Leukocyte-promoting factor is a cytokine/hormone that is produced by neutrophils when they encounter a foreign antigen. It stimulates the bone marrow to increase the rate of leukopoiesis, in order to replace the neutrophils that will inevitably be lost when they begin to phagocytose the foreign antigens. See also White blood cell, Leukocytosis, Complete blood count, Indium-111 WBC scan, Leukocyte extravasation.