What is an allergy?

Medically speaking, allergy has a very specific meaning: a reaction to something that your body doesn't like. Allergies are an abnormal sensitivity to a substance called an allergen that comes into contact with the body that the majority of other people can tolerate without difficulty. Usually, the immune system does a good job of telling the difference between the many non-toxic substances in our environment and the viruses, bacteria and other troublemakers that threaten health. For example, when harmful bacteria enter the respiratory system, the immune system mounts a defence in which protective proteins called antibodies attach themselves to the outer surface of the bacteria and target them for destruction. In most cases the immune system doesn't attack substances that don't harm us.

However, in people with allergies, the immune system sometimes makes a mistake and launches attacks against a perfectly harmless substance, such as pollen, animal hair and certain foods or drugs. While many mild allergies are more of an irritation than anything else, they can be disruptive enough to require ongoing attention and medical treatment. In a minority of cases allergies can cause a fatal reaction.