pmn

PMN most commonly refers to polymorphonuclear leukocytes, particularly neutrophils, which are the most abundant white blood cells in humans and play a central role in the innate immune response. They are produced in the bone marrow through granulopoiesis under the influence of cytokines such as G-CSF and GM-CSF, and enter the bloodstream as mature cells with a multi-lobed nucleus and cytoplasmic granules. In clinical practice, PMN is frequently used to denote neutrophils, though the broader term polymorphonuclear leukocytes can include other granulocytes.

Neutrophils are among the first responders to infection or tissue injury. They migrate from blood vessels to affected tissues via chemotaxis guided by chemokines and cytokines. Once at the site of infection, they kill pathogens primarily through phagocytosis, production of reactive oxygen species during the respiratory burst, and release of granule enzymes and antimicrobial peptides. In some situations they form neutrophil extracellular traps (NETs) to immobilize and kill microbes. Neutrophils circulate for only a few hours before moving into tissues, and their overall lifespan is typically around one to two days.

Clinical relevance and laboratory use: PMN counts are a standard part of complete blood counts. Neutrophilia, an elevated neutrophil count, often reflects acute bacterial infection, inflammation, stress, or steroid use. Neutropenia, a reduced neutrophil count, increases susceptibility to infections. A “left shift”—the presence of increased immature neutrophils—can indicate an impending or ongoing bacterial response. Disorders of PMN function, such as chronic granulomatous disease, involve defects in the neutrophil oxidative burst and impair microbial killing.