Human beings can be divided into groups according to certain factors in the blood which react with factors of a different type in other blood. The blood of some groups may be mixed without danger. In some cases, the fluid matter in the blood of one person will coagulate the red blood cells of another. This is of the greatest importance in blood transfusion. By testing the blood of one person with that of another, in many thousands of people, the groups were divided, and it is now known that group “O” blood can be mixed without any danger. The other classifications have been made to indicate when transfusions are safe and when they are not.More recently, the Rh factor has been discovered, and it is known that an Rh negative and an Rh positive together may produce a child that suffers promptly after birth with a condition that destroys the red blood cells. Under such circumstances, life is saved by transfusion into the child of a blood that is safe. In legal cases, the presence of the blood groups has been found useful because it can be shown that a certain man could not possibly be the father of a certain child.*14/318/5*