Allergies are caused by an “over reaction” to certain common substances in the environment known as ‘allergens.’ These allergens include pollens of various grasses, weeds, trees, mold spores, house dust, dust mites, and a variety of other substances including foods. Exposure to an allergen can be from inhalation, skin contact, or ingestion. Once your pet is exposed to an allergen, sensitivity is established and then every time your pet is exposed to the allergen, a reaction can occur.