A neutrophil is a type of white blood cell and a major component of innate immunity. Neutrophils are the most abundant circulating leukocytes in healthy adult humans, commonly constituting about 50–70% of the total. They provide rapid defense particularly against bacteria and fungi, and are among the earliest cells recruited to sites of infection or tissue injury. Their antimicrobial mechanisms include engulfing microorganisms, releasing granule contents, and producing extracellular traps. These activities are closely associated with inflammation and require regulation because they can also damage host tissues. (pmc.ncbi.nlm.nih.gov)
Structure and identification
On stained blood smears, mature neutrophils typically measure approximately 12–15 micrometres in diameter. Their nucleus usually contains three to five lobes connected by thin strands. “Polymorphonuclear” describes this segmented appearance: a neutrophil has one lobulated nucleus, not several separate nuclei. Its pale cytoplasm contains numerous granules. The name “neutrophil” reflects the relatively neutral staining characteristics of its specific granules in conventional blood stains. (ncbi.nlm.nih.gov)
Neutrophils contain several classes of intracellular granules. Primary, or azurophilic, granules contain myeloperoxidase and proteolytic enzymes, including neutrophil elastase. Secondary, or specific, granules contain antimicrobial substances such as lactoferrin; tertiary granules contain gelatinase and other components involved in migration. Secretory vesicles provide additional stores of membrane-associated molecules. These compartments differ in their contents and readiness for release, allowing neutrophils to deploy particular functions during recruitment and microbial killing. (pubmed.ncbi.nlm.nih.gov)
Production and life cycle
In adults, neutrophils develop predominantly in the bone marrow through hematopoiesis. Their maturation sequence includes myeloblast, promyelocyte, myelocyte, metamyelocyte, band neutrophil, and segmented neutrophil stages. Early precursors divide, whereas later stages undergo structural and functional maturation without further proliferation. Granules form at different developmental stages, and the nucleus progressively condenses and becomes segmented. (pmc.ncbi.nlm.nih.gov)
Production and release are regulated by cytokines, particularly granulocyte colony-stimulating factor (G-CSF). The marrow maintains a reserve of mature cells that can be mobilized when demand rises. Infection and inflammatory signals can increase production through emergency granulopoiesis. Neutrophils also occupy circulating and marginated vascular pools, including substantial populations retained within the lung microvasculature. Consequently, a blood count measures only part of the body's neutrophil population. (pmc.ncbi.nlm.nih.gov)
Neutrophils are relatively short-lived, but their survival depends on their location and activation state. Inflammatory signals can delay cell death, making a single fixed lifespan an inadequate description of all neutrophils. Their abundance reflects a balance among marrow production, release, vascular distribution, tissue migration, and clearance. (pmc.ncbi.nlm.nih.gov)
Recruitment to tissues
Recruitment from blood generally involves rolling along the vessel wall, activation, firm adhesion, crawling, and passage across the endothelium. Selectins help establish initial interactions, while activated integrins strengthen attachment. The precise requirements differ between vascular beds; migration through lung capillaries does not simply reproduce every step observed in other tissues. (pmc.ncbi.nlm.nih.gov)
Within tissues, chemotaxis directs neutrophils toward gradients of microbial products and host-derived attractants. These signals include chemokines and products of the complement system. Recruitment coordinates the arrival of antimicrobial cells at affected sites, but excessive or persistent accumulation can contribute to tissue injury. (pmc.ncbi.nlm.nih.gov)
Antimicrobial mechanisms
Phagocytosis is a central neutrophil function. The cell surrounds a microorganism and encloses it within a membrane-bound phagosome. Opsonization, in which microbes become coated with antibodies or complement components, improves recognition and uptake. Granules then fuse with the phagosome, delivering antimicrobial substances into the compartment containing the ingested organism. (pmc.ncbi.nlm.nih.gov)
During the respiratory burst, the NADPH oxidase complex generates superoxide and related reactive oxygen species. Myeloperoxidase uses hydrogen peroxide and chloride to produce hypochlorous acid. These oxidants act alongside nonoxidative mechanisms, including proteases, antimicrobial peptides, lysozyme, and iron-binding lactoferrin. Confining these reactions within phagosomes helps limit exposure of surrounding tissue to toxic products. (pmc.ncbi.nlm.nih.gov)
Activated neutrophils can also release neutrophil extracellular traps (NETs): extracellular networks of DNA and chromatin associated with granule proteins. Experimentally described in 2004, NETs can capture microorganisms and concentrate antimicrobial substances around them. NET release occurs through different pathways and should not automatically be equated with one universal cell-death mechanism. Its consequences depend on the stimulus and biological setting. (pubmed.ncbi.nlm.nih.gov)
Clearance and clinical significance
Many neutrophils undergo apoptosis after completing their functions. Macrophages remove them through efferocytosis, helping prevent uncontrolled release of intracellular contents and supporting resolution of inflammation. Conversely, persistent granule release, oxidant production, and extracellular chromatin can contribute to injury when neutrophil responses are inadequately controlled. (pmc.ncbi.nlm.nih.gov)
Blood tests report neutrophils as both a proportion of leukocytes and an absolute neutrophil count. Neutropenia denotes a reduced count; its causes include impaired production, increased destruction, and altered distribution. Some infections, inherited disorders, and chemotherapy can reduce neutrophil numbers. Severe neutropenia is associated with increased susceptibility to bacterial and fungal infection. Cell number and cell function are distinct: defects in adhesion, granule processing, or oxidant generation can impair defense even when neutrophils are present. (ncbi.nlm.nih.gov)