Neutrophils are a type of white blood cell, also known as leukocytes, and are key components of the immune system. They function as rapid responders, deploying quickly to sites of infection or inflammation. Making up approximately 55% to 70% of all white blood cells, neutrophils are the most abundant granulocyte. This highlights their significant role in the initial defense against invading microorganisms, such as bacteria, and in responding to tissue damage. Their unique structure directly supports these immediate and protective functions.
Distinctive Physical Traits
Neutrophils measure between 10 to 15 micrometers in diameter, larger than red blood cells. A defining characteristic is their multi-lobed nucleus, consisting of two to five segments connected by thin strands of chromatin. This segmented nucleus gives them their alternative name, polymorphonuclear leukocytes. The nucleus’s shape allows neutrophils flexibility, enabling them to deform and squeeze through narrow tissue spaces to reach infection sites.
The cytoplasm of a neutrophil appears pale and granular when stained. These granules are small, evenly distributed, and often take on a neutral pink or lilac color with common histological stains. The appearance of these cytoplasmic granules and the multi-lobed nucleus are key features distinguishing neutrophils from other white blood cells.
Internal Machinery
The specialized internal components of neutrophils, particularly their granules, are organized for effective immune responses. Neutrophils contain at least three distinct types of granules, each with unique contents and functions. These pre-formed packages allow for rapid deployment of antimicrobial agents upon activation.
Primary (Azurophilic) Granules
These are larger and more numerous. They contain potent hydrolytic enzymes and antimicrobial proteins, including myeloperoxidase (MPO), defensins, lysozyme, and elastase. MPO generates reactive oxygen species toxic to bacteria. Defensins are antimicrobial peptides that directly kill pathogens, while lysozyme breaks down bacterial cell walls. Elastase degrades bacterial proteins and modulates inflammation.
Secondary (Specific) Granules
These are smaller and more abundant than primary granules. They contain substances such as lactoferrin, collagenase, gelatinase, and NADPH oxidase. Lactoferrin sequesters iron, limiting bacterial growth. Collagenase and gelatinase contribute to extracellular matrix degradation. NADPH oxidase produces superoxide radicals for oxygen-dependent killing.
Tertiary (Gelatinase) Granules
These are the smallest and rapidly released upon activation. They contain matrix metalloproteinases (MMPs), such as gelatinase, which break down extracellular matrix components like collagen. This degradation is important for tissue remodeling and creating pathways for immune cell migration.
Beyond granules, neutrophils also contain mitochondria. While their role in energy production (ATP synthesis) is less pronounced compared to other cell types, as glycolysis is a primary energy pathway for neutrophils, they are more involved in regulating cell death processes, like apoptosis, and contribute to functions such as neutrophil extracellular trap (NET) formation, adhesion, and migration. The cytoskeleton, composed of actin and microtubules, is also highly dynamic within neutrophils. This internal scaffolding is responsible for maintaining cell shape, enabling cellular movement, and facilitating the engulfment of pathogens during phagocytosis. Other organelles, such as the Golgi apparatus and endoplasmic reticulum, are present for protein synthesis and packaging, supporting the production and sorting of the various proteins found within the neutrophil’s granules.
Structure’s Role in Action
The distinctive structural features of neutrophils directly enable their rapid and effective immune functions. The flexible, multi-lobed nucleus and dynamic cytoskeleton allow neutrophils to undergo significant shape changes. This adaptability is important for their rapid migration through tissues, a process known as chemotaxis, where they follow chemical signals. It also facilitates diapedesis, the process of squeezing between endothelial cells to exit blood vessels and enter inflamed tissues.
Once at the site of infection, the cell membrane and underlying cytoskeleton are key in phagocytosis, the process of engulfing pathogens. The neutrophil extends projections of its membrane, called pseudopodia, to surround and internalize microorganisms, forming a phagosome. The pre-packaged contents of the various granules are then delivered to this phagosome.
The enzymes and antimicrobial peptides from primary, secondary, and tertiary granules, such as myeloperoxidase, defensins, and lactoferrin, are released into the phagosome to destroy the engulfed pathogens. Neutrophils also employ neutrophil extracellular traps (NETs), releasing a meshwork of DNA and granule proteins into the extracellular space to trap and kill microbes outside the cell. The pre-existence of these diverse antimicrobial agents within granules allows for an immediate response upon encountering threats, linking neutrophil structure to their swift role in innate immunity.