Neutrophil

Neutrophils are the most abundant type of white blood cell in humans, forming a key part of the innate immune system. They are granulocytes with a multi-lobed nucleus (typically 2–5 lobes) and cytoplasmic granules that contain enzymes essential for antimicrobial defense. In healthy adults, circulating neutrophils account for about 40 to 70 percent of white blood cells. They have a short half-life in the bloodstream (roughly 6 to 8 hours) and rapidly migrate into tissues during infection or inflammation.

Neutrophils develop in the bone marrow from myeloid lineage stem cells through a maturation sequence that includes myeloblast, promyelocyte, myelocyte, metamyelocyte, band cell, and finally the segmented neutrophil. They enter the circulation and are recruited to sites of infection in response to chemotactic signals produced by microbes and damaged tissue, where they perform their antimicrobial functions.

At infection sites, neutrophils employ several mechanisms to eradicate pathogens: phagocytosis followed by intracellular killing; degranulation releasing proteases, defensins, and other enzymes; a respiratory burst generating reactive oxygen species; and the formation of neutrophil extracellular traps that immobilize and kill microbes. Migration is guided by chemokines such as interleukin‑8 (IL‑8) and complement component C5a.

Clinical relevance: Abnormal neutrophil counts or function can indicate disease. Neutrophilia occurs in bacterial infections and stress, while neutropenia can result from bone marrow suppression, certain drugs, or inherited disorders. Defects in neutrophil function, such as chronic granulomatous disease, impair microbial killing and predispose to infections. Routine evaluation uses complete blood count with differential and, if needed, specialized tests of neutrophil function.