Immunology

Neutrophil – Definition, Structure, Functions

What is Neutrophil? Features of Neutrophils Neutrophils low Level Neutrophil high Level Life span of Neutrophil Structure of Neutrophils Functions of Neutrophils ...

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Neutrophil - Definition, Structure, Functions
Neutrophil - Definition, Structure, Functions

What is Neutrophil?

  • Neutrophil, often referred to as heterophil or neutrocyte, is the most abundant form of granulocyte, comprising between 55 and 70 percent of white blood cells in the human body and aiding in the battle against infections.
  • They are present in varied quantities in the healthiest tissues. They are an essential component of the immune system, with roles that vary between organisms.
  • They differentiate into subpopulations of neutrophil killers and neutrophil cagers after developing from stem cells in the bone marrow. Segmented neutrophils and banded neutrophils are subtypes of neutrophils.
  • The name neutrophil is derived from Hematoxylin% eosin & histological features or preparations.
  • It has a nucleus with two to five lobes. In contrast to other white blood cells, they are not restricted to a specific area of circulation. Therefore, they are essential.
  • A neutrophil is terminally differentiated; after its discharge from the bone marrow, it neither splits nor alters its phenotype.
  • It begins with myeloblasts, which mature into promyelocytes, followed by myelocytes, metamyelocytes, bands, and lastly neutrophils.
  • When neutrophils are discharged from bone marrow, they have a 6-hour lifespan in the bloodstream. In tissues, it constitutes 50%.
  • The leukocyte adhesion cascade method transports neutrophils from the circulation to areas of infection or inflammation.
  • Neutrophil kinetics is the study of the movement of neutrophils and neutrophil precursors between the bone marrow, peripheral blood, and tissues.
  • The daily generation of neutrophils has been estimated to be between 0.9 and 1.0 109 cells/kg.
  • Neutrophils that fail to migrate into tissues undergo programmed cell death or apoptosis and are then eliminated by macrophages in the bone marrow and liver.
3D rendering of a neutrophil
3D rendering of a neutrophil | Image Source: Blausen Medical, CC BY-SA 4.0 https://creativecommons.org/licenses/by-sa/4.0, via Wikimedia Commons

Features of Neutrophils

  • The presence of fully functional neutrophils in tissues is essential for microbial infection defence.
  • Neutrophils have three primary antimicrobial functions: phagocytosis, degranulation, and release of nuclear material in the form of neutrophil extracellular traps.
  • By phagocytosis and the following fusion of the phagosome by lysosomes harbouring antimicrobial peptides, enzymes, and reactive oxygen intermediates, they can swallow and kill invading germs intracellularly (ROI). Neutrophils can also destroy microorganisms extracellularly by releasing antimicrobial peptides and enzymes from their granules.
  • Neutrophils are one of the first immune cells to respond when microbes such as bacteria and viruses enter the body.
  • Neutrophils are phagocytes that reside in the circulation.
  • They are among the initial inflammatory cells that move towards the site of inflammation.
  • They may flow easily within the walls of veins and into the body’s tissues to rapidly fight all antigens, such as bacteria, viruses, fungi, toxins, cancer cells, etc.
  • After being expelled from the bone marrow, approximately half of these cells are found along the walls of blood arteries, while the other half are established in bodily tissues.
  • Neutrophils deal with foreign infections by consuming them, a process known as phagocytosis, or by incorporating them into their own cells, a process known as endocytosis. Once the foreign infection penetrates the cell, it is fought by enzymes, resulting in the eradication of the infectious disease.
Neutrophils with segmented nuclei surrounded by erythrocytes and platelets. Intra-cellular granules are visible in the cytoplasm (Giemsa stained).
Neutrophils with segmented nuclei surrounded by erythrocytes and platelets. Intra-cellular granules are visible in the cytoplasm (Giemsa stained). | Image Source: Dr Graham Beards, CC BY-SA 3.0 https://creativecommons.org/licenses/by-sa/3.0, via Wikimedia Commons

Neutrophils low Level

  • When the neutrophil count is below 1500 cells/mm3 of blood volume, the neutrophil count is regarded to be low.
  • This disorder is also known as neutropenia. Mild neutropenia is characterised by neutrophil counts between 1000 and 1500 cells per millimetre. The range between 500 and 100 cells/mm3 is known as mild neutropenia. When neutrophil counts fall below 500 cells/mm3, severe neutropenia is seen.
  • It is common to detect a low neutrophil count after taking medicine, but it can also be caused by other factors or illnesses.
  • These are the reasons for the low neutrophil count:
    • The most significant cause of a low neutrophil count is the use of medications, particularly those administered during chemotherapy.
    • AIDS, TB, and hepatitis are some of the underlying disorders that depress the immune system and induce low neutrophil counts.
    • In a similar fashion, other disorders, such as cancer and related bone marrow diseases, can also cause a low neutrophil count.
    • Deficiencies in vitamin B12 and other minerals are a further cause of neutropenia.
    • Neutrophil counts are diminished by autoimmune disorders such as Crohn’s disease, lupus, and rheumatoid arthritis.

Neutrophil high Level

  • When the number of neutrophils exceeds 8,000 cells/mm3, the level is regarded to be high.
  • This disorder is known as neutrophilia. Neutrophilia can range from modest, infrequent neutrophilia to neutrophil leukocytosis, a more serious disorder.
  • Due to the fact that neutrophils are part of the immune system, a rise in neutrophil count is primarily caused by a bacterial infection, but other causes may also contribute.
  • These are the causes of the elevated neutrophil count:
    • The most prevalent cause of neutrophilia is a bacterial infection, particularly inflammation-causing pyogenic infections.
    • Inflammation is also accompanied by an increase in neutrophil count, typically after heart attacks or burns.
    • Ingestion of some medications, such as prednisone, and an increase in cortisol and adrenaline hormone levels also boost the number of neutrophils entering the bloodstream.
    • Neutrophilia is also a symptom of malignancies such as leukaemia.
    • In addition to splenectomy and appendicitis, surgical treatments are known to boost neutrophil count.

Life span of Neutrophil

  • According to various methodologies, the average lifespan of inactivated human neutrophils un the circulation is between 5 and 135 hours.
  • Upon activation, they marginate (place themselves near to the blood artery endothelium) and, in the majority of cases, undergo selectin-dependent capture followed by integrin-dependent adhesion before migrating into tissues, where they survive for 1–2 days.
  • Neutrophils have been shown to be discharged from a splenic reserve into the bloodstream following myocardial infarction.
  • Neutrophils are far more abundant than the longer-lived monocytes and macrophages. It is likely that a pathogen (disease-causing bacteria or virus) will encounter a neutrophil first.
  • Some specialists suggest that neutrophils’ short lifespan is an evolutionary adaptation.
  • The short lifespan of neutrophils decreases the spread of infections that parasitize phagocytes, as the longer such pathogens remain outside a host cell, the more likely they are to be eliminated by a component of the body’s immune system.
  • In addition, because neutrophil antimicrobial compounds can also cause damage to host tissues, their short half-life reduces host damage during inflammation.
  • After pathogen phagocytosis by macrophages, neutrophils will be eliminated. The cell surface proteins PECAM-1 and phosphatidylserine are implicated in this process.

Structure of Neutrophils

  • Neutrophil granulocytes attached to a surface have an average diameter of 12–15 micrometres (m) in peripheral blood smears. In suspension, the average diameter of human neutrophils is 8.85 m.
  • Along with the eosinophil and the basophil, they comprise the class of polymorphonuclear cells, so called for the multilobed shape of their nuclei (as compared to lymphocytes and monocytes, the other types of white cells).
  • The nucleus has the typical appearance of distinct lobes, which are joined by chromatin.
  • Neutrophil maturation results in the disappearance of the nucleolus, which occurs in just a few other types of nucleated cells.
  • Up to 17% of female human neutrophil nuclei carry an inactive X chromosome in a drumstick-shaped appendage.
  • The cytoplasm contains a tiny Golgi apparatus, sparse mitochondria and ribosomes, and no rough endoplasmic reticulum.
  • Additionally, the cytoplasm includes approximately 200 granules, of which one-third are azurophilic.
  • As neutrophils grow, their segmentation (many nucleus segments) will increase. Normal neutrophils contain three to five segments.
  • Hypersegmentation is abnormal and is a symptom of a number of diseases, most notably vitamin B12 deficiency. In a manual review of the blood smear, this is considered positive when the majority or all of the neutrophils have five or more segments.
  • Neutrophils are the most prevalent white blood cells in humans, accounting for 50–70% of all white blood cells (about 1011 generated daily) (leukocytes).
  • The reported normal range for human blood counts differs among laboratories, however a neutrophil count of 2.5–7.5 109/L is the typical range for neutrophils. People of African and Middle Eastern heritage may have normal but decreased white blood cell levels.
  • A report may classify neutrophils as segmented or banded neutrophils.
  • When inactive and circulating in the bloodstream, neutrophils are spherical. As they search for antigens, they transform into amorphous or amoeba-like forms and extend pseudopods upon activation.
  • In 1973, Sanchez et al. discovered that the ability of neutrophils to absorb germs is diminished when simple sugars such as glucose, fructose, sucrose, honey, and orange juice are consumed, whereas starches have no effect.
  • In contrast, fasting increased neutrophils’ phagocytic capacity to consume bacteria. It was concluded that sugar consumption impacted the function, but not the number, of phagocytes in bacteria-eating cells.
  • In 2007, researchers at the Whitehead Institute of Biomedical Research discovered that, when exposed to a variety of sugars on microbial surfaces, neutrophils reacted preferentially to certain types of sugars.
  • The neutrophils consumed and destroyed beta-1,6-glucan targets more so than beta-1,3-glucan targets.

Functions of Neutrophils 

  • About 40% of white blood cells and 60% of immune cells are granulocytes, with neutrophils being the most common type.
  • Moreover, these cells release a variety of proteins with antibacterial effects and the ability to modify tissue.
  • When an infection occurs, neutrophils operate as the body’s first line of defence by engulfing bacteria inside of specialised sacs called phagosomes and then hydrolyzing and killing them.
  • An further type of neutrophils, known as cager neutrophils, transports invading particles to the place where killer neutrophils can destroy them.
  • Due to their short lifespan, neutrophils die off while degrading foreign invaders. Thereafter, the bone marrow consistently generates new neutrophils.

References

  • Peter J. Delves, Seamus J. Martin, Dennis R. Burton, and Ivan M. Roitt(2017). Roitt’s Essential Immunology, Thirteenth Edition. John Wiley & Sons, Ltd.
  • Judith A. Owen, Jenni Punt, Sharon A. Stranford (2013). Kuby Immunology. Seventh Edition. H. Freeman and Company
  • Klin Lab Diagn. 2006; (2):34-36.
  • Rosales, C. (2018). Neutrophil: A Cell with Many Roles in Inflammation or Several Cell Types? Frontiers in Physiology, 9. doi:10.3389/fphys.2018.00113 
  • https://doi.org/10.1016/B978-1-4377-1738-9.00011-6
  • Rosales C. Neutrophil: A Cell with Many Roles in Inflammation or Several Cell Types? Front Physiol. 2018 Feb 20;9:113. doi: 10.3389/fphys.2018.00113. PMID: 29515456; PMCID: PMC5826082.
  • https://www.healthline.com/health/neutrophils#anc
  • https://www.nature.com/articles/3780067
  • https://www.medicalnewstoday.com/articles/323982
  • https://microbenotes.com/neutrophils/
  • https://www.cancer.gov/publications/dictionaries/cancer-terms/def/neutrophil
  • https://my.clevelandclinic.org/health/body/22313-neutrophils
  • https://www.vedantu.com/biology/neutrophils
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Microbiology Notes is an educational niche blog related to microbiology (bacteriology, virology, parasitology, mycology, immunology, molecular biology, biochemistry, etc.) and different branches of biology.

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