Neutrophils: The Predominant Leukocyte in Acute Inflammation

Neutrophils (polymorphonuclear leukocytes, PMNs) constitute 55–70% of circulating leukocytes in humans, making them the most abundant white blood cell. They are the first responders to sites of infection or sterile injury.

Key Features

• Morphology: Multi-lobed nucleus (2–5 lobes), fine cytoplasmic granules (azurophilic/primary, specific/secondary, gelatinase/tertiary).
• Lifespan: ~5–90 hours in circulation; days in tissues; constitutively undergo apoptosis.
• Production: Granulopoiesis in bone marrow (G-CSF driven); reserve pool (marginated) ~50% of total.

Antimicrobial Mechanisms

1. Phagocytosis: FcγR/CR-mediated engulfment → phagosome → NADPH oxidase (ROS) + granule fusion (MPO, elastase, cathepsin G, defensins).
2. Degranulation: Extracellular release of granule proteins.
3. NETosis: Neutrophil Extracellular Traps — chromatin + granular proteins trap/kill microbes; implicated in thrombosis, autoimmunity.
4. Cytokine Production: IL-1β, TNF-α, IL-8, LTB₄ — amplify recruitment.

Clinical Significance

• Neutrophilia: Stress, corticosteroids, infection, inflammation, malignancy, CML.
• Neutropenia: <1500/µL (mild), <1000 (moderate), <500 (severe/agranulocytosis). Causes: chemo, autoimmune, congenital (Kostmann), drugs, sepsis consumption.
• Functional Defects: CGD (NADPH oxidase mutation), LAD (integrin deficiency), Chediak-Higashi.

Diagnostic Markers

• CBC with differential (absolute neutrophil count = WBC × %neutrophils).
• Bandemia (>10% bands = left shift = acute bacterial infection).
• CD11b/CD66b upregulation (activation).
• Myeloperoxidase (MPO) staining, NBT test (CGD).

Neutrophils are the double-edged sword of immunity: essential for bacterial/fungal defense, but drivers of tissue injury in sepsis, ARDS, ischemia-reperfusion, and autoimmune disease.