A rare blood type among Vulcans, possessed by Ambassador Sarek and in hybrid form by Spock, whose scarcity forced a risky transfusion between father and son during heart surgery.
T-negative is the blood group of Sarek of Vulcan. It is uncommon even among Vulcans, and Spock has inherited it, although his blood also contains human elements that have to be filtered out before it can be given to a full Vulcan.
The type becomes a matter of life and death in the original series episode Journey to Babel. Sarek suffers a series of heart attacks while travelling aboard the Enterprise to a diplomatic conference and needs an operation for which the ship does not carry nearly enough T-negative blood. Spock volunteers as donor and proposes taking an experimental Rigelian drug to speed his own blood production, at some danger to himself.
Matters are complicated when Captain Kirk is stabbed by an Orion agent and Spock refuses to leave the bridge. Kirk feigns recovery to release him, and Doctor McCoy carries out the surgery while the ship is under attack.