Can I be allergic to one dog, but not another?

you're allergic to all dogs, not just one...the reason you're getting hives is because not all dogs have the same kind of coat or skin. retrievers are water dogs so their skin produces more oils. after you've been in contact with the new dog wash whatever parts of your body it touched and you should be fine. also, the more you bathe your dog the less it will be a problem (less oils present means smaller chance of allergic reaction).