How come I'm allergic to other people's cats but not my own, have I build an immunity to them?

I've always been slightly allergic to cats where I get itchy eyes and nose so growing up, we only had outdoor cats. Now that I'm older, I have indoor/outdoor cats and I'm not the least bit allergic to them. When I first brought home my new kitten from the shelter though, I was having allergy symptoms but they disappeared now. Whenever I go to a friend's house with indoor cats, I'm miserable. What's going on? Is there a medical explanation for this?