The immune system's function is to activate the body to fight against intruders such as parasites, bacteria, viruses and other foreign organisms and material that might harm it. This activation is a perfectly healthy response. It occurs where the body encounters the intruder, e.g. in the skin as a reddening, swelling and soreness. A special group of active substances in the body called antibodies and a variety of white blood cells are active in the normal immunological processes.
A defect in the immune system
Allergy - except contact allergy - is caused by the unnecessary production of antibodies against perfectly natural substances in our environment, such as pollens. The immune system reacts towards these in the same way it would react towards something harmful. Why the immune system "misreads" harmless agents is unknown, but you can only become allergic to a substance you have been exposed to before, a process called "sensitisation".
The possibility of becoming allergic is linked to both inheritance and environment. The process by which allergy is inherited is not fully understood. Even though other members of the family are allergic to pollens, a child may develop a quite different kind of allergy - e.g. house dust mite allergy.