Allergies to pets are caused by protein found in the animal's dander (dead skin cells), saliva or urine. For birds, many people are allergic to the excreta. The proteins from pets are carried on microscopic particles through the air. When inhaled, they trigger reactions in allergic people. As all dogs and cats possess these proteins, none of them is allergy-free. Though some breeds are considered more allergy-friendly, it is likely because they are groomed more frequently, a process that removes much of the dander. It is a common misconception that people are allergic to a dog or cat's hair, and it is falsely believed that an animal that sheds less will not cause a reaction.*
*Source: American Academy of Allergy Asthma & Immunology.