Hundreds or even thousands of ordinary substances can trigger allergic reactions. These are called \"allergens.\" Among the most common are plant pollens, molds, household dust (dust mites), animal dander, industrial chemicals, foods, medicines and insect stings.

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 that are inhaled, swallowed or come in contact with the skin.