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A blood type (also called a blood group) is a classification of blood based on the presence or absence of inherited antigenic substances on the surface of red blood cells (RBCs). There are four ABO types: A, B, AB and O. They can all have the antigen A,B, both or non. Rhesus-positive people also have the antigen Rh, whereas Rhesus-negative people have none. If you mix blood of several blood types together, it will get lumpy.