Blood types

Specific properties of human blood were demonstrated, classifying it into four types: O, A, B, and AB. Plasma of some types of blood introduced into some patients could clump red cells and block blood vessels, leading to death. This breakthrough significantly improved the safety and effectiveness of blood transfusions.
 Karl Landsteiner Austria 1900 Blood types

Specific properties of human blood were demonstrated, classifying it into four types: O, A, B, and AB. Plasma of some types of blood introduced into some patients could clump red cells and block blood vessels, leading to death. This breakthrough significantly improved the safety and effectiveness of blood transfusions.