An allergy is an abnormal immune reaction to substances that are ordinarily harmless. There are many ordinary substances that can trigger allergic reactions. Among the most common are pollens, molds, dust mites, animal dander, foods, medicines and insect stings. We get exposed to substances by inhaling them, ingesting them, or simply by having them come in contact with our skin. An allergic reaction may occur anywhere in the body, but usually appears in the skin, eyes, lining of the stomach, nose, sinuses, throat and lungs — places where special immune system cells are stationed to fight off invaders.