Neutrophil

Neutrophils are phagocytic white blood cells belonging to the innate immune system. They are the most common type of granulocyte, accounting for 40% to 70% of all human white blood cells. Their roles differ across animal species; in humans, they are involved in sterile inflammation, tissue repair, and cancer, and they display coordinated, collective behavior. They are also referred to as neutrocytes, heterophils, or polymorphonuclear leukocytes.

These cells originate from stem cells in the bone marrow and develop into two subpopulations: neutrophil-killers and neutrophil-cagers. Neutrophils are short-lived, surviving between 5 and 135 hours, and are highly mobile, able to reach tissue areas inaccessible to other cells or molecules. They can be divided into segmented neutrophils and banded neutrophils (or bands). Along with basophils and eosinophils, they belong to the polymorphonuclear cell (PMN) family.

The name "neutrophil" comes from how they stain in hematoxylin and eosin (H&E) preparations. While basophilic white blood cells stain dark blue and eosinophilic ones stain bright red, neutrophils take on a neutral pink color. Typically, their nucleus has 2 to 5 lobes.

As phagocytes, neutrophils are normally found in the bloodstream. During the acute phase of inflammation—especially from bacterial infection, environmental exposure, or some cancers—they are among the first immune cells to migrate to the inflamed site. They travel through blood vessels and interstitial space, guided by chemical signals like interleukin-8 (IL-8), C5a, fMLP, leukotriene B4, and hydrogen peroxide (H2O2) in a process called chemotaxis. They are the main cells in pus, giving it its whitish-to-yellowish color.

Neutrophils arrive at an injury site within minutes and are a hallmark of acute inflammation. Beyond fighting infection, they contribute to acute pain by releasing pro-inflammatory cytokines and other mediators that sensitize nociceptors, heightening pain perception. However, some pathogens are indigestible, so neutrophils may need help from other immune cells to clear certain infections.

**Structure**

When adhered to a surface, neutrophil granulocytes average 12–15 micrometers (μm) in diameter on peripheral blood smears. In suspension, human neutrophils average 8.85 μm. Along with eosinophils and basophils, they form the polymorphonuclear cell class, named for their multilobulated nucleus (unlike lymphocytes and monocytes). The nucleus has a characteristic lobed appearance, with lobes connected by chromatin. The nucleolus disappears as the neutrophil matures—a trait seen in only a few other nucleated cell types. Up to 17% of female human neutrophil nuclei have a drumstick-shaped appendage containing the inactivated X chromosome. In the cytoplasm, the Golgi apparatus is small, mitochondria and ribosomes are sparse, and rough endoplasmic reticulum is absent. The cytoplasm also contains about 200 granules, a third of which are azurophilic.

As neutrophils mature, their nuclei show increasing segmentation. A normal neutrophil has 3–5 segments. Hypersegmentation (most or all neutrophils with 5 or more segments) is abnormal and occurs in disorders like vitamin B12 deficiency, noted during manual blood smear review.

Neutrophils are the most abundant white blood cells in the human body, with about 10^11 produced daily, making up roughly 50–70% of all leukocytes. A standard normal blood count range is 2.5–7.5 × 10^9/L, though this varies between labs. People of African and Middle Eastern descent may have lower counts that are still normal. Reports may separate neutrophils into segmented and band forms.

When circulating in the bloodstream and inactive, neutrophils are spherical. Upon activation, they become more amorphous or amoeba-like, extending pseudopods to hunt for antigens.

**Reaction to Sugars**

The ability of neutrophils to engulf bacteria is reduced when simple sugars such as glucose, fructose, sucrose, honey, or orange juice are ingested, while starch ingestion has no effect. Fasting, conversely, strengthens their phagocytic capacity. This suggests that sugar ingestion alters the function, not the number, of phagocytes in engulfing bacteria. In 2007, researchers at the Whitehead Institute of Biomedical Research found that when given a choice of sugars on microbial surfaces, neutrophils reacted preferentially to some types—specifically, they engulfed and killed beta-1,6-glucan targets more readily than beta-1,3-glucan targets.

**Development**

**Life Span**

The average lifespan of inactivated human neutrophils in circulation has been reported as between 5 and 135 hours, depending on the method used. Upon activation, they marginate (position themselves next to blood vessel endothelium), undergo selectin-dependent capture, then integrin-dependent adhesion in most cases, after which they migrate into tissues, surviving there for 1–2 days. Neutrophils are also released into the blood from a splenic reserve following a myocardial infarction. The distribution ratio of neutrophils in bone marrow, blood, and connective tissue is 28:1:25. They are far more numerous than the longer-lived monocyte/macrophage phagocytes.