Neutrophil

Neutrophils are a vital type of phagocytic white blood cell. They are a key part of the innate immune system. This system serves as the body's first line of defense against invaders. In humans, neutrophils are the most abundant type of granulocyte. They make up between 40% and 70% of all white blood cells, also known as leukocytes.

Because they are so numerous, they are often the first responders to an infection. They participate in sterile inflammation, tissue repair, and even processes related to cancer.

These cells operate through a sophisticated process called chemotaxis. This is a method of movement where cells follow chemical gradients to reach a target. Neutrophils use specific cell surface receptors to detect various chemical signals. These signals include interleukin-8 (IL-8), C5a, fMLP, leukotriene B4, and hydrogen peroxide (H2O2).

To move, they use amoeboid movement, which allows them to crawl through tight spaces. They can even exhibit coordinated collective behavior known as neutrophil swarming. During swarming, they migrate in a highly organized way to cluster at a site of inflammation.

Once a neutrophil reaches an infection, it uses three main methods to attack microorganisms. The first is phagocytosis, which is the process of engulfing and ingesting particles or microbes. For this to work, targets must often be coated in opsonins through a process called antibody opsonization.

After ingestion, a phagosome forms inside the cell. The neutrophil then triggers a "respiratory burst" by activating the enzyme NADPH oxidase. This produces superoxide, which is converted into hydrogen peroxide and eventually hypochlorous acid (HClO). This acid is believed to have strong bactericidal properties to kill the bacteria. The second method is degranulation, which involves releasing soluble anti-microbials. The third method is the generation of neutrophil extracellular traps, or NETs.

Neutrophils belong to a family called polymorphonuclear cells (PMNs). This group also includes basophils and eosinophils. They are named for the unique shape of their nucleus, which is divided into multiple lobes. A normal neutrophil typically has a nucleus with 2 to 5 segments.

Scientists can identify them using a staining method called hematoxylin and eosin (H&E). While basophils stain dark blue and eosinophils stain bright red, neutrophils stain a neutral pink. They can also be divided into segmented neutrophils and banded neutrophils. In some medical conditions, such as vitamin B12 deficiency, cells may show hypersegmentation, meaning they have five or more segments.

These cells are produced in the bone marrow from stem cells. They differentiate into subpopulations, including neutrophil-killers and neutrophil-cagers.

Neutrophils are highly mobile and have a relatively short lifespan. In the bloodstream, they typically live between 5 and 135 hours. Once they migrate into tissues, they may survive for 1 to 2 days. This short life may be an evolutionary adaptation. It helps limit damage to host tissues from antimicrobial products. It also prevents pathogens that parasitize these cells from spreading too easily. After they finish their work, macrophages help remove them.

There are many specific details regarding their physical structure and daily production. The human body produces approximately 10^11 neutrophils every single day. In a blood smear, an inactivated neutrophil is spherical. However, once activated, they become amorphous or amoeba-like to hunt antigens. They can extend pseudopods to move more effectively. When adhered to a surface, they have an average diameter of 12 to 15 micrometers. In a liquid suspension, their diameter averages 8.85 micrometers.

Their cytoplasm contains about 200 granules, about one-third of which are azurophilic.

Neutrophils are also connected to how we experience physical sensations. During the acute phase of inflammation, they play a central role in combating infection. They also contribute to pain by releasing pro-inflammatory cytokines. These mediators can sensitize nociceptors, which leads to a heightened perception of pain. Additionally, neutrophils are the predominant cells found in pus. This is why pus has a characteristic whitish-to-yellowish appearance. While they are powerful defenders, some pathogens are indigestible. In these cases, neutrophils require the assistance of other immune cells to resolve the infection.