Neutrophils are a type of myeloid cell, a classification based on their origin and developmental pathway within the body’s blood-forming system. This article explores what neutrophils are, the broader myeloid cell category, and their developmental connection, detailing how they fulfill their protective functions.
What Neutrophils Are
Neutrophils are the most abundant type of white blood cell, or leukocyte, in humans, typically making up 55% to 70% of all white blood cells. They are characterized by a multi-lobed nucleus and a granular cytoplasm, which gives them their distinctive appearance under a microscope. These cells function as primary responders to infection and inflammation, rapidly migrating to affected areas. Neutrophils have a relatively short lifespan, circulating in the bloodstream for a few hours before migrating into tissues, where they survive for one to two days.
The Myeloid Cell Family
Myeloid cells represent a broad category of blood cells that originate from a common progenitor cell within the bone marrow. This common myeloid progenitor (CMP) is derived from hematopoietic stem cells, which are the foundational cells for all blood cell types. The myeloid cell family includes a diverse array of cells, such as monocytes, macrophages, basophils, eosinophils, mast cells, megakaryocytes (which produce platelets), and erythrocytes (red blood cells). These cells contribute to various physiological processes, including innate immunity, oxygen transport, and blood clotting.
Connecting Neutrophils to Myeloid Cells
The connection between neutrophils and myeloid cells lies in their shared developmental lineage through hematopoiesis, or blood cell formation, tracing back to multipotent hematopoietic stem cells (HSCs). These HSCs differentiate into various progenitor cells, including the common myeloid progenitor (CMP), which then gives rise to all cells of the myeloid lineage, including neutrophils.
Neutrophils specifically develop from the granulocyte-macrophage progenitor (GMP), which is a specialized subset derived from the CMP. This developmental pathway means that neutrophils are an integral part of the myeloid cell family, sharing a common ancestor with other myeloid cells. Their formation in the bone marrow from this specific lineage confirms their classification as myeloid cells.
How Neutrophils Fight Infection
Neutrophils employ several mechanisms to combat pathogens and clear infections. One method is phagocytosis, where neutrophils engulf and digest invading microorganisms like bacteria and fungi. After engulfing a pathogen into a specialized vesicle called a phagosome, the neutrophil fuses it with lysosomes containing digestive enzymes and reactive oxygen species to destroy the microbe.
Another defense strategy is degranulation, which involves releasing antimicrobial substances stored in their cytoplasmic granules into the extracellular space or directly into phagosomes. These substances include enzymes and proteins that break down microbial components and neutralize threats. Additionally, neutrophils can form Neutrophil Extracellular Traps (NETs), which are web-like structures composed of DNA and antimicrobial proteins extruded from the cell. These NETs ensnare and neutralize pathogens, preventing their spread and contributing to their elimination.