Blood type compatibility means that blood can be transfused into a person without fear of clumping or agglutination. Not all blood types are compatible with each other.
There are eight blood types: type A, B, AB and O and their negative forms. Those with type O Rh- are universal donors and can donate blood to anyone, while those who are type AB Rh+ are universal recipients and can receive any type of blood, according the official website for the Nobel Prize. Those with Rh+ blood types cannot donate to someone with Rh- blood; however, those with Rh- blood can donate to those with Rh+ blood.