From Kathleen MacNaughton, R.N.,
Your Guide to Asthma.
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About.com Health's Disease and Condition content is reviewed by Sanja Jelic, MD

Definition: An allergen is a substance that can trigger—or set off—an allergic reaction in the body. This reaction can cause symptoms in the nose, eyes, mouth, throat, skin, and airways. In people with allergy-induced asthma, the resulting asthma symptoms are most often wheezing, cough, chest tightness, and shortness of breath.

Get more detailed information on asthma allergens

Pronunciation: AL-lur-JEN

Also Known As: allergy triggers, asthma triggers

Examples: Grass, tree, and weed pollens are examples of allergens. So is pet dander, cockroach sheddings, mold spores, and dust mites.