Final answer:
Karl Landsteiner, an Austrian biologist and physician, discovered the major human blood groups in 1900, which revolutionized blood transfusions by allowing for safe and compatible transfusions.
Step-by-step explanation:
Karl Landsteiner, an Austrian biologist and physician, made significant discoveries related to blood transfusions in humans. In 1900, he discovered the major human blood groups and their importance in preventing transfusion reactions and deaths. Blood groups are determined by the presence or absence of specific marker molecules on the plasma membranes of erythrocytes. Landsteiner's discoveries allowed for the matching of patient-donor blood types, ensuring compatibility and safe transfusions.