This is the original "3 bits" explanation:

Your blood type is a 3-bit byte.  The three bits are: "A", "B", and
"Rhesus (Rh) Factor".  (The letter "O" just means "neither A nor B".)
The Rh factor is always listed at the end, with "+" if it's present,
or "-" if it's absent.  

Here are some blood types represented as 3-bit bytes:

    "0-"  ==> 000
    "AB+" ==> 111
    "A-"  ==> 100
    "B+"  ==> 011

The donation rule is: You cannot receive a bit you don't have.

This is why "0-" is the universal donor, and "AB+" the universal
recipient.  The more people you can donate to, the fewer you can
receive from, and vice versa.