How Blood Works

Blood Types

There are four major blood types: A, B, AB, and 0. The blood types are determined by proteins called antigens (also called agglutinogens) on the surface of the RBC.

U.S. Blood Type Distribution
According to the American Association of Blood Banking, these are the percentages of different blood types in the U.S. population:
  • A+: 34 percent
  • A-: 6 percent
  • B+: 9 percent
  • B-: 2 percent
  • AB+: 3 percent
  • AB-: 1 percent
  • O+: 38 percent
  • O-: 7 percent

There are two antigens, A and B. If you have the A antigen on the RBC, then you have type A blood. When B antigen is present, you have type B blood. When both A and B antigens are present, you have type AB blood. When neither are present, you have type O blood.

When an antigen is present on the RBC, then the opposite antibody (also called agglutinin) is present in the plasma. For instance, type A blood has anti-type-B antibodies. Type B blood has anti-type-A antibodies. Type AB blood has no antibodies in the plasma, and type O blood has both anti-type-A and anti-type-B antibodies in the plasma. These antibodies are not present at birth but are formed spontaneously during infancy and last throughout life.

In addition to the ABO blood group system, there is an Rh blood group system. There are many Rh antigens that can be present on the surface of the RBC. The D antigen is the most common Rh antigen. If the D antigen is present, then that blood is Rh+. If the D antigen is missing, then the blood is Rh-. In the United States, 85 percent of the population is Rh+ and 15 percent is Rh-. Unlike in the ABO system, the corresponding antibody to the Rh antigen does not develop spontaneously but only when the Rh- person is exposed to Rh antigen by blood transfusion or during pregnancy. When an Rh- mother is pregnant with an Rh+ fetus, then the mother forms antibodies that can travel through the placenta and cause a disease called hemolytic disease of the newborn (HDN), or erythroblastosis fetalis.