Blood type AB means that a person’s blood type is AB. In ABO blood group, there are four types of human blood types, namely blood type O, blood type A, blood type B, and blood type AB. The classification of the ABO blood group system is determined by the antigens contained on the red blood cell membranes. If there is A antigen on the red blood cell membrane, it is A blood type; if there is B antigen, it is B blood type; if there is both A antigen and B antigen, it is AB blood type; if there is neither A antigen nor B antigen, it is O blood type. So blood type AB means that the membrane of red blood cells contains both A antigen and B antigen, while the serum of such a person has neither anti-A nor anti-B antibodies.