While dander often gets the blame, dog allergens are also present in their saliva, urine, and even blood.
Pet dander is made up of tiny flakes of dead skin shed by dogs (and other animals with fur or feathers). It's microscopic, around 1-10 micrometers in size, making it easily airborne and difficult to see. Dog saliva, urine, and even blood contain an allergen protein, which can also trigger allergic or asthmatic attacks in some individuals.
There are seven named dog component allergens identified, so having a truly hypoallergenic pet is highly unlikely. A significant percentage of the population is mainly allergic to the dog allergen Can f 1.