This is related to the two types of antigens both A and B present on The RBCs either singly (blood group A and Blood group B respectively) or both together (as AB), but having no antigen on RBCs meaning "zero" antigen, refers to the Blood group "O" group.
The 4 blood types are based on the type of antigens. Blood type A has A antigens, B has B antigens and AB has both A and B antigens. Blood type O comes from the German word "Ohne", which means "without".
