No lines per say, but there's rules to abide by. Take only what you need, never drain, and no innocents. Willing donors are preferable, unwilling must be scum.
You'll hear a lot of others argue my logic on the last point, but really, do the others a favor and take out a mugger or rapist before you take out anyone else.
If all else fails, I'd be happy to donate some of my bagged with you.