Neutrophil granulocytes (commonly referred to as neutrophils) are a class of white blood cells and are part of the immune system.
Role in blood
Neutrophil granulocytes have an average volume of 330 femtoliters (fl) and a diameter of 12-15 micrometers (Ám) in peripheral blood smears. They are the most common PMNs (polymorphonuclear cells, see granulocytes), constituting about 99% of circulating PMNs. PMNs in turn account for 70% of all leukocytes in humans.
Other granulocytes are the eosinophil granulocyte and the basophil granulocyte.
Neutrophils are active phagocytes, capable of only one phagocytic event, expending all of their glucose reserves in an extremely vigorous respiratory burst. The respiratory burst involves the activation of an NADPH oxidase enzyme, which produces large quantities of superoxide, a reactive oxygen species. Being highly motile, neutrophils quickly congregate at a focus of infection, attracted by cytokines expressed by activated endothelium, mast cells and macrophages.
They are much more numerous than the longer-lived monocyte/macrophages. The first phagocyte a pathogen is likely to encounter is a neutrophil. Some authorities feel that the short lifetimes of neutrophils is an evolutionary adaptation to minimize propagation of those pathogens that parasitize phagocytes. The more time such parasites spend outside a host cell, the more likely they will be destroyed by some component of the body's defenses. However, because neutrophil antimicrobial products can also damage host tissues, other authorities feel that their short life is an adaptation designed to limit damage to the host during inflammation.
Role in disease
Low neutrophil granulocyte counts are termed "neutropenia". This can be congenital (genetic disorder) or due to acquired factors. It can also be a side-effect of medication, including chemotherapy.
Functional disorders are often hereditary. They are disorders of phagocytosis or deficiencies in the respiratory burst (as in chronic granulomatous disease, a rare immune deficiency).
In alpha 1-antitrypsin deficiency, the important neutrophil enzyme elastase is not adequately inhibited by alpha 1-antitrypsin, leading to excessive tissue damage in the presence of inflammation - most prominently pulmonary emphysema.
In Familial Mediterranean fever (FMF), a mutation in the pyrin (or marenostrin) gene, which is expressed mainly in neutrophil granulocytes, leads to a constitutionally active acute phase response and causing attacks of fever, arthralgia, peritonitis and - eventually - amyloidosis.