You must notice that AB0 antigens are on the cell membrane of red blood cells and that the respective antibodies are in the plasma. When a person receives blood because he is having surgery for example, they only give him red blood cells. So let´s think the following: a person with group A (john) has antigenA on red blood cells and antiB antibodies in his plasma. A person with group 0 (mary) has antiA and antiB antibodies in his plasma and has no antigens on red blood cells. So if JOhn receives red blood cells from mary, he will NOT be receiving antiA nor antiB antibodies and therefore there wont be hemolysis