Allergen An allergen is a medical word for anything that causes an allergic reaction. Allergens can be many different things, including grass, dust mites, or animal dander - the stuff that flakes off dogs' and cats' skin. Back to Articles Related Articles Note: All information is for educational purposes only. For specific medical advice, diagnoses, and treatment, consult your doctor. © 1995-2021 KidsHealth®. All rights reserved. Images provided by The Nemours Foundation, iStock, Getty Images, Veer, Shutterstock, and Clipart.com.