AB blood type and O blood type will have hemolysis reaction because the mechanism of hemolysis reaction is antigen antibody reaction. AB blood type contains A antigen and B antigen on the red blood cell membrane, and O blood type contains anti-A antibody and anti-B antibody in the serum. When type O blood and type AB blood are mixed, the anti-A and anti-B antibodies in type O blood will combine with the A and B antigens in type AB blood, causing the red blood cells to agglutinate and hemolysis will occur. In the past, people said that O blood is the universal blood giver and AB blood is the universal blood recipient, and only in the last resort can heterotypic blood be transfused, but clinically, the principle of transfusing homotypic blood is still used.