Yorkies are often called hypoallergenic dogs but there is no such thing as a dog that no person can have an allergic reaction to.
The significant allergen for most people is a protein found in the dog's saliva and dander, produced by the sebaceous glands. "Even if you get a hairless dog, it's still going to produce the allergen," Dr. Wanda Phipatanakul, chair of the Indoor Allergen Committee for the American Academy of Allergy, Asthma, and Immunology is quoted in the newsmagazine U.S. News & World Report as saying.How hypoallergenic a particular dog is for a particular person may vary with the individual dog and the individual person.
Although there is no scientific evidence to support the belief that some dog breeds are hypoallergenic, many people, especially dog breeders, spread rumor as fact for self-benefit. Breeds that shed less are believed to be hypoallergenic, since it is believed that the dog's dander and saliva stick to the hair and are not released into the environment; though this has never been proven.
If a person is only mildly allergic, they may be able to tolerate a specific dog, possibly of one of the allegedly hypoallergenic breeds. Dr. Thomas A. Platts-Mills, head of the Asthma and Allergic Disease Center at the University of Virginia, explained that there are cases in which a specific dog (not breed) might be better tolerated by a specific person, for unknown reasons. "We think there really are differences in protein production between dogs that may help one patient and not another," Dr. Platts-Mills said.
All dogs shed, and all dogs produce dander and saliva in some degree. Even if a dog sheds very little or has little dander, inhaling dog hair or dander, or being licked by a dog, can trigger a reaction in a sensitive person.
I have one of the dogs that is often called hypoallergenic, he's a Maltese. He doesn't shed at all
and I think that is pretty cool for reasons other than allergy (e.g. not having hair everywhere).