Karl Landsteiner. Portrait of the Austrian-US pathologist and Nobel Laureate Karl Landsteiner (1868-1943). In 1901 Landsteiner discovered three of the major human blood groups: A, B and O, and how to differentiate between them. His colleagues then discovered the fourth, AB. Blood of group A has the marker protein A on its blood cells, group B has B, AB has A and B and O has neither. The correct blood type must be given in transfusions to avoid an immune reaction. Landsteiner joined the Rockefeller Institute, USA, in 1922, where he was involved in the discovery of the M, N and MN blood factors and the Rhesus (Rh) blood groups. He was awarded the Nobel Prize for Medicine in 1930.