It's not really the dog the person is allergic too but instead it's the dander from the dog that gets into the air that makes someone allergic to the dog.
Dogs shed hair even if you cannot see the tiny bits of hair floating in the air but those tiny strands of hair are floating in the air and when you get near the dog you begin to sneeze or have watery eyes.
It doesn't matter whether the dogs hair is long or short because if you're allergic to dogs then you'll still be allergic to the dog even with short hair.
You can also be allergic to the dogs dead skin that are flakes or even the urine or saliva of the dog.
Some dogs shed less so sometimes you may be able to tolerate a hypoallergenic dog but not always.