Karl Landsteiner, an Austrian immunologist, discovered the main human blood types (the ABO system) in 1900–1901.

Key discovery

  • The ABO blood group system (types A, B, AB, and O) was identified by Karl Landsteiner while he was working in Vienna around 1900–1901.
  • He noticed that when he mixed blood samples from different people, some combinations clumped (agglutinated), revealing that there were distinct blood groups.

Why it mattered

  • Before his work, blood transfusions were extremely risky because nobody understood why some transfusions killed patients while others succeeded.
  • Landsteiner’s discovery allowed doctors to match compatible blood types, making transfusions far safer and saving countless lives.

Beyond ABO

  • Later, in 1940, Landsteiner and Alexander Weiner helped identify the Rh blood group system (the “positive” or “negative” part of a blood type, like O negative).
  • For these fundamental contributions to immunology and transfusion medicine, Landsteiner received the Nobel Prize in Physiology or Medicine in 1930.