Allergy is the word used to describe an adverse (bad) reaction that the body has to a particular substance in the environment. Most substances that cause allergies are not harmful and have no effect on people who are not allergic.
Any substance that triggers an allergic reaction is called an allergen. Some of the most common allergens include pollen, house dust mites, mould and pets. Less common allergens include nuts, fruit and latex.
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.
Common allergic disorders include asthma, eczema and hay fever. Symptoms can include sneezing; wheezing, coughing and skin rashes (see Allergies - Symptoms for more information about allergic reactions).
The symptoms depend on how you came into contact with the allergen. For example, you may experience problems with your airways if you breathe in pollen.
Allergies are very common. According to the British Allergy Foundation, one in three people in the UK suffer from an allergy at some time in their lives. The numbers are increasing every year and as many as 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 (see Allergies - Causes for more information on the common causes of allergies).
Allergic reactions do not happen the first time you come into contact with an allergen, but at a later point of contact. This is because the body’s immune system has to develop sensitivity to the allergen before you can become allergic to it.
Typical allergic reactions involve irritation and inflammation (swelling) in the body. Symptoms may include:
- sinus pain (feelings of pressure or pain high up in the nose, around the eyes and at the front of the skull)
- runny nose
- nettle rash/hives
- itchy eyes, ears, lips, throat and palate (roof of mouth)
- shortness of breath
- sickness, vomiting and diarrhoea
It is important to remember that these symptoms can also be caused by other conditions.