Who can donate to blood type A-? 1. O-,A- 2. O+, O-, A- 3. A+, A- 4. AB-, O-
Remember that antibody of the recipient reacts again the antigen of the donor. A- will have antibody B and also anti rhesus antibody. So positive blood groups cannot donate blood to the negative blood groups.
Theoretically in the emergency situations if you dont have other options Rh+ can be donated to the Rh- patient since the Rh Ab react slower so even incompativble Rh can give you time necessary to stabilize patient