Blood types were first discovered in the very early 1900s, with the main ABO blood groups identified in 1900–1901 and the Rh factor added in 1940.

Quick Scoop: Key Dates

  • 1900–1901: Austrian physician Karl Landsteiner shows that human blood falls into distinct groups (A, B, C later renamed O, and later AB), founding the ABO system.
  • 1907: Jan Janský classifies four human blood groups (I–IV), corresponding to modern O, A, B, AB.
  • 1927 onward: Many other blood group systems (like MN, P) are added as transfusion medicine develops.
  • 1940: Landsteiner and Alexander Wiener describe the Rh blood group system (positive/negative), critical for safe transfusions and pregnancy care.

Why it mattered

Before this discovery, doctors assumed all blood was the same, and transfusions often ended in dangerous reactions because of unseen incompatibilities. Once blood types were understood, matching donor and recipient types dramatically improved safety and turned transfusion into a routine, life‑saving procedure.

In modern terms: blood types moved from a deadly mystery to a mapped-out system between about 1900 and 1940, and we’ve been adding finer details ever since.

TL;DR: When were blood types discovered? Around 1900–1901 for ABO, with major expansion (including Rh) by 1940.

Information gathered from public forums or data available on the internet and portrayed here.