Also known as a white blood cell, leukocyte was a type of cell in the immune system that defended the body against infections and other foreign bodies.
A symptom of rejection was an increase in white blood cells. (TNG: "The Host")
During the final stages of the Teplan blight, production of leukocytes increased. (DS9: "Inter Arma Enim Silent Leges")
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")