I don't think I really realized how allergic to cats I was until I was old enough to move out of my parents' house -- and then moved back in. My parents had always had cats, and I had always had sinus problems. I was constantly stuffy and blowing my nose, and I can remember being treated as a kids for sinus infections that would just never go away.
When I moved out the first time, for the first time in my life I didn't have any pets. I never noticed anything different -- but when I moved back in, I realized (when all my problems returned) that my sinuses had been clear. Only this time, I no longer was accustomed to having cats -- I hesitated to say I had built up an immunity over years of living with them, but that's sure what it felt like. I'd lost that while I was living on my own, and I had to suffer through all the sinus problems in order to get it back again.
Ever since, I've known I'm allergic to cats, but it's never stopped me from having them. The next time I moved out, I got a cat of my own, and I've always had at least one living with me ever since. I've learned that some cats, for whatever reason (shedding more? different kind of fur?) bother me more than others -- for example, when we went over to our friends' house, I was miserable, sneezing and stuffy, and when we left I made the mistake of touching my eyes. Instant itchy eyes! I used to hate that, and I had forgotten how lucky I was that the cats I have right now don't seem to cause it.
So what's the difference? I'm not certain, but I think it's how much the cat sheds. My cats and our friends' cats are all indoor cats, so the problem isn't what's on their fur from outside. They are all long-haired too, and the cats I had growing up were short-haired and still caused it. The one thing I noticed was that my friends' cats shed a lot while I was petting them, whereas my cats shed very little. Luck of the draw, I suppose (though my first cat, who died a couple years ago, shed like crazy -- and caused me all manner of grief).
Knowing this, I'll probably always look for cats that don't shed as much, but even if my next cat causes me more problems than the ones I've currently got, I'd never let that come between us. There are always such things as allergy pills (the once-a-day variety works great), and as I've learned, the body tends to become accustomed and stop reacting as strongly.
So if you are allergic to cats, don't despair -- it doesn't mean you can't live with them and love them just as much as anyone else!