Near-immediate, potentially catastrophic destruction of red blood cells (RBCs) as soon as incompatible antibodies interact with those RBCs. Intravascular hemolysis is most commonly caused by antibodies that are capable of activating the complement system, so as a result, the antibodies are most often IgM antibodies that react at body temperatures rather than IgG. Intravascular hemolysis leads to the formation of schistocytes (red cell fragments) and typically results in significant increases in lactate dehydrogenase (LDH) and indirect bilirubin levels, and sharp declines in the patient’s haptoglobin level. In blood banking, intravascular hemolysis accounts for the very serious and often deadly consequences of an acute hemolytic transfusion reaction, seen most often with clerical or laboratory errors leading to a patient receiving ABO-incompatible blood. Most other clinically significant blood group antibodies do not cause intravascular hemolysis, however. Other blood group systems like the Rh, Kell, and Duffy systems have antibodies that are primarily IgG, and as a result, extravascular hemolysis is more common.