Allergies occur when your immune system reacts to a foreign substance such as pollen, mold or pet dander.
Your immune system produces proteins known as antibodies. These antibodies protect you from unwanted invaders that could make you sick or cause an infection. When you have allergies, your immune system makes antibodies that identify your particular allergen as something harmful, even though it isn't.
When you inhale the allergen or come into contact with it, your immune system responds and produces an inflammatory response in your nasal passages or lungs. Prolonged or regular exposure to the allergen can cause the ongoing (chronic) airway inflammation associated with asthma.
Cats and dogs
Allergens from cats and dogs are found in skin cells the animals shed (dander), as well as in their saliva, urine and sweat and on their fur. Dander is a particular problem because it is very small and can remain airborne for long periods of time with the slightest bit of air circulation. It also collects easily in upholstered furniture and sticks to your clothes.
Pet saliva can stick to carpets, bedding, furniture and clothing. Dried saliva can become airborne.
So-called hypoallergenic cats and dogs may shed less fur than shedding types, but no breed is truly hypoallergenic.
Rodents and rabbits
Rodent pets include mice, gerbils, hamsters and guinea pigs. Allergens from rodents are usually present in hair, dander, saliva and urine. Dust from litter or sawdust in the bottom of cages may contribute to airborne allergens from rodents.
Rabbit allergens are present in dander, hair and saliva.
Pet allergy is rarely caused by animals that don't have fur, such as fish and reptiles.