Allergen is a substance which is capable of causing an allergic reaction in our body.
Allergies are harmful medical conditions may result when we eat or touch a particular substance i.e., after exposure via ingestion, inhalation, injection, or contact with skin. In other words it is an inappropriate response of the immune system to an allergen. Thus the substances to which such an immune response (allergic reaction) is produced are called allergens. An allergen is not a dangerous pathogen.
Some of the common examples of allergens are mites in dust, pollens, animal dander, animal hair etc.
While some allergens affect only the exposed area, others are absorbed or injected into the bloodstream.