An allergy is an overreaction by the immune system to substances in the environment that is harmless to most people. These substances are known as allergens and include dust mites, animal dander, pollens, insects, moulds, foods and some medicines. Atopy is a genetic (inherited) tendency to develop allergic diseases. People with atopy are said to be atopic. When atopic people are exposed to allergens they can develop an immune reaction that leads to allergic inflammation (redness and swelling).