Allergy is caused by allergens. These are substances which are usually harmless, but can cause allergic reactions in certain people.
Allergy symptoms are the result of a chain reaction that starts in the immune system. Your immune system controls how your body defends itself. For instance, if you have an allergy to pollen, your immune system identifies pollen as an invader or allergen. Your immune system overreacts by producing antibodies called Immunoglobulin E (IgE). These antibodies travel to cells that release histamine and other chemicals, causing an allergic reaction with symptoms.