People with cat allergies have allergic reactions after coming into contact with cats. A cat allergy occurs when the body's immune system overreacts to a normally harmless substance associated with the cat. The protein enters the body and starts a cascade of events that triggers the immune system and eventually leads to an allergic reaction.

Contrary to popular perception, it is not the cat’s hair that causes most allergic reactions. Rather, it is protein in flakes of skin (dander) and dried saliva, urine and blood that trigger reactions. While many animals have proteins in their saliva that can trigger allergies, cat allergies are the most common. This is because cats groom themselves continually, which results in a great deal of saliva on the animal and in the animal's environment.

According to the American Academy of Allergy, Asthma and Immunology (AAAAI) nearly 10 million pet owners are allergic to their animals. Cat allergies may not develop quickly, sometimes appearing after more than two years of exposure. Kittens produce less dander than adult cats which is why people with cat allergies, especially children, may be able to tolerate kittens but not adult cats.

Direct contact with a cat is not needed to develop or trigger an allergy. Flakes from dander, saliva and urine proteins are extremely lightweight and easily become attached to larger particles, which can then become airborne or stick to surfaces or objects. These allergen particles remain in an environment for weeks or months after a cat has departed. In addition, cat saliva proteins dry on the cat’s hair and become airborne when the cat is stroked.

Cat allergens are even more likely to be transported through the air than dog dander. Cat dander is lighter and even stickier than dog dander, which makes it more likely that the proteins will stick to other material and be carried farther in the air. Cat allergens can be airborne for extended periods of time, even several months.

A cat does not have to live within a home for its allergens to be present. Owners that keep their cats outside may bring the cat allergens inside with them on their clothing. Studies have indicated that non-cat owners had significantly higher levels of cat dander on their clothing at the end of the working day after coming into contact with cat owners.

In some cases, the amount of cat allergen found on these individuals was quite high. Therefore, people can experience an allergic reaction to a cat without ever sharing an environment with a cat.

Cat allergen also continues to affect the lungs long after exposure. One study found that cat allergen can impair lung function in people with asthma for up to 22 hours after the animal is gone.

Some people have allergies to fleas. As a result, a flea-infested cat may trigger an allergic reaction. In this situation, it is not the cat but the fleas that are the source of the allergy. People with flea allergies develop intense itching and reddening at the bite site.

Cats and dogs are the primary causes of animal allergies. This is likely due to the fact that they are the pets most commonly kept inside of the house. However, other furred or feathered animals (e.g., ferrets, hamsters, guinea pigs, monkeys, horses, rabbits, birds) also cause allergies. People with cat allergies can safely keep pets without fur or feathers, such as fish, snakes and turtles.