A person with an allergy has an immune system that over-reacts to a substance, such as a drug, dust, food, plant material or animal dander. The immune system is made up of special cells that fight infection.

When the body is exposed to the substance, the cells release chemicals into the blood stream. The chemicals cause inflammation of the skin, nose, throat, lungs or intestines. Allergy symptoms include itching, hives, rash, diarrhea, sneezing, runny nose, or wheezing.