what causes dog dander
1 Once they take hold, mold and mildew shed tiny spores -- and these spores trigger indoor allergy symptoms.  Pet Dander: If you have pet allergies, youâre not actually allergic to cat or dog hair. Instead, the allergic reaction is caused by a tiny protein in an animalâs saliva. Even homes without pets may contain dander. Thatâs because pet dander is sticky and light.