Karl Landsteiner was an Austrian immunologist and pathologist, born on June 14, 1868. He is best known for discovering the ABO blood group system, which categorizes human blood into four main types: A, B, AB, and O. This groundbreaking work laid the foundation for safe blood transfusions and significantly advanced the field of transfusion medicine. In addition to his work on blood groups, Landsteiner also discovered the Rh factor, another important blood classification system. His contributions earned him the Nobel Prize in Physiology or Medicine in 1930. Landsteiner's research has had a lasting impact on medicine, saving countless lives through improved blood compatibility.