The substance in a food that causes an allergic reaction in certain people is called an allergen. Allergens are normally proteins, and there is usually more than one kind of allergen in each food.
Most allergic reactions to food are mild, but sometimes they can be very serious. If someone has a food allergy they can react to just a tiny amount of the food they are sensitive to.
The symptoms of an allergic reaction can vary and the reactions can be more or less severe on different occasions. Even if you try very hard to avoid the food that you are sensitive to, you may still eat it by accident.
The most common symptoms of an allergic reaction include:
coughing
dry, itchy throat and tongue
itchy skin or rash
nausea and feeling bloated
diarrhoea and/or vomiting
wheezing and shortness of breath
swelling of the lips and throat
runny or blocked nose
sore, red and itchy eyes
Symptoms can appear within minutes, or up to several hours after someone has eaten the food they are allergic to. Generally, you won't experience all of these symptoms at the same time and bear in mind that some of the symptoms of an allergic reaction can also be symptoms of other illnesses.
People with severe allergies can have a reaction called anaphylaxis (pronounced anna-fill-axis), sometimes called anaphylactic shock. When someone has an anaphylactic reaction, they can have symptoms in different parts of the body at the same time, including rashes, swelling of the lips and throat, difficulty breathing and a rapid fall in blood pressure and loss of consciousness.
Anaphylaxis can be fatal if it isn't treated immediately, usually with an injection of adrenaline (epinephrine). This is why it's extremely important for someone with a severe allergy to take their medication with them wherever they go.
The first symptoms of anaphylaxis can develop within minutes of eating the food, but other symptoms, such as severe breathing difficulties, can develop up to several hours later. Many anaphylactic reactions can be misleadingly mild at first, so it's better to be cautious and not underestimate the danger. People with severe allergies who also have asthma are more likely to have a severe reaction affecting the lungs.
Anaphylaxis can also be caused by other things, such as bee and wasp stings, and drug allergy, but food allergy is one of the most common causes. In the UK and Europe, peanuts, milk, eggs and fish are the most common foods to cause anaphylaxis. Nuts, sesame seeds and shellfish can also cause it.
Certain foods, particularly fruit and vegetables, can cause reactions such as itching or rashes when they touch the lips and mouth. This is called oral allergy syndrome and is a symptom of food allergy. These reactions usually happen in people who have hayfever and are sensitive to pollen, for example pollen from birch, grass, or plants in the daisy family such as ragweed and mugwort. This is because the allergens in these types of pollen are also found in some fruit and vegetables.
For example, a person who is sensitive to ragweed pollen may develop rashes or blisters on the lips when they eat melon, and someone who is sensitive to birch tree pollen may react to apples. Cooking often destroys the allergens that cause this kind of reaction in fresh fruit and vegetables so, for example, people who react to raw apples might be able to eat cooked apples.
Exercise-induced food allergy
This is an unusual condition where someone has an allergic reaction when they take exercise within a couple of hours of eating a particular food. People who are sensitive in this way may normally be able to eat the food with only a mild reaction, or no reaction at all, but they can have a severe reaction (including anaphylaxis) if they eat it just before they exercise. Experts don't fully understand why some people react this way.
Sometimes, if someone has an allergy to one thing, they can develop an allergy to other things. This can happen because they have a tendency to develop allergies (which is known as being atopic). In this case they can react to a number of unrelated allergens, for example peanuts and cats. Other people can react to different foods that contain either the same allergen or an allergen with a very similar structure, which means they can cause similar allergic reactions. This is known as allergic cross-reactivity.
Allergic cross-reactivity means that someone may suffer an allergic reaction even when they're avoiding the foods they know they're allergic to. For example, if someone is allergic to peanuts, they might react to other foods in the legume family such as soya, lupins, peas, lentils and beans. Allergic cross-reactions can also happen between certain fruit or vegetables and latex (known as latex-food syndrome), or the pollens that cause hayfever.
There isn't a cure for food allergy or intolerance, so the only way to prevent a reaction is to avoid the food you are sensitive to.
Many children with food allergy grow out of it. However, people with allergies to peanuts, nuts, fish and shellfish normally have the condition for life.
It's important for anyone who thinks they might have a food intolerance or a food allergy to talk about it to their GP, who can then refer them for tests and a full diagnosis, if appropriate. Once the diagnosis has been made, they should avoid the food or foods they are sensitive to.
If you have a severe food allergy, you will probably be prescribed antihistamines and a pen containing adrenaline (known as epinephrine). This is to inject yourself if you have a reaction. Never go anywhere without your medication.
If you, or your child, are prescribed an adrenaline pen, your health professional will show you how to use it. If you're not sure what to do, ask them for advice.
People who have a severe food allergy should also wear a bracelet or necklace giving details of their allergy, so medical staff will know about it in an emergency.
Make sure your family, friends and work colleagues know about your food allergy and what to look out for when buying food for you. If you have a severe allergy, you should also make sure they know what to do if you have a reaction.
If you have a child who has been diagnosed with a food allergy, it's especially important that you and anyone who looks after them (including teachers and the parents of friends who they might visit) know how to avoid the foods they are sensitive to and what to do if they have an allergic reaction, including how to use their adrenaline pen.
Schools should have plans to deal with an emergency, but you should discuss your child's needs in detail with their school.
We don't know exactly why some people have a food allergy and others don't. But someone is more likely to develop a food allergy if they have a parent, brother or sister with an allergy of any type.
Children under three years old are more likely to develop allergies than adults. Many food allergies begin in childhood but disappear as the child gets older. However, some people never outgrow their allergy, and others develop a food allergy as adults.
Eating habits, for example how often a person has eaten a particular food, are also thought to be important. This might be why allergies to particular foods are more common in countries where they are eaten a lot, for example, fish allergy in Scandinavian countries, rice allergy in Japan and peanut allergy in the US. The way a food is prepared may also be important. For example, people with peanut allergy can react to smaller amounts of roasted peanuts than raw or boiled peanuts.
But eating habits don't seem to be the whole story. For example, peanut allergy is either very rare or unheard of in Indonesia and parts of Africa, even though peanuts are commonly eaten in these countries. Some experts believe that allergy is now more common in some countries than others because of lifestyle. People in more affluent westernised countries seem to be more likely to develop all types of allergy than people in developing countries.
The number of people who suffer from food allergy in developed countries seems to have increased in recent years, but we don't have definite information about this. If food allergy has been increasing, this would match recent increases in allergic diseases such as asthma, eczema and hayfever.
We don't know exactly how many people in the UK have a food allergy. About 20 to 30% believe they are intolerant to one or more foods. However, tests show that only about 1 to 2% of the adult population in the UK has a food allergy - about 1 million people. We think about 10 deaths a year in the UK are caused by food allergy.
Visit the Allergy UK website