Neutrophils are the body’s most numerous type of white blood cell (leukocyte), acting as the primary defense against invading pathogens. They constitute between 40% and 70% of all circulating white blood cells. Classified as granulocytes, these cells are distinguished by numerous small granules within their cytoplasm. Their nucleus has a characteristic multi-lobed shape, leading to the alternative name polymorphonuclear leukocytes.
The Role of Neutrophils in Acute Immunity
Neutrophils function as the immune system’s first responders, initiating the rapid defense seen during acute inflammation. When tissue is damaged or an infection takes hold, neutrophils are quickly mobilized from the bloodstream to the site of the threat. Chemotaxis is the process where neutrophils detect and follow a chemical gradient created by signals released from pathogens and injured host cells, such as interleukins and complement factors.
The cells migrate out of the blood vessels and into the affected tissue by squeezing between the endothelial cells lining the vessel walls. They arrive within minutes of the initial injury, forming the bulk of the cellular infiltrate that characterizes the early inflammatory response. This massive deployment ensures that a localized threat is contained and neutralized before it can spread.
Specific Methods Neutrophils Use to Eliminate Threats
Once at the site of infection, neutrophils employ several techniques to neutralize invaders. The most direct method is phagocytosis, which involves the cell engulfing a pathogen, such as a bacterium, into an internal pouch called a phagosome. Fusing the phagosome with internal granules releases powerful enzymes and reactive oxygen species, which collectively destroy the captured microbe. This process is often referred to as a “respiratory burst” due to the rapid consumption of oxygen required to generate these destructive molecules.
Neutrophils also utilize degranulation, releasing their antimicrobial contents directly into the external tissue environment. These granules contain various proteins and peptides, including myeloperoxidase and lactoferrin, that are toxic to surrounding microbes. This external release is often used to combat pathogens that are too large to be engulfed by a single cell.
A third mechanism is the formation of Neutrophil Extracellular Traps (NETs), a process known as NETosis. This involves the neutrophil expelling a sticky, web-like mesh composed of its own decondensed DNA and associated antimicrobial proteins. These nets physically trap and concentrate bacteria and fungi, preventing them from disseminating and simultaneously killing them with the embedded toxins. This method is a form of cellular suicide, sacrificing the neutrophil to protect the host.
Origin and Lifespan
The continuous supply of neutrophils is maintained by myelopoiesis, a process occurring within the bone marrow. They differentiate from hematopoietic stem cells, undergoing maturation before being released into circulation. The human body produces an estimated \(10^{11}\) cells daily to sustain the immune system’s function.
Once in the bloodstream, neutrophils are short-lived, typically circulating for only a few hours. Their lifespan extends to about one to two days once they migrate into tissue to fight an infection. This brief existence ensures that the potent contents of the neutrophil are quickly cleared after the threat is neutralized, and macrophages then engulf the spent cells to resolve the inflammation.
What Neutrophil Counts Reveal About Health
The Absolute Neutrophil Count (ANC) is a common blood test metric providing insights into a person’s health. Neutrophilia (a high neutrophil count) usually indicates an active infection, most often bacterial, or significant inflammation. Other causes include physical stress, tissue damage, and the use of certain medications like corticosteroids.
Conversely, neutropenia (a low neutrophil count) weakens the body’s ability to fight infection. This condition may result from suppressed bone marrow production (e.g., during chemotherapy) or certain autoimmune diseases. Severe neutropenia (ANC below 500 cells per microliter) places an individual at higher risk for life-threatening infections, and monitoring the ANC is fundamental for managing compromised patients.