Neutrophils

Neutrophils are the most abundant white blood cells in most mammals. They form an essential part of the innate immune system and serve as the first line of defence against pathogens and are a key mediator of the inflammatory response. Like other granulocytes, neutrophils are generated in the bone marrow. They are short-lived and highly mobile. They form part of the polymorphonuclear cells family (PMNs) together with basophils and eosinophils.

The name neutrophil derives from staining characteristics on hematoxylin and eosin (H&E) histological or cytological preparations. Whereas basophilic white blood cells stain dark blue and eosinophilic white blood cells stain bright red, neutrophils stain a neutral pink. Immature neutrophils have a distinct band-shaped nucleus which changes into a segmented nucleus following maturation.

Neutrophils are a type of phagocyte (see also macrophage) and are normally found circulating around the body in the bloodstream. In the acute phase of inflammation, particularly as a result of bacterial infection and some cancers, neutrophils migrate towards the site of inflammation following chemical signals expressed by activated endothelium, mast cells, and macrophages such as Interleukin-8 (IL-8), C5a, fMLP, Leukotriene B4 and hydrogen peroxide in a process known as chemotaxis.

Neutrophils are recruited to the site of injury within minutes following trauma, and are the hallmark of acute inflammation. Neutrophils combat infection by releasing cytokines, which in turn amplify inflammatory reactions by several other cell types. Neutrophils can also directly attack pathogens by phagocytosis, releasing soluble anti-microbial enzymes (degranulation) and generation of neutrophil extracellular traps (NETs).

• Phagocytosis – antibody coated (opsonized) pathogens are ingested forming a phagosome inside the neutrophil. Reactive oxygen species and hydrolytic enzymes are released into the phagosome to kill the pathogen.
• Degranulation – Neutrophils store various anti-microbial proteins in granules. These are released into the extracellular space by a process known as degranulation. Granules are classified as primary, secondary or tertiary depending on their contents.
• Neutrophil extracellular traps – activated neutrophils release web-like structures of DNA. These neutrophil extracellular traps (NETs) comprise a web of fibres composed of chromatin and serine proteases that trap and kill extracellular microbes.