My sister was very allergic to cats when she was a kid. Then one day she just sort of turned up with this cat. My sister's friends cat had kittens and this was one of the kittens.

Anyway she was afraid my parents would say no so she hid the cat in her room for 2 or 3 days before my parents noticed. By which point it was somewhat of a done deal.

Anyway 16 years later when the cat died my sister was no longer allergic, so if you spend enough time around them you do eventually become desensitized. Or at least some people seem to.