the.com/blood type

means A classification of blood based on the presence or absence of specific antigens on red blood cells, most commonly organized into the ABO and Rh systems (like O negative or AB positive).

from The concept emerged in the early 1900s when an Austrian scientist noticed that mixing blood from different people sometimes caused it to clump — a discovery that explained why transfusions had been fatally unpredictable for centuries. He sorted blood into groups based on this clumping reaction, work that later earned a Nobel Prize. The Rh factor, the other half of the familiar plus-or-minus label, was identified decades later and named after the rhesus monkeys used in the research. 'Type' itself comes from Greek 'typos,' meaning a mark or impression — fittingly, since that's exactly what sits stamped on every red blood cell.