How blood is defined..

Four fold table- Landsteiner’s ABO blood-type system

In this system, you either have base substance A in your blood or not, AND you either have base substance B or not. Four possible blood types are thus defined -- or hypothesized. At one extreme, if neither A nor B is present, you have blood type 0, the universal donor. At the other extreme, if both are present, you have type AB – the universal receiver.