Neutrophils are the most abundant white blood cells in human circulation and serve as the immune system’s rapid-response force, arriving at sites of infection or injury within minutes. They kill invading microbes through several distinct strategies, recruit other immune cells, and help set the stage for tissue repair. But these same cells, when poorly regulated, contribute to a striking range of diseases, from autoimmune conditions and chronic wounds to cardiovascular disease and acute lung injury. Understanding what neutrophils do well and where they go wrong reveals how tightly the immune system walks the line between protection and self-inflicted damage.
From Bone Marrow to the Site of Infection
Neutrophils are produced in vast numbers in the bone marrow, with the body generating billions each day to maintain a constant supply. When infection or tissue damage triggers an alarm, the cytokine G-CSF rapidly mobilizes neutrophils into the bloodstream. This emergency release depends on a chemokine called KC (also known as CXCL1), which spikes in the blood and acts through the receptor CXCR2 on neutrophil surfaces. In mice lacking this receptor, G-CSF fails to mobilize neutrophils at all, demonstrating how tightly this exit from the marrow is controlled.1PubMed Central. G-CSF-mediated thrombopoietin release triggers neutrophil motility and mobilization from bone marrow via induction of Cxcr2 ligands
Once in the bloodstream, neutrophils need to find their way out of blood vessels and into the affected tissue. This process, called extravasation, follows a carefully orchestrated sequence of adhesive interactions between neutrophils and the endothelial cells lining blood vessel walls.2PubMed Central. Mechanisms of neutrophil transendothelial migration The neutrophils first roll along the vessel wall, then grip more firmly, crawl along the surface, and finally squeeze between or through endothelial cells. Two adhesion molecules on endothelial cells, ICAM-1 and ICAM-2, play distinct roles here. Neutrophils preferentially stick to ICAM-1, but ICAM-2 serves a different, more surprising purpose: it guides neutrophils to preferred exit sites along the vessel. Without ICAM-2, neutrophils lose the ability to crawl against the direction of blood flow and cannot locate these transmigration “hotspots,” making their exit from the vessel much less efficient.3PubMed Central. Intercellular adhesion molecule 2 regulates diapedesis hotspots by allowing neutrophil crawling against the direction of flow
Killing Pathogens Up Close
Once neutrophils reach the infected tissue, their primary job is to engulf and destroy invaders. They swallow bacteria whole in a process called phagocytosis, trapping the microbe inside an internal compartment. What happens next is dramatic: the neutrophil floods that compartment with reactive oxygen species, highly toxic molecules that damage and kill the trapped pathogen. The bulk of these reactive oxygen species come from an enzyme called NADPH oxidase, which generates superoxide in large quantities.4PubMed Central. The roles of NADPH oxidase in modulating neutrophil effector responses This sudden flood of toxic molecules is often called the “oxidative burst,” and it is one of the most potent weapons in the body’s antimicrobial arsenal.
Alongside reactive oxygen species, neutrophils deploy a battery of antimicrobial proteins stored in internal granules. When activated, neutrophils release these granules either into the compartment holding the engulfed microbe or into the surrounding tissue. One of the most important granule proteins is myeloperoxidase, stored in a class of granules called azurophilic granules. Myeloperoxidase catalyzes the production of hypochlorous acid, which is essentially the active ingredient in household bleach, along with other potent oxidants.5PubMed Central. Myeloperoxidase: Regulation of Neutrophil Function and Target for Therapy This gives you a sense of how aggressively neutrophils attack: they are manufacturing bleach-like chemicals inside your tissues to kill bacteria.
Neutrophil Extracellular Traps
Perhaps the most visually striking weapon in the neutrophil toolkit is something discovered only in the early 2000s. In response to certain stimuli, neutrophils can expel their own DNA, unfurling it into web-like structures studded with antimicrobial proteins. These structures, called neutrophil extracellular traps or NETs, physically snare bacteria, fungi, and other pathogens, preventing them from spreading while exposing them to concentrated antimicrobial molecules.6PubMed. Neutrophil Extracellular Traps: The Biology of Chromatin Externalization For the neutrophil, this is typically a one-way trip: the process of forming NETs, called NETosis, usually kills the cell.
The mechanics of NET formation involve an elaborate series of internal events. One pathway relies on neutrophil elastase escaping from the azurophilic granules and entering the nucleus, where it partially breaks down specific histone proteins. Myeloperoxidase then cooperates with neutrophil elastase to further unravel the tightly packed chromatin, independent of its usual oxidant-producing function.7PubMed Central. Neutrophil elastase and myeloperoxidase regulate the formation of neutrophil extracellular traps A separate, complementary pathway depends on the enzyme PAD4, which chemically modifies histones through a process called citrullination. This modification strips positive charges from the histone proteins, weakening their grip on negatively charged DNA and causing the chromatin to dramatically expand.8PubMed Central. PAD4 takes charge during neutrophil activation: Impact of PAD4 mediated NET formation on immune‐mediated disease PAD4 is required for the nuclear envelope to rupture and the DNA to be expelled from the cell.9PubMed Central. NETosis proceeds by cytoskeleton and endomembrane disassembly and PAD4-mediated chromatin decondensation and nuclear envelope rupture
Fueling the Attack With a Metabolic Switch
Neutrophils are unusual cells metabolically. At rest, they rely heavily on glycolysis, breaking down glucose for energy much the way many cells do. But when they activate and need to generate a massive oxidative burst, they undergo a remarkable metabolic rearrangement. Activated neutrophils divert essentially all of their glucose-6-phosphate into a pathway called the pentose phosphate pathway, and they even reverse the normal flow of upper glycolysis to maximize recycling of sugar intermediates through this pathway. The result is a metabolic configuration that produces the maximum possible yield of NADPH, the molecule that fuels NADPH oxidase and drives superoxide production.10PubMed Central. Switching to the cyclic pentose phosphate pathway powers the oxidative burst in activated neutrophils
When this metabolic switch is disrupted, the consequences are severe: the oxidative burst is greatly suppressed, NET formation drops, and the neutrophil’s ability to kill pathogens falls apart. This finding has reshaped how researchers think about neutrophil biology. These cells are not metabolically passive. They have the flexibility to completely rewire their energy metabolism in real time to meet the demands of an immune response.
Coordinating With the Broader Immune System
Neutrophils were long regarded as simple killing machines that showed up, did their job, and died. That view has changed considerably. Neutrophils communicate extensively with other immune cells, releasing granule contents, cytokines, and chemokines that shape the behavior of both innate and adaptive immune responses.11PubMed Central. Extracellular Mechanisms of Neutrophils in Immune Cell Crosstalk They can influence how dendritic cells present antigens, modulate T cell responses, and recruit additional immune cells to the site of infection. This crosstalk means that neutrophils are not just effector cells operating in isolation. They are actively steering the direction and intensity of the immune response around them.
This communication also matters for winding down inflammation. When neutrophils have done their job, they undergo programmed cell death, and macrophages engulf these dying neutrophils in a process called efferocytosis. This cleanup is far from passive waste disposal. The act of a macrophage eating an apoptotic neutrophil actively promotes anti-inflammatory signaling, prevents the neutrophil from bursting open and spilling its toxic contents, and helps dampen the overall immune response.12PubMed Central. Clearance of apoptotic neutrophils and resolution of inflammation When this process fails, the consequences can be significant, as uncleared neutrophils release their destructive cargo into the surrounding tissue.
Neutrophils Run on a Clock
One of the more surprising discoveries in neutrophil biology is that these cells follow a circadian rhythm. Neutrophils do not behave the same way at all hours of the day. Their trafficking patterns, functional capacity, and clearance from the bloodstream are regulated by an internal clock driven by the gene Bmal1. This gene controls expression of the chemokine CXCL2, which drives changes in neutrophil properties through the CXCR2 receptor. As neutrophils age in the circulation over the course of a day, a competing signal through the CXCR4 receptor opposes these changes. The net effect is that neutrophils are shuttled into tissues preferentially at night (in mice), where their antimicrobial activity is boosted, while during the day the vascular system is better protected from neutrophil-mediated damage.13Immunity. A Neutrophil Timer Coordinates Immune Defense and Vascular Protection
This circadian regulation has real clinical implications. Research on stroke outcomes has found that during the inactive phase (night in mice, when neutrophils display a more activated, NET-prone state), these cells cause microvascular stalling and reduce blood flow through collateral vessels. These fluctuations disappeared when neutrophils were depleted or when the circadian clock was disrupted.14PubMed Central. Circadian Control of Neutrophils Drives Collateral Perfusion and Stroke Outcome This raises the possibility that the timing of certain cardiovascular events could be partly shaped by the daily oscillation in neutrophil behavior.
When Neutrophils Drive Disease
The same aggressive tools that make neutrophils effective against pathogens can cause serious problems when deployed inappropriately or excessively. NETs, in particular, have emerged as a common thread linking neutrophil activity to a range of diseases.
In autoimmune conditions like systemic lupus erythematosus and a group of blood vessel diseases called ANCA-associated vasculitis, patients show excessive NET formation that correlates with how active their disease is. The NETs are immunogenic, meaning the immune system forms antibodies against the very components neutrophils release, creating a vicious cycle. Autoantibodies in ANCA-associated vasculitis target myeloperoxidase or proteinase-3, while in lupus they target double-stranded DNA, histones, and nucleosomes, all components that are abundantly present in NETs.15PubMed Central. Clinical Implications of Excessive Neutrophil Extracellular Trap Formation in Renal Autoimmune Diseases The two diseases even involve distinct forms of NETosis. In vasculitis, NET formation unfolds over hours and is a lytic, cell-destroying process dependent on reactive oxygen species and PAD4. In lupus, NETs form rapidly within minutes through a non-lytic mechanism involving immune-complex signaling, and the resulting NETs carry distinctive immunogenic cargo including oxidized mitochondrial DNA.16PubMed Central. Intrinsically Distinct Role of Neutrophil Extracellular Trap Formation in Antineutrophil Cytoplasmic Antibody-Associated Vasculitis Compared to Systemic Lupus Erythematosus
In cardiovascular disease, NETs have been detected in both atherosclerotic plaques and arterial blood clots. They activate endothelial cells, antigen-presenting cells, and platelets, fueling a pro-inflammatory loop that may play a causal role in plaque formation and arterial clotting, not merely a bystander one.17PubMed. Neutrophil Extracellular Traps in Atherosclerosis and Atherothrombosis
Lung Injury and Respiratory Infections
The lungs are a particularly vulnerable site for neutrophil-mediated damage. In severe influenza, neutrophils and their NETs infiltrate the lungs in large numbers and contribute directly to acute lung injury. Experiments comparing mice depleted of macrophages versus neutrophils are revealing: macrophage-depleted animals showed excessive neutrophil infiltration, alveolar damage, and progression to full acute respiratory distress syndrome with pulmonary edema and hemorrhage. Neutrophil-depleted animals, by contrast, showed only mild lung pathology. NETs were found tangled within alveoli in areas of tissue injury, linking them directly to lung damage.18PubMed Central. Excessive neutrophils and neutrophil extracellular traps contribute to acute lung injury of influenza pneumonitis
A similar pattern was observed during severe COVID-19 infections, where autopsy findings revealed heavy neutrophil infiltration in pulmonary capillaries and exudation into the alveolar space. Severe cases progressing to acute respiratory distress syndrome, multi-organ damage, and death appeared linked to neutrophil-driven hyperinflammation and clotting within blood vessels.19PubMed Central. Activated neutrophils in the initiation and progression of COVID-19: hyperinflammation and immunothrombosis in COVID-19 These findings have made neutrophil-targeting therapies a growing area of interest in acute respiratory medicine.
Not All Neutrophils Behave the Same
For decades, neutrophils were treated as a single, uniform cell type. That assumption has eroded. One particularly interesting subpopulation is the low-density neutrophils, which show up in the same blood fraction as mononuclear cells (lymphocytes and monocytes) when blood is separated by density. In healthy people, these low-density neutrophils are rare, but their numbers spike dramatically in various diseases.20PubMed Central. Low-Density Neutrophils: Enigmatic Cells in Health and Disease
What makes low-density neutrophils particularly puzzling is that they seem to behave differently depending on the disease context. In lupus, they are hyperactive: prone to producing NETs and secreting large amounts of pro-inflammatory cytokines. In cancer, by contrast, they often function as immunosuppressive cells, dampening the immune response rather than amplifying it. This dual personality suggests that the low-density neutrophil category is itself a mixed bag, containing cells with activated, mature, and immature features that shift depending on what is driving their expansion.21Journal of Leukocyte Biology. Low-density granulocytes in the pathogenesis of inflammatory disease The lack of a single reliable surface marker to identify these cells has made studying them difficult, and researchers are still working to determine whether low-density neutrophils in health, pregnancy, and disease represent the same or fundamentally different cell populations.
Chronic Wounds and Tissue Damage
In acute infections, neutrophils are supposed to arrive, do their job, die, and get cleaned up by macrophages. In chronic wounds, this lifecycle goes wrong. Neutrophils persist at the wound site instead of undergoing their normal programmed death, and their continued activity tips the balance from repair toward destruction. A key culprit is MMP-8, a collagen-degrading enzyme secreted primarily by neutrophils. In chronic wounds, MMP-8 levels can be 50 to 100 times higher than in normally healing wounds.22Metalloproteinases In Medicine. Matrix metalloproteinases in impaired wound healing – Section: MMPs in chronic wounds
The damage goes beyond simple collagen breakdown. MMP-8 also degrades fibronectin, growth factors like TGF-β and PDGF, and proteins that normally inhibit tissue-degrading enzymes. Meanwhile, neutrophil-derived elastase activates additional matrix-degrading enzymes, creating a cascading destructive cycle. The result is that the equilibrium between building new tissue and breaking down old tissue collapses, and the wound stalls. Studies in stressed mice have confirmed this pattern, showing that higher and more prolonged neutrophil numbers at the wound site correspond to overexpression of MMP-8 and poor collagen architecture in the healing tissue.23PubMed Central. MMP-8 overexpression and persistence of neutrophils relate to stress-impaired healing and poor collagen architecture in mice This connection between stress, prolonged neutrophil activity, and impaired healing is one reason chronic wounds remain so difficult to treat.
Neutrophils in the Tumor Microenvironment
Cancer has added another layer of complexity to the neutrophil story. Tumor-associated neutrophils can promote tumor growth by releasing factors that encourage the formation of new blood vessels, a process called angiogenesis. Tumors need a blood supply to grow, and neutrophils appear to assist in building one, releasing both conventional and unconventional pro-angiogenic factors.24PubMed Central. The Good, the Bad, and the Ugly: Neutrophils, Angiogenesis, and Cancer This has led researchers to explore whether therapies targeting neutrophils could complement existing anti-angiogenic cancer treatments.
The picture is not entirely one-sided, though. Some tumor-associated neutrophils can adopt an anti-tumor phenotype, killing cancer cells directly. The factors that determine which way a neutrophil will swing in the tumor microenvironment are still being sorted out, but the distinction between pro-tumor and anti-tumor neutrophil states mirrors the broader recognition that neutrophils are far more functionally diverse than the textbooks once suggested.
Targeting Neutrophils Therapeutically
Given how many diseases involve excessive or misguided neutrophil activity, the idea of dampening neutrophil recruitment has attracted sustained interest. One major target is the CXCR2 signaling pathway, the same receptor that controls neutrophil release from the bone marrow and guides their migration to inflamed tissues. Blocking CXCR2 could reduce the accumulation of neutrophils at sites where they are doing more harm than good, potentially preventing the prolonged tissue damage seen in chronic inflammatory disorders.25PubMed Central. The role of CXCR2 in acute inflammatory responses and its antagonists as anti-inflammatory therapeutics
The challenge, of course, is that you cannot simply shut down neutrophil activity without crippling the body’s defense against infection. People with genetic deficiencies in neutrophil function, such as chronic granulomatous disease (where NADPH oxidase does not work), suffer from severe, life-threatening infections. Any therapeutic approach needs to fine-tune neutrophil behavior rather than eliminate it. Strategies under investigation include targeting specific NET formation pathways (particularly PAD4 inhibitors), blocking individual adhesion molecules to reduce neutrophil infiltration in specific tissues, and exploiting the circadian variation in neutrophil phenotype to time interventions for maximum benefit with minimum immune suppression. The growing appreciation for neutrophil heterogeneity suggests that selectively targeting harmful neutrophil subsets while leaving protective ones intact may eventually be possible, though reliable markers to distinguish the two remain a work in progress.