At first glance, one person’s blood looks no different from another’s, but appearances can be dangerously deceptive. Early attempts at carrying out blood transfusions in humans were highly unpredictable, often triggering a hazardous and potentially fatal reaction. Examining the underlying cause of such bad blood between people led Karl Landsteiner to discover the existence of human blood groups, for which he was awarded the 1930 Nobel Prize in Physiology or Medicine.

Extracting blood samples from his research staff, and seeing whether one person’s red blood cells clumped together when mixed with blood serum from another’s, Landsteiner discovered that reactions occurred when a recipient possessed natural antibodies against a donor’s blood cells. On this basis, Landsteiner found that people could be classified into three groups, now known as A, B and O, with a fourth group AB discovered soon after. A more detailed investigation revealed that each group was distinguishable by the presence or absence of a particular set of molecules, or antigens, lying on the surface of red blood cells. Tying all the pieces of evidence together, Landsteiner showed that adverse reactions occurred when anyone carrying antibodies to unique antigens found in other blood groups received these blood types from donors.

These discoveries removed a great deal of the risk from blood transfusions. Establishing and matching patients’ blood groups in advance could prevent a donor from receiving incompatible blood. Crime scene investigations were also handed a helpful new tool, as dried blood samples could now be typed. Once researchers realized that blood groups are inherited across generations, Landsteiner’s discoveries also helped to uncover previously unseen paths through human life. These newly revealed pathways found application both in an anthropological sense, by analyzing the way in which the distribution of blood groups varies across geographic populations, and in a legal sense, by establishing parental association through paternity tests.