Invisible and unseen particles or substances in the environment can cause allergic reactions for some people. They exist in many forms, including pet dander, mold, and pollen. Weeds, grass, and trees release allergens in the air, while pet dander comes from the proteins and skin cells that mammals generally shed. Dust mites, too, are a form of allergen and live sneakily in your upholstery, bedding, and carpet. This means it is possible to suffer from ...