There are 48 blood type systems, of which ABO (giving A, B, AB, and O) and Rh (+/-) can be combined to form the 8 common types.
There are effectively millions of types because all the systems combined combinatorially, but most antigens beyond ABO and Rh don't cause that much of an issue, so in emergency cases, they just go with them.
There are effectively millions of types because all the systems combined combinatorially, but most antigens beyond ABO and Rh don't cause that much of an issue, so in emergency cases, they just go with them.