[al-er-juhn, -jen]
noun Immunology.
any substance, often a protein, that induces an allergy: common allergens include pollen, grasses, dust, and some medications.

1910–15; aller(gy) + -gen Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
allergen (ˈæləˌdʒɛn)
any substance capable of inducing an allergy

Collins English Dictionary - Complete & Unabridged 10th Edition
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Word Origin & History

1912, from allergy (q.v.) on model of antigen.
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Medical Dictionary

allergen al·ler·gen (āl'ər-jən)

  1. A substance, such as pollen, that causes an allergy.

  2. See antigen.

al'ler·gen'ic (-jěn'ĭk) adj.
The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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American Heritage
Science Dictionary
allergen   (āl'ər-jən)  Pronunciation Key 
A substance, such as pollen, that causes an allergic reaction.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Encyclopedia Britannica


substance that in some persons induces the hypersensitive state of allergy and stimulates the formation of reaginic antibodies. Allergens may be naturally occurring or of synthetic origin and include pollen, mold spores, dust, animal dander, insect debris, foods, blood serum, and drugs. Identification of allergens is made by studying both the site of symptoms (e.g., inhalants such as molds, pollens, and dander usually affect the eyes, nose, and bronchi; cosmetics often affect the skin of the face and hands) and the time that symptoms appear (e.g., seasonal allergy to pollen). See also antigen; reagin.

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.
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Example sentences
While some individuals build up immunities from multiple exposures to an
  allergen, others will develop sensitivities.
It's a deadly allergen to some people, but irresistible to rodents.
It turns out that a fragrance used in many shampoos is a common allergen that
  may even cause eczema.
The researchers found there were no differences in allergen levels between
  hypoallergenic dogs and non-hypoallergenic ones.
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