Allergies to animals are a common cause of rhinitis and asthma. Household pets (most often cats and dogs) are the principal cause of animal allergies, but other animals such as rabbits and horses or rodents such as rats, guinea pigs and hamsters are also frequent triggers of animal allergies. Allergy causing proteins from animals known as allergens are often found in public places such as schools or offices where animals are not normally permitted. These allergy proteins stick to clothing and shoes and can travel from place to place contaminating indoor environments.
Dander (the skin that flakes off cats and dogs) gives rise to small airborne allergy particles which produce allergic symptoms when inhaled into the nose or lungs, or after coming in contact with the mucous membranes of the nose, throat or eyes. Saliva from pets may also be a source of allergen. Some people develop hives when licked by dogs. Rodents shed protein in their urine, which can be very allergenic. The urine becomes aerosolized and stays airborne for long periods of time.
There is no such thing as a non-allergenic cat or dog. Some smaller species of dogs may shed less allergen in the indoor environment than a large hairy dog such as a golden lab or German shepherd.
Some but not all studies suggest that frequent washing of cats or dogs may reduce the amount of allergen shed from the pet.