What essential process in blood transfusions was discovered by Karl Landsteiner in 1901?

Karl Landsteiner's discovery of blood types in 1901 was a fundamental advancement in the field of transfusion medicine. He identified the different blood groups and classified them as types A, B, AB, and O based on the presence or absence of specific antigens on red blood cell surfaces. This classification is crucial because it directly addresses the compatibility of donor and recipient blood during transfusions.

Prior to Landsteiner's work, blood transfusions were often performed without knowledge of blood type compatibility, which could result in severe and sometimes fatal reactions if incompatible blood was transfused. His identification of the blood groups allowed for safer transfusion practices, significantly improving patient outcomes and laying the groundwork for modern blood transfusion protocols. By providing a clear system for matching blood types, Landsteiner's contribution transformed medical practice and saved countless lives.

The other options pertain to different aspects of medical advancements: sterile techniques and the invention of the blood transfusion machine relate more to the methods of performing transfusions rather than the understanding of blood itself. The identification of blood-borne pathogens is relevant to infection control but is not the focus of Landsteiner's discovery. Thus, understanding the classification of blood types is the key aspect that makes option B the correct answer