What do the different blood types mean?

Question

Which ones are compatible and why?

Answer 1

This is a very common question. Look before you ask.

But I'll explain anyhow.

The blood types are named after the antigen's on the blood cells. The antigens are A and B (and D).

Blood type A had antigens A, blood type B has B, AB has both and O has no antigens.

All blood types come in Rh + or Rh -. Rh stands for Rhesus type, and refers to the D antigen.

As for compatibility:
AB can accept blood from A,B, AB and O
A can accept blood from A or O
B can accept blood from B or O
O can accept blood from O
To take in to account Rh + and -, blood types with + can accept from + or - and - can accept from only -. So AB+ is universal acceptor (can accept from any blood type) and O- is universal donor (can donate to any blood type)

Answer 2

Type A is compatible to Type A, AB
Type B is compatible to Type B, AB
Type O is compatible to Type O, A, B, AB

It depends on what antigens they have on the blood cells. If the blood cells see other blood cells that have antigens that are unknown to them, they start killing them off.
Type O is the universal donor
Type AB is the universal receiver