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 1 in 4 people in the UK at some point in their lives.
They're particularly common in children. Some allergies go away as a child gets older, although many are lifelong.
Adults can develop allergies to things they were not 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 are not allergic to them.
Symptoms of an allergic reaction
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.
Getting help for allergies
See a 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.
A GP can help determine whether it's likely you have an allergy.
If they think 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, they may refer you to an allergy specialist for testing and advice about treatment.
Find out more about allergy testing
How to manage an allergy
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.
There are also several medicines 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 does not react to it so severely.
What causes allergies?
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 1 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.
Is it an allergy, sensitivity or intolerance?
A reaction produced by the body's immune system when exposed to a normally harmless substance.
The exaggeration of the normal effects of a substance. For example, the caffeine in a cup of coffee may cause extreme symptoms, such as palpitations and trembling.
Where a substance causes unpleasant symptoms, such as diarrhoea, but does not involve the immune system.
People with an intolerance to certain foods can typically eat a small amount without having any problems.
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.
Main allergy symptoms
Common symptoms of an allergic reaction include:
Itchy, red, watering eyes
Raised, itchy, red rash (hives)
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.
Severe allergic reaction (anaphylaxis)
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.
Find your nearest NHS allergy clinic
The tests that may be carried out are described on this page.
Skin prick testing
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.
If you're allergic to the substance, an itchy, red bump will appear within 15 minutes.
Most people find skin prick testing not particularly painful, but it can be a little uncomfortable. It's also very safe.
Make sure you do not 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.
Do not 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.
Allergy testing kits
The use of commercial allergy-testing kits isn't recommended.
These tests are often of a lower standard than those provided by the NHS or accredited private clinics, and are generally considered to be unreliable.
Allergy tests should be interpreted by a qualified professional who has detailed knowledge of your symptoms and medical history.
The treatment for an allergy depends on what you're allergic to. In many cases, a 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 medicines to control your symptoms.
Avoiding exposure to allergens
The best way to keep your symptoms under control is often to avoid the things you're allergic to, although this is not always practical.
For example, you may be able to help manage:
Medicines for mild allergies are available from pharmacies without a prescription.
But always ask a 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.
Do not use them for more than a week at a time, as using them for long periods can make your symptoms worse.
Lotions and creams
Red and itchy skin caused by an allergic reaction can sometimes be treated with over-the-counter creams and lotions, such as:
Steroid medicines 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 a 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's 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 does not react to it so severely.
This will not necessarily cure your allergy, but it'll make it milder and mean you can take less medicine.
Treating severe allergic reactions (anaphylaxis)
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.
Treating specific allergic conditions
Use the links below to find information about how specific allergies and related conditions are treated: