From here
Blood types, as people commonly know, has a classification which is simply based on two things:
* ABO group
* Rhesus factor
Now, the ABO group, depends on two antigens; antigen A and antigen B. which lie on the surface of the red blood cell (RBC):
* A person having an A antigen on his RBC cells will show a blood type of A
* A person having a B antigen on his RBC cells will show a blood type of B
* A person having both A & B antigens on his RBC cells will show a blood type of AB
* A person having neither of those antigens will show a blood type of O
Second, the rhesus factor, which depends on a single antigen; antigen D, which also lies on the surface of the RBC:
* A person having a D antigen is called an Rh positive, e.g. A, B and D antigens’ presence exhibit a blood type of AB+ (universal acceptor)
* A person without the D antigen is called an Rh negative, e.g. Neither A, nor B, nor D antigens’ presence exhibit a blood type of O- (universal donor).
Both factors combine to form the blood types as we know them today.