It was in 1901 in Vienna that a seemingly innocent laboratory experiment brought about an irreversible shift in the world of contemporary medicine. An Austrian physician-scientist Karl Landsteiner found out that when he combined blood samples belonging to different people on his table, some combinations would cause red blood cells to float freely. In contrast, others would cause them to clump.The visual hint became the discovery of blood groups in humans. Until then, the transfusion procedure between donors and recipients was associated with significant risks, as many such procedures ended in death. Thus, a routine observation brought from a hazardous practice into a predictable science that helped save countless lives thereafter.A simple laboratory test with a huge impactThe discovery made by Karl Landsteiner was rather the outcome of curiosity regarding the process taking place during blood interaction between different persons. As described in a historical paper by the Iranian Journal of Public Health, some combinations of blood caused agglutination, while other combinations did not.In order to figure out why this was so, he isolated the red blood cells from the liquid part of the blood known as serum for each individual. Then he conducted tests on the red blood cells of one individual using the serum of another. Such an approach helped reveal a clear repetitive pattern. Blood was shown to be different for every person, regardless of the fact that the liquid looked the same under a microscope. Using the results of such tests, Landsteiner was able to classify the human blood into different types, which he originally called A, B, and C.Why were the blood transfusions so dangerous beforeBefore the findings of Dr Landsteiner, why one transfusion worked perfectly well on one individual while being deadly for another remained unknown. Medical professionals used to attribute the failure of the transfusion procedures to insufficient surgical practices or excessive blood loss. In truth, the problem was associated with the immune system of the human body.If an individual received a mismatched blood sample, his/her body would recognise it as a hostile foreign substance. Therefore, the immune response would act aggressively against it, leading to dangerous immune reactions in the patient's body, which could be lethal to the patient. The lab test conducted by Landsteiner clarified why it was dangerous for a patient to receive incompatible blood. It showed that the agglutination effect is caused by immunological activity, and it means that antibodies in the serum of one individual fight against the red blood cells of another individual. Thus, Landsteiner managed to reveal the rules of blood compatibility. Blood groups from laboratory tests to safe transfusionsThe identification of blood groups allowed medical practitioners to find a way to use the knowledge in practice. By recognising the groups, they were able to make sure that there would be no aggressive immune response and that a transfusion would be safe for a patient.The time that elapsed from the initial scientific discovery to the practical implementation in hospitals was extraordinarily brief. According to the historical account published in the Journal of Anaesthesiology Clinical Pharmacology, while the classification of the ABO system took place approximately at the turn of the century, the very first cross-match test done before the transfusion was completed in a hospital was already in 1907. The mere span of seven years clearly demonstrates that the invention was not left to lie forgotten in an academic publication. Cross-matching has become the solution to a life-and-death medical issue. With this method, physicians got the ability to pose a highly relevant question before undertaking a procedure: Do these particular blood samples fit one another?An invaluable legacy born out of meticulous observationThe discovery of a medical marvel is usually associated with either a lucky coincidence or a flash of genius inspiration. Nevertheless, the history of blood type discovery tells a different story. Instead of witnessing any kind of accident, Landsteiner simply observed carefully what other people were overlooking, and checked and rechecked his observations numerous times.Over a century on, the knowledge that he pioneered is absolutely fundamental to emergency medicine, surgery, and blood donation. He may have failed to unravel all the mysteries of blood, but he certainly provided the modern world of medicine with a solid foundation on which to build. Thanks to his years of hard work at the lab bench, there could be no more talk of blood being one simple fluid.