To prevent rejection, your a, b, & rh blood-type antigens have to be compatible.
A donor cannot have any blood-type antigen
that you don't already have. In other words: ab is compatible with o, a, b and ab. A with a or o, b with b or o, and o only with o. Rh+ matches rh+ or rh-, rh- matches only rh-. It is often harder to find a match for the other major "histocompatibility antigens".