An allergy is an adverse reaction that the body has to a particular food or substance in the environment.

Most substances that cause allergies are not harmful and have no effect on people who are not allergic.

The allergic response

Any substance that triggers an allergic reaction is called an allergen. Some of the most common allergens include:

• grass and tree pollen (hay fever)
• dust mites
• animal dander (tiny flakes of skin or hair)
• food allergy (particularly fruits, shellfish and nuts)

An allergy develops when the body’s immune system reacts to an allergen as though it is a threat, like an infection. It produces antibodies to fight off the allergen, in a reaction called the "immune response". 

The next time a person comes into contact with the allergen, the body "remembers" the previous exposure and produces more of the antibodies. This causes the release of chemicals in the body that lead to an allergic reaction.

Symptoms of an allergy can include sneezing, wheezing, itchy eyes, skin rashes and swelling.

The nature of the symptoms depend on the allergen. For example, you may experience problems with your airways if you breathe in pollen.

Seeing your GP 

If you think you have an allergy, see your GP.

Depending on your symptoms, the condition of your skin and any medication you are taking, you may be offered further tests to identify the allergen.

Read more about the diagnosis of allergies.

How common are allergies?

Allergies are very common. According to Allergy UK, one in four people in the UK suffers from an allergy at some point in their lives. The numbers are increasing every year and up to half of those affected are children.

The reason for the rise is unclear. Some experts believe it is associated with pollution. Another theory is that allergies are caused by living in a cleaner, germ-free environment, which reduces the number of germs our immune system has to deal with. This causes it to overreact when it comes into contact with harmless substances.

Read more about the common causes of allergies.

Managing an allergy

In some cases, the most effective way of managing an allergy is to avoid all contact with the allergen causing the reaction.

There are also several medications available to help control the common symptoms of many allergies.

Read more about treating an allergy and preventing an allergic reaction.