Every cat's protein in their dander and saliva is a little bit different. I have a very strong cat allergy, and have since I was 12 years old. But when we were first married, Paul "borrowed" one of the cats from his mother's house to be a mouser for the apartment for a few days. He was too lonely to bother with any mouse-hunting, as he had lived in a house with other cats since birth, so he glued himself to me for a couple days, and I had no allergic reaction to him. He was the only cat I have ever encountered that didn't trigger my allergies, usually I can tell if a cat has been in a place before I ever see the cat itself, from respiratory and skin reactions.