When a person is allergic to a certain foreign substance, his or her immune system will trigger an exaggerated reaction in response to body contact with this substance. This is an exaggerated reaction because non-allergic people’s immune systems will view these foreign substances as harmless and will not respond to them. These foreign substances that trigger allergic reactions are called allergens, and a person who inappropriately reacts to these substances are said to be allergic or “atopic.” Examples of allergens include pollens, dust mites, molds, dander, and foods.