19. Karl Landsteiner
Karl Landsteiner was an Austrian physician who began to investigate the properties of blood. His fascination revolved around the fact that when two different blood types were mixed together, the red blood cells would clump as the result of a disease or unknown disorder.
Using his own blood samples, Landsteiner was able to discover that humans have a variety of antigens in the blood that attack red blood cells containing adverse antigens. This would result in the clumping effect. Before his death, Landsteiner was able to help advance the field of science on why blood transfusions between different blood types failed.