The predictability of ABO antibodies appearing in serum lacking the corresponding antigens makes ABO typing a simple process in most cases. However, the importance of getting it right cannot be stressed enough when a patient will be transfused with blood from a donor. If a patient receives donor cells containing A or B antigens and the transfused patient's serum contains the corresponding antibody, the donor cells will be destroyed almost immediately, causing a severe (hemolytic) and sometimes fatal reaction. Therefore, it is of utmost importance to thoroughly understand the ABO blood group system.
In addition to red cells, ABO antigenic determinants (epitopes) are found in many tissues, body fluids, and other cells, including endothelial cells and platelets. Because ABO antigens are so widely expressed, ABO antigens are also a major consideration in solid organ and bone marrow transplants.