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.
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")
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")