Allergy is the exaggerated and out-of-place reaction of the immune system to external substances or stimuli that are not harmful to the body, so the reaction actually damages the body instead of helping it.
The may be pollen and foods, as well as insect stings, drugs and almost any other substances.
You may read more here:
www.nlm.nih.gov/medlineplus/allergy.html http://www.mayoclinic.com/health/allergy/AA99999 