Is it true that cat allergy is impossible if the cats stay indoors?

Not true: Both indoor and outdoor cats can cause allergies. Sometimes a person can develop tolerance to their own cat's dander, and so not be allergic to their own cat after a while, but allergic to others There are things that can help. Remove other allergens: avoid fragrances in the house and on skin, clean often, avoid carpets and drapes, clean litterbox often. Air filter systems can help too.