A system used to group human blood into different types, based on the presence or absence of certain markers on the surface of red blood cells. The four main blood types are A, B, O, and AB. For a blood transfusion, the ABO blood group system is used to match the blood type of the donor and the person receiving the transfusion. People with blood type O can donate blood to anyone and are called universal donors. People with blood type AB can accept blood from all donors and are called universal recipients. People with type A or B can receive matching blood or type O blood.