The ABO blood groups result from the presence or absence of two
antigens, A and B, on the surface of the red blood cells. The immune
system produces an antibody in the plasma for the antigen not present.
Usually, it is necessary for exposure to an antigen to occur before
antibodies are produced; however, in this instance the antibodies are
already present. Type A blood has the A antigen on its red blood cells and
anti-B antibodies in the plasma. Type B blood has the B antigen on its red
blood cells and anti-A antibodies in the plasma. Type AB blood has both A
and B antigens on the red blood cells and no antibodies in the plasma.
Finally, Type O blood has neither A or B antigens on the red blood cells
and there are both A and B antibodies in the plasma. The relationships of
the ABO blood groups to the antigens and antibodies are summarized in
the table below.