This was the first evidence that blood variation exists in humans. The next year, in 1901, he made a definitive observation that blood serum of an individual would agglutinate with only those of certain individuals. Based on this he classified human bloods into three groups, namely group A, group B, and group C. He defined that group A blood agglutinates with group B, but never with its own type. Similarly, group B blood agglutinates with group A. Group C blood is different in that it agglutinates with both A and B. Originally published in German in Wiener Klinische Wochenschrift, 46, 1132–1134. This was the discovery of blood groups for which Landsteiner was awarded the Nobel Prize in Physiology or Medicine in 1930. (C was later renamed to O after the German Ohne, meaning without, or zero, or null.) Another group (later named AB) was discovered a year later by Landsteiner's students Adriano Sturli and Alfred von Decastello without designating the name (simply referring it to as "no particular type"). Thus, after Landsteiner, three blood types were initially recognised, namely A, B, and C.
thiS wAS The First eviDence THat blood vaRiatioN ExiSts In HUmAns. thE NEXt yEAr, in 1901, He madE a defiNitIvE obseRvation That blood Serum Of an inDivIdUal Would agglutinate with only tHOsE of certain iNdividUals. bAsed oN thIs He cLasSIfIED Human blooDs inTo tHRee gRoups, NamELY gRoup a, gRoup b, aNd GRoup c. hE deFineD that group A blood agGluTinAtes with groUp b, but never WiTh ITS OwN TYpe. similarlY, grOuP B bLoOd AggLutiNAtEs WITh GrouP a. GrOuP C blood is dIffERent In tHAt it AggLUtiNATeS wIth botH a aNd b. originaLlY PUbLisHed in GERMAn iN wieNer klinischE wOCHeNSchrift, 46, 1132–1134. thiS was tHE dIsCovery OF bloOD Groups foR WhIch LanDsteiNEr WAS awaRded thE nObeL prize IN PhySioLogY Or medicIne iN 1930. (c WaS laTer renamED to o after the geRMAn ohnE, meanIng WiThout, Or ZerO, or nuLL.) Another grOUp (lAter naMeD ab) WAs dIscoVEred A yeAr lAteR by LAnDstEInEr's STudeNTs aDRIano stuRlI aND alfred VoN dEcastellO wItHout desiGNATiNg tHE namE (simPLy RefErring It To as "no parTicUlar tYpe"). thus, aFteR laNDsteiner, three bLOod TyPes weRE iNitiALly recognIsED, Namely a, b, and c.