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:
grass and tree pollen – an allergy to these is known as hay fever (allergic rhinitis)
animal dander (tiny flakes of skin or hair)
food – particularly nuts, fruit, shellfish, eggs and cow's milk
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 allergies
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.
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.
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.
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 isn't always practical.