Can O− donate blood to B−?

Yes, O− can donate red cells to B−. The hospital’s crossmatch test confirms every unit before it is given, but the ABO and Rh rules allow it. ## Why O− works for B− Your red cells carry the no A, no B antigens and no RhD antigen, nothing for the recipient to attack. Compatibility is decided by what is on the donor red cell versus the antibodies in the recipient’s plasma. ## Who else O− can help O− red cells can go to: O−, O+, A−, A+, B−, B+, AB−, AB+, every single patient in the country, which is why O− is called the universal red cell donor and why O− shelves must never run empty. Plasma works the opposite way: O plasma only suits O patients, plasma compatibility is the mirror of red cells (see red cells vs plasma). ## Before any transfusion Even a compatible-on-paper unit goes through the crossmatch, the final lab check that mixes your cells with the patient’s plasma. Rules plus crossmatch together are why transfusion is safe by design.