Many of you no doubt know enough about blood types to appreciate just how little Jim seems to know about them. For those who don't, the simple version is as follows:

The main classification scheme for blood types (there are additional ones of lesser importance) is the combination of the ABO and Rh systems, which record the presence of specific antigens in the blood. In short, the letters A and B indicate the presence of the so-called type A and type B antigens respectively. A person with type AB blood has both, while a person with O has neither. The Rh designation records the presence of the Rh antigen as Rh-positive, and the absence as Rh-negative.

The presence of antigens restricts who you can safely donate blood to. If you transfuse blood containing any given antigen to someone whose blood doesn't already contain it, that causes severe problems. Type O-negative blood (i.e. Type O which is also Rh-negative) contains no antigens, so can safely be given to anyone. On the other hand, if you have type AB-positive blood, then you already have every antigen, so it doesn't matter what type the blood given to you is. For this reason, type O-negative is called the universal donor blood type, while AB-positive is called the universal recipient.

As mentioned already, there are other additional antigens and blood typing systems which come into play occasionally, but these are relatively rare.

Transcript

[SFX]: BUZZ
Qui-Gon: Hello?
Obi-Wan: Did you by any chance happen to do something like, oh, I don't know, transfuse your blood into an NPC with an unknown blood type?
Qui-Gon: Don't worry. I'm O negative.
GM: You put your blood type on your character sheet? Let me see that.
R2-D2: Universal donor? To optimise your capacity to accept medical treatment, you should have taken universal recipient.
Qui-Gon: Isn't O negative good for everything?
R2-D2: No, you need AB positive.
Qui-Gon: Near enough.
Shmi: {appearing in doorway} Anakin just told me the strangest thing.
Qui-Gon: It was a complete accident!
Shmi: You and my son are now blood-twins. It's so generous of you to take our burdens as your own.
Qui-Gon: Sharing burdens is what I do.
Obi-Wan: I can attest to that.