The naturally occurring anti-A and anti-B antibodies against the antigens present on red blood cells. If incompatible blood is transfused, these antibodies destroy the recipient’s red blood cells through agglutination and hemolysis. Patients with type A blood cannot receive blood from a donor with type B or AB blood, because they have anti-B antibodies. Patients with type B blood cannot receive type A or AB blood, because they have anti-A antibodies. Patients with type 0 blood cannot receive type A, B, or AB blood, because they have anti-A and anti-B antibodies. Type O patients have no A or B antigens and are called universal donors because they can donate blood to any of the other groups. Type AB patients are universal recipients because they lack both anti-A and anti-B antibodies, and thus can receive blood from donors with any blood type.