An Allergen is a substance that causes an allergic reaction in individuals sensitive to it.

Other definition:
An Allergen is a material that, as a result of coming into contact with appropriate tissues of an animal body, induces a state of allergy or hypersensitivity; generally associated with idiosyncratic hypersensitivities.

Other definition:
An Allergen is a substance capable of causing an allergic reaction because of an individual's sensitivity to that substance.