Neutrophils are a type of white blood cell, also known as leukocytes, that play an important role in the body’s immune system. They are the most abundant kind of white blood cell in humans, making up 40% to 70% of all white blood cells. These cells are a first line of defense against invading microorganisms like bacteria and viruses. When infection or inflammation occurs, neutrophils are among the first immune cells to respond, quickly traveling to the affected site.
How Neutrophils Are Formed
Neutrophils originate in the bone marrow from hematopoietic stem cells (HSCs), precursor cells that develop into all blood cell types. This process, called granulopoiesis, involves a series of differentiation steps. HSCs first give rise to multipotent progenitors, which then develop into common myeloid progenitors (CMPs).
CMPs then differentiate into granulocyte-macrophage progenitors (GMPs), committed to forming neutrophils and monocytes. From GMPs, neutrophil-committed proliferative precursors emerge, undergoing several stages of maturation within the bone marrow. These stages include myeloblasts, promyelocytes, myelocytes, metamyelocytes, and band neutrophils, before becoming mature segmented neutrophils ready for release into the bloodstream. The bone marrow continuously produces a vast number of neutrophils, with a normal adult generating about 100 billion daily. This provides a large reserve for rapid response to infections or inflammation.
Their Role in Defense
Neutrophils rapidly migrate to affected tissues in response to infection and inflammation. Highly mobile, they move through blood vessels and interstitial spaces, guided by chemical signals released at sites of tissue damage. Once at the site, they employ several mechanisms to combat pathogens.
One primary mechanism is phagocytosis, where neutrophils engulf invading microorganisms, dead cells, and tissue debris. The engulfed particles are contained within a phagosome, which fuses with neutrophil granules to create an acidic environment and deliver antimicrobial molecules, destroying the pathogen. Neutrophils also release antimicrobial substances from their granules into the extracellular space through degranulation. These substances, including lactoferrin, lysozyme, and collagenase, kill pathogens and modulate the immune response.
Neutrophils can also form Neutrophil Extracellular Traps (NETs) in a process called NETosis. NETs are web-like structures composed of DNA, histones, and antimicrobial proteins released from the neutrophil. These traps physically ensnare and immobilize microorganisms, preventing their spread and facilitating their destruction. These combined actions make neutrophils effective against a wide range of invading microbes.
What Happens When Neutrophil Levels Change
Abnormal neutrophil counts can impact the body’s ability to fight infection and manage inflammation. Neutropenia refers to lower-than-normal levels of neutrophils in the blood, which can make it harder for the body to fight infections. This condition can arise from various factors, including genetic conditions, infections, cancer, or medical treatments like chemotherapy. Symptoms of neutropenia often stem from infections, such as fever, sore throat, swollen lymph nodes, or mouth ulcers. Severe neutropenia can be life-threatening if infections are not promptly treated.
Conversely, neutrophilia indicates an elevated number of neutrophils in the bloodstream. This increase occurs in response to acute infections, inflammation, physical or emotional stress, or certain medications. For instance, most bacterial infections lead to a high neutrophil count as the body increases its immune response. While neutrophilia may not always present specific symptoms, the underlying condition causing it can lead to fever, fatigue, swelling, or persistent infections. Monitoring neutrophil levels is therefore important for diagnosing underlying health issues and guiding appropriate medical management.