If you're in a fight - WIN!
Do almost anything and everything to win; bite, kick, gouge, tear - if someone attacks you do everything possible to either escape or disable your attacker. If someone has attacked you - especially with a knife - fighting by the Queensbury rules is probably going to end up with you in a lot more pain than you should be.
As a caveat, if exploiting their weakness would result in killing them or causing permanent lasting harm - then you're probably going to face jail time. So, er, don't do that. (Maybe threaten to do that, though...)
It's all about reasonable force. Which is a massive legal grey area. But if you think you're in danger - do whatever it takes to get yourself out of there.