Neutrophil

Neutrophils are a type of white blood cell that engulf pathogens and belong to the innate immune system. They are the most common 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 group behavior. Other names for neutrophils include neutrocytes, heterophils, and polymorphonuclear leukocytes.

These cells originate from stem cells in the bone marrow and develop into two subtypes: neutrophil-killers and neutrophil-cagers. They have a short lifespan, ranging from 5 to 135 hours, and are highly mobile, capable of reaching tissue areas inaccessible to other cells or molecules. Neutrophils can be classified as segmented or banded (bands). Along with basophils and eosinophils, they belong to the polymorphonuclear cell (PMN) family.

The term "neutrophil" comes from how these cells stain in hematoxylin and eosin (H&E) preparations. Unlike basophils, which stain dark blue, and eosinophils, which stain bright red, neutrophils take on a neutral pink color. Typically, their nucleus has 2 to 5 lobes.

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

Neutrophils arrive at an injury site within minutes and are a defining feature of acute inflammation. Besides 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**

On a peripheral blood smear, adhered neutrophils average 12–15 micrometers (μm) in diameter. In suspension, human neutrophils average 8.85 μm. Along with eosinophils and basophils, they are classified as polymorphonuclear cells due to their multilobed nucleus (unlike lymphocytes and monocytes). The nucleus has a characteristic lobed shape, with lobes connected by chromatin. As neutrophils mature, the nucleolus disappears—a feature seen in only a few other nucleated cell types. In up to 17% of female human neutrophils, the nucleus contains a drumstick-shaped appendage that holds the inactivated X chromosome. The cytoplasm contains a small Golgi apparatus, sparse mitochondria and ribosomes, and no rough endoplasmic reticulum. It also holds about 200 granules, one-third of which are azurophilic.

As neutrophils mature, their nuclei show increasing segmentation. A normal neutrophil has 3 to 5 segments. Hypersegmentation (most or all neutrophils with 5 or more segments) is abnormal and occurs in certain disorders, notably vitamin B12 deficiency. This is detected during a 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. The normal blood count range varies by lab, but a typical standard is 2.5–7.5 × 10^9/L. People of African and Middle Eastern descent may have lower counts that are still normal. A report may separate neutrophils into segmented and band forms.

When circulating in the bloodstream and inactive, neutrophils are spherical. Once activated, they become more amorphous or amoeba-like and can extend pseudopods while hunting for antigens.

**Reaction to Sugars**

The ability of neutrophils to engulf bacteria is reduced after ingesting simple sugars such as glucose, fructose, sucrose, honey, and orange juice, while starch ingestion has no effect. Fasting, however, strengthens their phagocytic capacity. This suggests that sugar intake 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 certain types. They engulfed and killed beta-1,6-glucan targets more readily than beta-1,3-glucan targets.

**Development**

**Life Span**

The average lifespan of inactive human neutrophils in circulation has been reported, using different methods, to be between 5 and 135 hours. Upon activation, they marginate (position themselves next to blood vessel endothelium), undergo selectin-dependent capture, and then integrin-dependent adhesion in most cases. After this, they migrate into tissues, where they survive for 1 to 2 days. Following a myocardial infarction, neutrophils are also released into the blood from a splenic reserve. The distribution ratio of neutrophils in bone marrow, blood, and connective tissue is 28:1:25. Neutrophils are far more numerous than the longer-lived monocyte/macrophage phagocytes.