Also known as a white blood cell or white corpuscle, a leukocyte was a type of blood cell in the immune system that defended the body against infections and other foreign bodies.
In 2155, Elizabeth had an elevated white blood cell count. (ENT: "Terra Prime")
After the Odan Trill symbiont was implanted in Commander William T. Riker, there was an increase in white blood cells. (TNG: "The Host")
The leukocytes in Elim Garak's body were altered by his cranial implant, leaving toxins to build up in his body. In 2370, Julian Bashir and Jabara believed they could synthesize Cardassian leukocytes, although not in enough time to save Garak's life. (DS9: "The Wire")
During the final stages of the Teplan blight, production of leukocytes increased. (DS9: "Inter Arma Enim Silent Leges")