Packed red blood cells red cells with both are called AB. Except in unusual cases like infants or seriously immunocompromised individuals, all people will have antibodies to any ABO blood type that isn't present on their own red blood cells, and will have an immediate hemolytic reaction to a unit that is not compatible with their ABO type. In addition to the A and B antigens, there are rare variations which can further complicate transfusions, such as the Bombay phenotype. The Rh blood group system consists of nearly around 50 different antigens, but the one of the greatest clinical interest is the "D" antigen,
ABO blood group system letter O for "ohne", meaning without or zero; Landsteiner chose the latter.) This classification was adopted by the National Research Council and became variously known as the National Research Council classification, the International classification, and most popularly the "new" Landsteiner classification. The new system was gradually accepted and by the early 1950s, it was universally followed. The first practical use of blood typing in transfusion was by an American physician Reuben Ottenberg in 1907. And the large-scale application started during the First World War (1914-1915) when citric acid was developed as blood clot prevention. Felix Bernstein demonstrated the correct blood