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Type A has A antigens.
Type B has B antigens.
Type AB has both A and B antigens.
Type O lacks both A and B antigens.
Initially, Landsteiner called it "C," but the name was later changed to "O" from the German word "ohne," meaning "without," signifying the absence of these antigens. This is why Type O blood is unique—it's compatible with all blood types in emergencies, making it a universal donor.
Still, in German it's called "Blutgruppe Null" - so, zero after all...