A transfusion is a medical procedure where typed whole blood removed from a donor is given intravenously to a recipient. It is a common treatment for blood loss of any type and for anemia. it is also routine during surgery to replace blood lost during the procedure.
Giving a patient blood of the wrong type will result in the new blood being immediately attacked by antibodies from the immune system. Type O- blood, however, will not result in this reaction in any patient as the blood cells lack any of the proteins that give rise to an immune response. A patient with type AB+ blood can accept any type of blood as they have no antibodies for any of the blood type markers.
Transfusions pose a risk of blood borne illness, and as a result all donated blood has to be screened for viral, bacterial and parasitic diseases. Hepatitis C and HIV were particular concerns in the past, but infection by either through transfusion is rare now.