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, they 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 aren't 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.
allergy – a reaction produced by the body's immune system when exposed to a normally harmless substance
sensitivity – 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
intolerance – where a substance causes unpleasant symptoms, such as diarrhoea, but doesn't involve the immune system; people with an intolerance to certain foods can typically eat a small amount without having any problems