Everyone’s blood falls into one of four groups, or types: A, B, AB, or O.

The type of blood depends on the presence, or absence, of certain substances on red the blood cells. Blood types are inherited.

In blood transfusions, the people giving and those receiving the blood must belong to the same blood type (blood group), or a dangerous reaction will take place from the agglutination that occurs when blood of a different group is present.

One exception is that group 0 Rhesus-negative blood can be used in an emergency for anyone.