Allergies happen when your body is very sensitive to certain substances. Your body attacks harmless things from the environment as if they were infections or something dangerous. These things are called allergens. There are lots of different allergens. For example hayfever sufferers are allergic to pollen - tiny grass seeds floating in the breeze. The white blood cells in your body make complicated chemicals called antibodies to try and get rid of these allergens. When these antibodies start beating up on the allergen, a chemical reaction happens and stuff called histamine gets released. Histamine is what makes you sneeze and have runny eyes. Hayfever sufferers get sneezy, wheezy and headachy when they breathe in pollen and other allergens. Other people get all itchy or swell up when they get near cat hair or dust mites. Normally antibodies are great - they're your body's way of protecting itself against bacteria and viruses. But in an allergy they start uneccessarily attacking the wrong target.
Britain has 6 million hayfever sufferers!