阅读理解
When you donate blood, a small amount is usually taken in advance to test your blood type．If you are O＋, the O is your ABO type and the＋is your Rh type．It is possible to be A,B,AB, or O as well as Rh＋or Rh-．
The ABO system was discovered by Karl Landsteiner in the early 1900s．He was awarded the Nobel Prize for his achievements．There are four basic types of blood in the ABO system：A,B,AB and O．Everybody is born with one of these four types of blood．We get blood type, just like hair color and height from parents．Because of the substances(物质)contained in each type, the four groups must be transfused carefully．If two different blood types are mixed together, it may put a person into an extremely dangerous situation．Basically, A and B cannot be mixed．A and B cannot receive AB, but AB may receive A or B．In an emergency, type O blood can be given because it is most likely to be accepted by all blood types，so it is often called the universal donor．However，there is still a risk．For the opposite reason，AB is sometimes called the universal receiver．However，because there can be so many reactions in transfusions，patients usually receive only salt until their blood can be matched as exactly as possible in the blood bank of the hospital．
There is a relationship between your blood type and your nationality．Among the Europeans, about 45 percent have type O while 42 percent have type A．The rarest is type AB．Other races have different percentage．For example, some American Indian groups have nearly 100 percent type out of 100 donors in the world．
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