You have been badly injured in a car accident. It is necessary to give you a blood transfusion because you lost a great deal of blood in the accident. However, special care must be taken in selecting new blood for you. If the blood is too different from your own, the transfusion could kill you. There are four basic types of blood: A, B, AB and O. A simple test can show a person’s blood type. Everybody is born with one of these four types of blood. Blood type, like hair color and height, is received from parents. Because of substances contained in each type, the four groups must be transfused carefully. Basically, A and B cannot be mixed. A and B cannot receive AB, but AB may receive A or B. O can be given to any other group, hence , it is often called the universal donor. For the opposite reason, AB is sometimes called the universal recipient. However, because so many reactions can happen in transfusions, patients usually receive only salt or plasma until their blood can be matched as exactly as possible in the blood bank of a hospital. In this way, it is possible to keep away from any bad reactions to the transfusions. There is a relationship between your blood type and your nationality. Among Europeans and people of European ancestry, about 42% have type A while 45% have type O. The rarest is type AB. Other races have different percentages. For example, some American Indian groups have nearly 100% of type O.