Early experiments in replacing blood lost by one person with the blood of another seldom succeeded and sometimes proved fatal. Blood transfusion as a practical procedure began after it had been discovered that there are sharply different types or groups of blood. One classification recognized four types called for convenience A, B, AB and O. Between persons of the same blood type, blood can be exchanged safely. But where blood types differ, the following rules apply; persons of types A and B can receive blood from those of type AB: but persons of these first two types cannot exchange blood. Type AB can receive from persons of all other types (universal receivers), but can give to none outside their own group. Those of type B can receive from no other groups, but can give to all others (universal donors. The so called Rhesus factor (given that name because the experimental then work on clotting was done with Rhesus monkeys) is also important in blood transfusions. It may be dangerous to transfer Rhesus – positive blood to a Rhesus- negative person. The wrong type of blood introduced into a patient causes his red cells to clump and clot together blocking the capillaries and killing him.

Though experiments with blood transfusion, the transfer of blood or blood components into a person’s blood stream, have been carried out for hundreds of years it was not until 1901, when the Austrian Karl Landsteiner discovered human blood groups , that blood transfusion became safer. Mixing blood from two individuals can lead to blood clumping or agglutination. The clumped red cells can crack and cause toxic reactions. This can have fatal consequences. Karl Landsteiner discovered that blood clumping was an immunological reaction which occurs when the receiver of a blood transfusion has antibodies against the donor blood cells. Karl Landsteiner’s work made it possible to determine blood types and thus paved the way for blood transfusions to be carried out safely. For this discovery he has awarded the Nobel Prize in Medicine in 1930.