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.
Note: All information is for educational purposes only. For specific medical advice, diagnoses, and treatment, consult your doctor.
Images provided by iStock, Getty Images, Corbis, Veer, Science Photo Library, Science Source Images, Shutterstock, and Clipart.com