Many aspects of the human body remained unclear to us till the advancement of medical science. The reason why certain blood transfusions were successful while others could be deadly was discovered only in 1900 by Karl Landsteiner at the University of Vienna. He discovered the ABO blood group system by mixing the red cells and serum of each of his staff. Landsteiner demonstrated that serum of some people agglutinated or caused clumping of red blood cells of others.
From these early experiments, he identified three blood groups which he called A, B, and C. The last one was later renamed ‘O’ from the German term ‘Ohne’ which means ‘without’, similar to ‘zero’ or ‘null’ in English.
The fourth blood group AB, which is rarer than the others, was discovered a year later. Landsteiner was awarded the Nobel Prize in Physiology or Medicine in 1930 for his work.
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