allergy

Some food allergies result in immediate, severe and even life-threatening symptoms (such as severe peanut allergy), whereas others cause symptoms that may take longer to develop (for example, gluten allergy, also known as coeliac disease).

If you have a food allergy, you may have symptoms that include:

• itchy or swollen lips, mouth, tongue and throat
  
• skin reactions (eg swelling and itching, a rash around your mouth, eczema and flushing)
  
• wheezing or shortness of breath
  
• diarrhoea, feeling sick, vomiting and bloating
  
• coughing
  
• a runny nose
  
• sore, red and itchy eyes

These symptoms may be caused by problems other than a food allergy. If you have any of these symptoms, see your doctor for advice.

If you're severely allergic to nuts or peanuts, your allergy may be triggered just by touching or being near someone who is eating them or has done recently. You may also have a reaction as a result of being in the same room where nuts have been. Using cutlery or crockery that has been contaminated with traces of nuts may also cause an allergic reaction.

Complications of food allergy

Some people develop a severe, whole-body allergic reaction called anaphylaxis, or anaphylactic shock; a rare but potentially fatal allergic reaction. Symptoms can include dizziness, a rapid pulse, a rapid drop in blood pressure and swelling of the airways and throat, making it difficult to breathe. If this happens you could lose consciousness if not treated.

Causes of food allergy

The exact reasons why some people are allergic to various foods aren't fully understood.

For some people with a food allergy, sensitivity may develop before birth while they are in their mother's womb (uterus). It's also possible that this may happen as a result of breastfeeding (although this isn't the case for peanuts). Ask your doctor or your midwife or health visitor for information.

If you have other allergies or conditions such as eczema or asthma, you may be more likely to develop a food allergy. If someone else in your family has these conditions, this can also increase your risk of having an allergy, although it may not be to the same thing.

There is some evidence that some babies may become sensitised to nuts as a result of being treated with skin treatments for eczema or other allergic conditions that contain peanut oil, or from peanut oil found in certain sorts of formula milk and nipple creams.

Diagnosis of food allergy

Your doctor will ask about your symptoms and examine you. He or she may also ask you about your medical history.

If an allergy seems likely, your GP may refer you to a specialist allergy clinic for tests that may include the following.

• Skin prick tests - an extract of the suspect food is put on a small patch of skin, usually on your forearm or back, and a very small, fine scratch is made. If redness and swelling develops around the scratch, the test is positive for that food.
  
• Elimination and challenge diets - where you remove the suspected allergy food from your diet and replace it with another food. If your symptoms improve, a diagnosis can usually be made. To confirm this diagnosis, your doctor may ask you to re-introduce the food back into your diet. If you have had severe allergy symptoms in the past, this will be done under medical supervision in hospital.
  
• Blood tests - these are useful if you have a severe food allergy and are at risk of anaphylaxis from the tests above or if you have extensive eczema or dermatitis and can't have a skin test. The RAST (radioallergosorbent) test measures levels of food-specific antibodies in your blood.

Treatment of food allergy

Some people outgrow their food allergies (especially children) but this is less likely with allergies to nuts and fish.

There isn't a cure for food allergy so the only way to prevent a reaction is to avoid the food you're allergic to.

If you have a mild food allergy, it may be possible to treat this with antihistamines. However, it's important that you only take medicines for your allergy on the advice of your doctor. Always read the patient information leaflet that comes with your medicine and if you have any questions, ask your pharmacist for advice.

Emergency treatment

If you have a severe food allergy, you may be at risk of anaphylaxis and will need to take certain precautions. Despite reading menus and ingredients lists carefully, it's easy to eat a problem food by accident. Even a tiny amount of a problem food can trigger a severe reaction.

• Ask your doctor for advice about what to do if you have an allergic reaction.
  
• Let your family, friends and work colleagues know about your allergy, and how they can help you in the event of an emergency.
  
• Wear a bracelet or carry a card that gives details about your allergy.
  
• Carry a pre-loaded adrenaline syringe. Adrenaline is a hormone (a chemical that occurs naturally in your body) that relaxes muscles and so helps to reduce any swelling and make breathing easier. Single doses of adrenaline are available in the form of an EpiPen or AnaPen that consist of a sterile syringe of adrenaline that is ready to be used in an emergency. You will need to go to hospital immediately if you have anaphylaxis, even if using adrenaline has helped to reduce the reaction.
  
• If your child has a severe allergy, you must make sure that all their carers and teachers have the information to react correctly in an emergency.

Living with a food allergy

If you have a food allergy, it's important to check the packaging of foods carefully to find out if they contain what you're allergic to. There are laws that food manufacturers have to work to that mean that all pre-packed food sold in the UK and the European Union must say on the label whether it contains an allergen, such as eggs or nuts. Attempts are being made to extend this to other foods that aren't pre-packed.

If you're allergic to eggs, you won't be able to have the vaccine for yellow fever. Your GP is also likely to advise you not to have the flu vaccine.

For answers to frequently asked questions on this topic, see Common questions.

For sources and links to further information, see Resources.