Blood transfusion: The transfer of blood or blood components from one person (the donor) into the bloodstream of another person (the recipient). Blood transfusion may be done as a lifesaving maneuver to replace blood cells or blood products lost through bleeding or due to depression of the bone marrow. Transfusion of one's own blood (autologous) is the safest method but requires advanced planning, and not all patients are eligible for it. Directed donor blood allows the patient to receive blood from known donors. Volunteer donor blood is usually most readily available and, when properly tested, has a low risk of side effects.