The first and most important blood groups were dis­co­ve­r­ed in 1901 by Karl Land­stei­ner and are clas­si­fied today as A, B, AB and O. The­se and other blood groups play an espe­cial­ly important role in the trans­fu­si­on of red blood cells (ery­thro­cytes) but are as good as irrele­vant to the trans­plan­ta­ti­on of stem cells.