A bone marrow transplant can change a patient’s blood type, because the donor’s stem cells take over red cell production. This episode explains why that is possible: a blood type is the set of antigens on red blood cells, policed by antibodies in the plasma, and the International Society of Blood Transfusion recognizes 49 blood group systems, including Kell, Duffy, Kidd, and Lewis. Many of those variants trace to disease. One malaria parasite uses the Duffy antigen to enter red cells, so Duffy negative blood became common in people of African descent, and type O cells resist the sticky rosetting caused by malaria infection.

Karl Landsteiner first noted serum clumping in a short footnote in 1900 and defined the A, B, and O groups in 1901. Then Jan Janský in 1907 and William Moss in 1910 created Roman numeral systems that ran in opposite directions, causing confusion in hospitals until the League of Nations standardized the letters in 1928. The episode covers acute hemolytic reactions, hemolytic disease of the newborn and Rho(D) immune globulin, and Japan’s and South Korea’s blood type personality beliefs.

• Frontline Waffen-SS troops had their blood group tattooed under the left arm
• Rh negative mothers make anti-D antibodies only after exposure to Rh positive blood
• Type O is the universal red cell donor, but type AB plasma is the universal plasma donor
• People with the Bombay phenotype lack the H antigen and react even to O negative blood
• Blood type emojis were added to Unicode because of the personality trend, not for medicine