People with dog allergies have allergic reactions after coming into contact with dogs. Contrary to popular perception, it is not the dog’s fur that causes most of these reactions.

A dog allergy occurs when the body's immune system overreacts to a normally harmless substance associated with the dog, usually proteins in flakes of skin (dander), urine, saliva or blood. The protein enters the body and starts a cascade of events that triggers the immune system and eventually leads to an allergic reaction.

How this reaction manifests itself often depends on the individual. However, an allergic reaction can affect the skin, gastrointestinal tract, respiratory system, or even the heart and blood vessels. Symptoms of allergies are sometimes confused with other causes, such as colds or the flu. In addition, dog allergies can trigger asthma attacks in susceptible people. Studies have shown that up to 67 percent of children with asthma are sensitive to animal allergens.

Direct contact with a dog is not needed to develop an allergy. Flakes from dander and dried saliva and urine proteins are extremely lightweight and easily become attached to larger particles, which can become airborne or stick to surfaces or objects. These particles remain in the environment for weeks or even months after a dog has departed. Researchers have documented the presence of dog allergens in homes, office buildings and other environments where dogs did not live. In some cases, the amount of dog allergen found was quite high. Therefore, people can experience an allergic reaction to a dog without ever having any direct contact or sharing an environment with the animal.

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

According to the American Academy of Allergy, Asthma and Immunology (AAAAI), nearly 10 million pet owners are allergic to their animals. Dog allergies may not develop quickly, and it is not unusual for dog allergies to take more than two years to develop. Puppies produce less dander than adult dogs which is why people, especially children, with dog allergies may be able to tolerate puppies but not adult dogs.

Dogs and cats 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 dog allergies can safely keep pets without fur or feathers, such as fish, snakes and turtles.