In an emergency with no time to cross match, which blood type is transfused, and why?

1. A transfusion reaction happens when recipient antibodies meet a matching antigen on the donated erythrocytes. 2. The safe donor is therefore the one whose red cells display the fewest antigens. 3. Type O erythrocytes display neither the A nor the B antigen. 4. Rh negative erythrocytes additionally lack the D antigen. 5. Type O negative cells therefore present nothing for anti-A, anti-B or anti-Rh antibodies to attack, which is why that type is the universal donor. 6. Type AB positive is the universal recipient, not the universal donor, because its usefulness comes from its plasma lacking antibodies rather than its cells lacking antigens. 7. Type O positive still carries the D antigen and could agglutinate in a previously sensitised Rh negative recipient, so it is not the emergency choice. _Source: OpenStax Anatomy and Physiology (CC BY 4.0), Ch 18 "The Cardiovascular System: Blood", section 18.6 Blood Typing_