Jo...
I have a very mild allergy to cats... And I have had cats all my life. It's mild enough I can usually ignore it. BUT - whenever Possum noses me (and snots on me) - I get hives wherever his wet little nose touched. AND, most dogs do this to me too - especially when they lick me. Hives everywhere and I itch, within seconds.
Now here's a neat thing... Bubbles no longer does this to me. He did at first... Now, not at all. At first I thought it was what he was eating... But it can't be, because other dogs that eat the same thing make me itch.
I think it's the saliva, to be honest... I am just semi-immune to certain dogs.