Blood type, sometimes also referred to as blood group, was a characteristic of Human red blood cells determined by the presence of specific surface antigens. Human blood type designations commonly used two systems for determining blood transfusions: the AB designation and the Rh factor designation, although there were other systems in use for other purposes. There were four AB types and two Rh types, yielding eight types: O+, A+, B+, AB+, O−, A−, B−, and AB−. O was the designation used when the blood cells in question possessed neither A nor B antigens. O-positive was the most common blood type in most early 21st century Human populations. (ENT: "Carpenter Street")

Detroit prostitute Georgia Tandy had a blood type of O-negative. Captain Archer told Loomis and T'Pol that his blood type was B-negative. (ENT: "Carpenter Street") Likewise, Julian Bashir had type B-negative blood. (DS9: "In Purgatory's Shadow")

Doctor Phlox found it easy to synthesize Burzaan's blood type for a transfusion after the Eska was attacked by a drayjin. (ENT: "Rogue Planet")

Vulcans seem to have used a similar system for blood type designation, as both Spock and Sarek had T-negative blood. It was a rare type, even for a Vulcan. (TOS: "Journey to Babel")

In 2267, at a loss to understand why Pavel Chekov was not aging like the rest of the Gamma Hydra IV landing party, James T. Kirk wondered aloud whether it was "his youth, his blood type, his heritage, his glands, his genes?" (TOS: "The Deadly Years")

Curzon Dax and Jadzia Dax both had different blood types. In 2369, Bashir cited this as evidence of how different Trill are to each other, despite carrying the others memories. (DS9: "Dax")

After being changed into a Solid by the Great Link in 2372, Odo's blood type was O-negative. (DS9: "Broken Link")

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