There are four main blood types in the ABO blood group system: A, B, AB, and O. These blood types are determined by the presence or absence of two antigens, A and B, on the surface of red blood cells. People with blood type A have only A antigens, people with blood type B have only B antigens, people with blood type AB have both A and B antigens, and people with blood type O have neither A nor B antigens.

Blood type is important in blood transfusions, organ donations and transplants.