Oh boy... well, just think of the weirdest thing in Fringe and then multiply that by ten and overexplain it by five thousand words. That's probably what Fate/ canon is like.

Grainne's a Servant, which is a spirit inhabiting a magic vessel that looks like a normal human body, but it also means she's kind of immune to well, everything in her universe except other Servants and some magical artifacts of power. Servants are typically called by magi to fight in a contest called the Holy Grail War. The reward is supposed to be a wish for the winning team.

In LR she's susceptible to things because of the nerfs, but she'll be more resistant than a normal living person.