Multiple realities
(covers information from several alternate timelines)

Blood type was a characteristic of blood cells determined by the presence of specific surface antigens. Knowledge of these types in essential in performing blood transfusions.

In Humans, red blood cells determine blood types (which commonly used two systems for determining blood transfusions: the ABO designation and the Rh factor designation). In all there eight Human blood types: O+, A+, B+, AB+, O−, A−, B−, and AB−. (ENT: "Carpenter Street")

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

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")

In a rogue mission to destroy Humanity, Damron and a small group of Xindi-Reptilians traveled back in time to Earth, 2004, where they attempted to use a bio-reactor to synthesize a virus for their bio-weapon. The bio-weapon required samples of all eight types of Human blood to be properly calibrated. The Reptilians hired a man named Loomis to obtain samples of all these blood types for them. This plan was, however, revealed by Daniels to Jonathan Archer, and he and T'Pol were also sent to 2004 Detroit to stop the Reptilians. By the time of their arrival, the Reptilians had "only modified the virus for six [of the eight Human] blood types," before they were stopped. According to T'Pol, even with six blood types, the Reptilians would have been able to "infect three quarters of Earth's population," and as such, would mean that it was "unlikely Humans [would] pose a threat to them in the future." (ENT: "Carpenter Street")

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")

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