An allergy is a specific reaction of the body’s immune system to pollens, mold, dust mites, foods, medicines, or other commonly found substances. These substances do not bother most people, but for people who are allergic to them, they react in a number of ways like sneezing, watering eyes, coughing, and swelling. People with allergies are often found to react to more than one substance.
As many as 40 to 50 million Americans have some sort of allergies. Allergies are a major causes of illness and disability in the United States. 35 million of these Americans have reactions due to airborne allergens. The most common allergy in American is a pollen allergy, commonly called hay fever.   Airborne allergens also cause respiratory symptoms like asthma.
A great booklet on allergies can be found here:
http://www3.niaid.nih.gov/topics/allergicDiseases/PDF/airborne_allergens.pdf