Transfer of blood taken from one person into the circulation of another to restore blood volume, increase hemoglobin levels, or combat shock .
Once the blood-group antibodies (see {{link=ABO blood-group system">ABO blood-group system , Rh blood-group system ) were discovered, blood typing of donors and recipients rendered transfusion safe. In exchange transfusion, all or most of the blood is removed and replaced with another's blood. Undesirable reactions to transfusion are not uncommon.