Problem:

I know you can purchase dog and cat breeds that are hypoallergenic so those who are prone to pet allergies will have reduced allergies or none at all. These breeds are specifically bred for that trait, and as a result are typically pricey compared to another breed. These breeds are not necessarily hairless or short-haired, so I do not think it has to do with the amount of fur shed.

Question: What makes these animals different from other breeds in their inability to cause allergies?

Please explain in detail.