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Neutrophils

. In the marrow, mitotically active myeloid progenitors mature over a period of 7 to 10 days to give rise to large numbers of neutrophils. Under steady-state conditions, most of these newly formed neutrophils die without ever being released into the blood, constituting a storage pool that can be called upon during times of increased need. Upon release from the marrow, neutrophils enter the circulating pool, the only pool that is measured clinically. On average, neutrophils circulate for less than 24 hours before migrating into tissues, where they may survive for up to 3 days. At any given time, roughly half of the neutrophils in the blood are adherent to vessel walls; these cells are referred to as the marginal pool. Finally, some circulating neutrophils are sequestered in the spleen. Normally, <5% of the neutrophils in the body are in the circulating pool, which turns over quickly. As a result, stimuli that alter the release of neutrophils from the marrow, neutrophil margination, or the migration of neutrophils into tissues can have rapid and sometimes profound effects on the peripheral blood neutrophil count.

Neutrophils are a type of white blood cell (leukocyte)

Neutrophils are short-lived immune cells that are never present in normal, healthy tissues.
Tthese cells circulate inactive in the blood where they compose between 40-60% of circulating leukocytes.

🔬 Structure and Lifespan

Neutrophils possess highly lobulated nuclei, characteristically with four to five separate lobes, justifying their classification as PMNs (PolyMorphoNuclear leukocytes). I
The Neutrophil cytoplasm possesses a variety of visible granules which represent lysosomes containing a variety of toxic and inflammatory mediators, justifying their classification as granulocytes.

Lifespan: Short — typically 6–10 hours in circulation, up to 1–2 days in tissues

🛡️ Main Functions of Neutroph

They play a central role in the innate smmune system, which provides the body’s first line of defense against infection.il

They are rapidly recruited, within hours, to sites of inflammation where they quickly phagocytose cellular debris and any present pathogens.
Internalized debris and microbes are destroyed by exposure to a variety toxic materials such as degradative enzymes and reactive oxygen species.
Neutrophilic inflammation is often accompanied by significant host tissue damage as neutrophil-derived toxic materials often leak into the surrounding tissue.

Phagocytosis (Engulfing Microbes)
- Neutrophils rapidly move to infection sites and engulf bacteria, fungi, and debris. As such, the presence of Neutrophils in a tissue indicates on going Acute Inflammation.
- Inside the cell, lysosomal enzymes and reactive oxygen species (ROS) destroy the ingested microbes.
Chemotaxis (Migration to Infection Sites)
- They are attracted by chemical signals such as C5a, IL-8, leukotriene B4, and bacterial products (formylated peptides).
- This guides them from the bloodstream into infected or inflamed tissue.
Degranulation
- Neutrophils release antimicrobial substances from their granules, including:
  - Myeloperoxidase
  - Defensins
  - Lysozyme
  - Lactoferrin
- These substances help kill and digest microorganisms extracellularly.
Formation of NETs (Neutrophil Extracellular Traps)
- In severe infections, neutrophils can release web-like structures of DNA and enzymes to trap and neutralize microbes — a process called NETosis.
Inflammatory Regulation
- Neutrophils release cytokines and chemokines that amplify or regulate the inflammatory response.

⚖️ Clinical Notes

Neutrophilia: Increased neutrophil count, often due to bacterial infection, stress, or inflammation.
Neutropenia: Decreased neutrophil count, often due to chemotherapy, bone marrow suppression, or severe infection, increasing risk of infection.

Would you like me to include a diagram or summary table of neutrophil functions and mediators for quick review (like for study or teaching use)?

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Leukocytes