Allergy to cats is the best known allergy to animals.
A protein produced on the feline's skin is responsible for the allergy; it is also present in the saliva, urine, tears and scales (flakes of skin). This allergen can be carried in clothing. It can be found in significant quantities, even in places where no cats live. A child predisposed to allergies can therefore become sensitised to cats, without necessarily being in contact with them.
Allergies to dogs are far less frequent than to cats since the sensitising protein produced by the dog's skin is far less allergenic.
Other pets such as rodents may be responsible for an allergy as can farm animals (particularly horses which can cause violent reactions).