A person has been badly injured in a car accident. It is necessary to give him a transfusion (输血) because he has lost much blood in the accident. However, special care must be taken in selecting new blood for him. If the blood is too different from his own, the transfusion can kill him.
    There are four basic blood groups: A, B, AB and O. A single test can tell us a person's blood group. Everybody is born with one of these four blood groups. People's blood group, like hair colour, is inherited (继承) from parents.
    Because the substances contained in each group are different, transfusion must be given 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, so it is often called the universal donor. On the contrary, AB is sometimes called the universal recipient (接受者). However, before being given the transfusion, patients usually receive only salt or plasma (血浆) until their blood can be matched as exactly as possible. In this way, it is possible to avoid any bad results of the transfusion.
    There is a relationship between our blood groups and our nationalities. Among the Europeans, about 42 percent are blood group A while 45 percent are blood group O. The rarest is blood group AB. However, some American Indian groups are nearly 100 percent blood group O.