The ABO blood group is based on the presence (or absence) of two major agglutinogens (antigen) on red blood cell membranes - agglutinogen A and agglutinogen B. A person's erythrocytes contain one of four agglutinogen combinations as a result of inheritance: only A, only B, both A and B, or neither A nor B.
A person with only agglutinogen A has type A blood. A person with only agglutinogen B has type B blood. An individual with both agglutinogen A and B has type AB blood. A person with neither agglutinogen A nor B has type O blood. Thus, all humans have one of four possible ABO blood types - A, B, AB, or O.