In the late 19th century, a blood transfusion could save a patient or make the blood clump, the kidneys fail, and the patient die, and no one knew why. This episode follows Karl Landsteiner, born in Vienna in 1868 to a Jewish family and left fatherless at six, who earned his medical degree in 1891 and then spent years studying chemistry in Würzburg, Munich, and Zurich. Back at the University of Vienna’s Pathological Anatomical Institute he performed about 3,600 autopsies over a decade. In 1900 he began mixing serum and red cells from different people and showed that clumping was a predictable immune reaction, defining groups A, B, and a third he called C, now type O.

In 1907 Reuben Ottenberg in New York used that map for the first safe matched transfusion. In 1909 Landsteiner and Erwin Popper showed polio was infectious by passing infected material through a bacteria proof Chamberland filter. After World War I he worked as a prosector in The Hague and in a factory making old tuberculin, until Simon Flexner brought him to the Rockefeller Institute in 1923, at age 55. There he found the M, N, and P groups and, in 1937, the Rh factor.

• Why type O cells carry no A or B antigens for antibodies to grab, and type AB patients can receive any type
• The M, N, and P groups became a standard tool in paternity cases
• How the Rh factor explained hemolytic disease in newborns
• His conversion to Catholicism in 1890 and his failed 1937 lawsuit to keep his biography out of Who’s Who in American Jewry
• The 1930 Nobel Prize, and World Blood Donor Day held each June 14, his birthday