Health A to Z
An allergy is a reaction the body has to a particular food or substance.
Allergies are very common. They're thought to affect more than one in four people in the UK at some point in their lives.
They are particularly common in children. Some allergies go away as a child gets older, although many are lifelong. Adults can develop allergies to things they weren't previously allergic to.
Having an allergy can be a nuisance and affect your everyday activities, but most allergic reactions are mild and can be largely kept under control. Severe reactions can occasionally occur, but these are uncommon.
Substances that cause allergic reactions are called allergens. The more common allergens include:
Most of these allergens are generally harmless to people who aren't allergic to them.
Allergic reactions usually happen quickly within a few minutes of exposure to an allergen.
They can cause:
Most allergic reactions are mild, but occasionally a severe reaction called anaphylaxis or anaphylactic shock can occur. This is a medical emergency and needs urgent treatment.
Read more about the symptoms of allergies.
See your GP if you think you or your child might have had an allergic reaction to something.
The symptoms of an allergic reaction can also be caused by other conditions. Your GP can help determine whether it's likely you have an allergy.
If your GP thinks you might have a mild allergy, they can offer advice and treatment to help manage the condition.
If your allergy is particularly severe or it's not clear what you're allergic to, your GP may refer you to an allergy specialist for testing and advice about treatment.
Read more about allergy testing.
In many cases, the most effective way of managing an allergy is to avoid the allergen that causes the reaction whenever possible.
For example, if you have a food allergy, you should check a food's ingredients list for allergens before eating it. The Food Standards Agency has more information about food allergen labelling.
There are also several medications available to help control symptoms of allergic reactions, including:
For some people with very severe allergies, a treatment called immunotherapy may be recommended.
This involves being exposed to the allergen in a controlled way over a number of years, so your body gets used to it and doesn't react to it so severely.
Allergies occur when the body's immune system reacts to a particular substance as though it's harmful.
It's not clear why this happens, but most people affected have a family history of allergies or have closely related conditions such as asthma or eczema.
The number of people with allergies is increasing every year. The reasons for this are not understood, but one of the main theories is it's the result of living in a cleaner, germ-free environment, which reduces the number of germs our immune system has to deal with.
It's thought this may cause it to overreact when it comes into contact with harmless substances.
Symptoms of an allergic reaction usually develop within a few minutes of being exposed to something you're allergic to, although occasionally they can develop gradually over a few hours.
Although allergic reactions can be a nuisance and hamper your normal activities, most are mild. Very occasionally, a severe reaction called anaphylaxis can occur.
Common symptoms of an allergic reaction include:
The symptoms vary depending on what you're allergic to and how you come into contact with it. For example, you may have a runny nose if exposed to pollen, develop a rash if you have a skin allergy, or feel sick if you eat something you're allergic to.
See your GP if you or your child might have had an allergic reaction to something. They can help determine whether the symptoms are caused by an allergy or another condition. Read more about diagnosing allergies.
In rare cases, an allergy can lead to a severe allergic reaction, called anaphylaxis or anaphylactic shock, which can be life-threatening.
This affects the whole body and usually develops within minutes of exposure to something you're allergic to.
Signs of anaphylaxis include any of the symptoms above, as well as:
Anaphylaxis is a medical emergency that requires immediate treatment. Read more about anaphylaxis for information about what to do if it occurs.
If you think you have an allergy, tell your GP about the symptoms you're having, when they happen, how often they occur and if anything seems to trigger them.
Your GP can offer advice and treatment for mild allergies with a clear cause.
If your allergy is more severe or it's not obvious what you're allergic to, you may be referred for allergy testing at a specialist allergy clinic.
The tests that may be carried out are described below.
Skin prick testing is one of the most common allergy tests.
It involves putting a drop of liquid onto your forearm that contains a substance you may be allergic to. The skin under the drop is then gently pricked with a needle. If you are allergic to the substance, an itchy, red bump will appear within 15 minutes.
Skin prick testing is painless and very safe. Make sure you don't take antihistamines before the test, as they can interfere with the results.
Blood tests may be used instead of, or alongside, skin prick tests to help diagnose common allergies.
A sample of your blood is removed and analysed for specific antibodies produced by your immune system in response to an allergen.
Patch tests are used to investigate a type of eczema known as contact dermatitis, which can be caused by your skin being exposed to an allergen.
A small amount of the suspected allergen is added to special metal discs, which are then taped to your skin for 48 hours and monitored for a reaction.
If you have a suspected food allergy, you may be advised to avoid eating a particular food to see if your symptoms improve.
After a few weeks, you may then be asked to eat the food again to check if you have another reaction.
Don't attempt to do this yourself without discussing it with a qualified healthcare professional.
In a few cases, a test called a food challenge may also be used to diagnose a food allergy.
During the test, you're given the food you think you're allergic to in gradually increasing amounts, to see how you react under close supervision.
This test is riskier than other forms of testing, as it could cause a severe reaction, but is the most accurate way to diagnose food allergies. And challenge testing is always carried out in a clinic where a severe reaction can be treated if it does develop.
The treatment for an allergy depends on what you're allergic to. In many cases, your GP will be able to offer advice and treatment.
They'll advise you about taking steps to avoid exposure to the substance you're allergic to, and can recommend medication to control your symptoms.
The best way to keep your symptoms under control is often to avoid the things you're allergic to, although this isn't always practical.
For example, you may be able to help manage:
Read more about preventing allergic reactions.
Medications for mild allergies are available from pharmacies without a prescription, but always ask your pharmacist or GP for advice before starting any new medicine, as they're not suitable for everyone.
Antihistamines are the main medicines for allergies. They can be used:
Antihistamines can be taken as tablets, capsules, creams, liquids, eye drops or nasal sprays, depending on which part of your body is affected by your allergy.
Decongestants can be used as a short-term treatment for a blocked nose caused by an allergic reaction.
They can be taken as tablets, capsules, nasal sprays or liquids. Don't use them for more than a week at a time, as using them for long periods can make your symptoms worse.
Red and itchy skin caused by an allergic reaction can sometimes be treated with over-the-counter creams and lotions, such as:
Steroid medications can help reduce inflammation caused by an allergic reaction. They're available as:
Sprays, drops and weak steroid creams are available without a prescription. Stronger creams, inhalers and tablets are available on prescription from your GP.
Immunotherapy may be an option for a small number of people with certain severe and persistent allergies who are unable to control their symptoms using the measures above.
The treatment involves being given occasional small doses of the allergen – either as an injection, or as drops or tablets under the tongue – over the course of several years.
The injection can only be performed in a specialist clinic under the supervision of a doctor, as there is a small risk of a severe reaction. The drops or tablets can usually be taken at home.
The aim of treatment is to help your body get used to the allergen so it doesn't react to it so severely. This won't necessarily cure your allergy, but it will make it milder and mean you can take less medication.
Some people with severe allergies may experience life-threatening reactions, known as anaphylaxis or anaphylactic shock.
If you're at risk of this, you'll be given special injectors containing a medicine called adrenaline to use in an emergency.
If you develop symptoms of anaphylaxis, such as difficulty breathing, you should inject yourself in the outer thigh before seeking emergency medical help.
Read more about the treating anaphylaxis.
The best way to prevent an allergic reaction is to avoid the substance that you're allergic to, although this isn't always easy or practical.
Below is some practical advice that should help you avoid the most common allergens.
One of the biggest causes of allergies are dust mites, which are tiny insects found in household dust. You can limit the number of mites in your home by:
Concentrate your efforts of controlling dust mites in the areas of your home where you spend the most time, such as the bedroom and living room.
You can find more information on allergies in the home on the Allergy UK website.
It's not the pet fur that causes an allergic reaction. Instead, it's flakes of their dead skin, saliva and dried urine.
If you can't permanently remove a pet from the house, you could try:
If you're visiting a friend or relative with a pet, ask them not to dust or vacuum on the day you're visiting, as this will stir up the allergens into the air. Taking an antihistamine medicine about an hour before entering a pet-inhabited house can also help reduce your symptoms.
The Allergy UK website has more information about domestic pet allergies.
Tiny particles released by moulds can cause an allergic reaction in some people.
You can help prevent this by:
By law, food manufacturers must clearly label any foods that contain something that's known to cause allergic reactions in some people. By carefully checking the label for the list of ingredients, you should be able to avoid an allergic reaction.
People with food allergies most often experience an allergic reaction while eating out at a restaurant. You can avoid this by:
Remember, simple dishes are less likely to contain "hidden" ingredients. If you're not sure about a dish, don’t risk it
Pollen allergies – more commonly known as hay fever – are caused when trees and grasses release pollen into the air. Doctors often call hay fever allergic rhinitis.
Different plants pollinate at different times of the year, so the months that you get hay fever will depend on what sort of pollen(s) you are allergic to. Typically, people are affected during spring (trees) and summer (grasses).
To help keep your hay fever under control, you can:
Read more about preventing hay fever.
If you've ever suffered a bad reaction to an insect bite or sting, it's important to take precautions to minimise your risk.
When you're outdoors, particularly in the summer, you could:
Read more about preventing insect bites and stings.
If you're at risk of experiencing a severe allergic reaction (anaphylaxis), make sure you carry two adrenaline auto-injectors with you everywhere.
Wearing a MedicAlert or Medi-Tag medallion or bracelet can make others aware of your allergy in an emergency.
Consider telling your teachers, work colleagues and friends, so they can give you your adrenaline injection in an emergency, while waiting for an ambulance.
Read more about preventing anaphylaxis.