Introduction 

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.




Hay fever advice

Hay fever is an allergy to pollen that affects around one in four people. An expert explains how it's diagnosed, the symptoms and treatment.

Media last reviewed: 19/03/2013

Next review due: 19/03/2015

Allergy, sensitivity or intolerance

Allergy  this is a reaction produced by the body’s immune system when it encounters a normally harmless substance.

Sensitivity  this is the exaggeration of a normal side effect produced by contact with a substance. For example, the caffeine in a cup of coffee may cause extreme symptoms, such as palpitations and trembling, when it would usually only have this effect when taken in much larger doses.

Intolerance  this is where a substance (such as lactose) causes unpleasant symptoms (such as diarrhoea) for a variety of reasons, but does not involve the immune system. People with an intolerance to certain foods can typically eat a small amount without having any problems. In contrast, people with a food allergy will have a bad reaction even if they come into contact with a tiny amount of the food to which they are allergic.

Page last reviewed: 01/04/2014

Next review due: 01/04/2016