The most common blood type is O positive. There are four blood group types: A, B, AB and O. These groups are determined by the presence or absence of two antigens, A and B, on the red blood cell surface.
Group A has only the A antigen on red cells, while group B has only the B antigen on red cells.
Group AB has both A and B antigens on red cells, while group O has neither A nor B antigens on red cells.
A third antigen, known as the Rh factor, can be present and positive or absent and negative.
The universal red cell donor has type O negative blood and the universal plasma donor has type AB positive blood. AB negative is the least common blood type.