Before 1900, blood transfusion had an unpredictable outcome. In that year Landsteiner showed that the blood serum from one patient would often cause the red blood cells of another to ‘clump’ (agglutinate). He went on to show that all human blood can be grouped in terms of the presence or absence of antigens (A and B) in the red cells and the corresponding antibodies in the serum. Either antigen may be present (blood groups A and B) or both (AB) or neither (O), giving four groups of this kind. Using this idea, simple tests for grouping blood samples and R Lewisohn’s discovery in 1914 that sodium citrate prevents clotting – and helped by refrigeration – blood banks and blood transfusion were widely used by the time of the Second World War. Other blood antigens were later found, for example the MNP system (in 1927) and the Rhesus factor (1940), both discovered by Landsteiner and his co-workers and both (like the A, B, AB and O types) inheritable. Other blood group systems have since been found. The complexity of blood types now known leads to millions of blood-type combinations.

Landsteiner’s work won him a Nobel Prize in 1930 and has been valuable not only for safe transfusion but also in paternity cases, in forensic work and in anthropology for tracing race migration.

See also the Chronology of major events in science.