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")