Well, it is difficult to generalise. I mean, some people can be bitchy in quite a funny way - without a nasty edge. Some are just mean.
But I can answer your final question - the best way to react is to be very nice and kind. Squeeze her hand and say "I'm so happy we are friends..." and ask her for more advice, etc.
Never go to her level - even if you get one up on her you will find yourself mired at her level of awfulness.