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Neutrophils are a type of white blood cell or leukocyte which form an early line of defence against bacterial infections. They are most numerous type of leukocyte and belong to a group called phagocytes. They are part of the innate immune system and are involved in the inflammatory response.

Neutrophils are produced in huge numbers in response to infection, trauma, infarction (cell death due to lack of blood supply), emotional distress or other stimuli. They cruise around the blood stream waiting to be called to a site where damage is happening. Once there, they kill the invading bacteria and other noxious substances, usually dying in the process themselves. The method they use to kill invaders is called phagocytosis which involves engulfing and digesting the "enemy" cell.

As with all leukocytes, neutrophils are produced and partially matured in the bone marrow. They are very short lived lasting anything from a few hours to a few days. They are released in a form known as a "band" neutrophil which matures into a "segmented" neutrophil in the blood. During acute bodily stress, even less mature forms, known as "myelocytes", are released from the marrow. High neutrophil counts in the blood, especially with the presence of myelocytes, are an indicator of bodily stress. Cigarette smoking and obesity both increase the neutrophil count.

Neutrophils locate damage sites through immune system messenger proteins called chemokines, specifically they are attracted by alpha chemokines. Macrophages, which are usually the first leukocyte to arrive at the scene of an attack release these chemokines which causes the inside layer of the surrounding blood vessels (the endothelium) to produce "adhesion" proteins that specifically bind to receptors on the neutrophils. Once there, the neutrophil squeezes through the endothelium and migrates to the damage site by following the gradient of chemokines.

Neutrophils contain supplies of highly toxic substances including peroxidases, hydrolytic enzymes, and defensins
(antibiotic-like proteins) which they keep in "cytotoxic granules". When they engulf invading cells, they release these granules which poisons both the invader and the neutrophil. They can also cause wide spread destruction by releasing these granules in a kind of scorced earth policy.

The highly restrictive and complex way in which neutrophils are coerced to leave the blood, together with their short life expectancy, prevents these highly dangerous cells from reeking damage to healthly cells of the body.

Neutrophil Links:

Neutrophils: The Bacteria Slayers of the Innate Immune System
Blood cells and the CBC

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