Abedeus said:
Exactly. O- goes to the AB- most of the time, since AB- is extremely rare and only O- and AB- can help them. O- is in the biggest need, then O+, AB- and AB+. As and Bs are not that needed, but still useful.
edit: Crap, I confused mixing blood types with mixing Rh-/Rh+. Still, it's bad.
Let's just forget the Rh factor for a second, since it's just confusing (although it's really, really important, and you really want to take it into account, especially where women are concerned).
You are correct regarding all the different types and who can receive what. What I was saying however, is that you want to try to respect the different types
as much as possible when doing a blood transfusion. AB being the rarest, we still want to have AB blood on hand when it's needed. Sure O blood can help AB, but if you use up all the O blood to help an AB patient, then you end up in a pickle when an O person comes along. Just because an AB can O blood, doesn't mean that AB can opt out for giving blood, it's a very slim excuse.
In one sentence:
All blood types are welcome and needed.