Allergies develop when the immune system treats something ordinarily harmless - pollen, pet fur, dust mites, particular foods or an insect sting - as a threat and mounts a defensive reaction against it. The result can be anything from a bout of sneezing and an itchy rash through to swelling and breathing difficulties in more serious cases.
For most people, a mild allergy is kept in check with antihistamines bought from a pharmacy and by simply avoiding the trigger once it's identified. A genuinely severe reaction - anaphylaxis - is different: it needs emergency treatment straight away, so anyone at risk should carry the medication they've been prescribed and know how to use it. Getting a proper diagnosis from a GP or allergy clinic is far more reliable than trying to work out the cause yourself, especially with food or drug allergies.
Our page on hay fever covers one of the most widespread seasonal allergies in more detail.
Common questions
What causes an allergic reaction?
An allergic reaction occurs when the body's immune system mistakes a harmless substance - such as pollen, pet fur, dust mites or a particular food - for a threat and overreacts to it.
How urgent is a severe allergic reaction?
A severe reaction, known as anaphylaxis, is a medical emergency requiring immediate treatment. Milder allergies are usually kept in check with antihistamines and by steering clear of known triggers.