Drag a donor bag onto the patient. You'll find out right away whether that blood is safe to transfuse, and why. On touch screens, drag with your finger, or tap a bag and then tap the patient. Move on to a new patient whenever you're ready — each one is a random blood type, with anywhere from one to three safe donors to find.
Quick rule: a patient can only safely receive red cells whose antigens are already present on their own cells. Rh− patients should only receive Rh− blood. Type O− carries no A, B, or Rh antigens at all, which is why it's the universal donor.