The AB0-System is the most important blood type system apart from the Rhesus-System. Based on individual antigens on the surface of erythrocytes the blood types A, B, AB, and 0 are defined. Thus blood type A contains type-A-antigens, blood type B type-B-antigens, blood type AB both antigen types and blood type 0 none of them (see line 2). Human serum contains antibodies agains missing antigens (see line 3). If the red blood cells come in contact with each antibodies to which they expless complimentary antigens, it leads to clotting (see line 4).