Only with the discovery of the blood groups by Karl Landsteiner, the transfusion of compatible blood became possible but the use of blood group O greatly restricted its full deployment. Another important step forward was the proposal of using sodium citrate to prevent the formation of blood clots by Ruben Ottenberg. Still other, unknown blood ‘groups’ created rare but significant transfusion reactions, especially in pregnant women. With the identification of the Rhesus (Rh) antigen (D), the problem of incompatibility between D-negative mothers and their offspring was solved with a simple injection.