Neutrophils are a type of white blood cell that play a significant role in the body’s defense system. These cells are the most abundant kind of white blood cell, making up 55% to 70% of the total white blood cell count. They are a component of the innate immune response, the body’s immediate, non-specific defense against harmful invaders.
Neutrophils as Immune First Responders
Neutrophils act as the body’s initial responders to infection or injury, migrating to affected areas quickly. They are attracted by chemical signals released by damaged tissues and other immune cells, accumulating rapidly at the site of inflammation. Once there, neutrophils engage in several mechanisms to neutralize threats, particularly bacteria and fungi.
One primary method is phagocytosis, where neutrophils engulf pathogens, foreign particles, or dead cells. After engulfing the target, they form a specialized compartment called a phagosome, which then fuses with granules containing antimicrobial substances and enzymes. These enzymes and reactive oxygen species work together to break down and destroy the ingested microbes within the cell.
Neutrophils also release antimicrobial factors directly into the extracellular space. They can form neutrophil extracellular traps (NETs), which are web-like structures composed of decondensed chromatin, DNA, and various proteins. These NETs ensnare and kill microbes outside the cell, preventing their spread and enhancing their elimination by other immune cells like macrophages.
Production and Lifespan of Neutrophils
Neutrophils are continuously produced in the bone marrow, the soft, spongy tissue found inside bones. This process, known as granulopoiesis, involves hematopoietic stem cells differentiating into progenitor cells, which then mature into neutrophils. The bone marrow maintains a large reserve of mature neutrophils, ready for release into the bloodstream.
The journey from a precursor cell to a mature neutrophil takes about 12 to 17 days before they enter circulation. Once released into the bloodstream, neutrophils have a relatively short lifespan. While older estimates suggested they lived for less than a day, recent studies indicate an average circulatory lifespan of approximately 5.4 days.
After circulating in the blood, neutrophils migrate into tissues or are cleared by other cells, such as macrophages. This constant turnover necessitates the continuous production of billions of new neutrophils daily to maintain the body’s defense capabilities.
Understanding Neutrophil Counts in Health and Disease
Neutrophil levels in the blood are measured as part of a complete blood count (CBC) with differential. A normal neutrophil count ranges between 2,500 and 7,000 cells per microliter of blood. Deviations from this range can indicate underlying health conditions.
An elevated neutrophil count, known as neutrophilia, suggests the body is actively responding to a challenge. Common causes include bacterial infections, widespread inflammation, and physical or emotional stress. Conditions such as rheumatoid arthritis, gout, tissue damage from burns or trauma, and certain medications like corticosteroids can also lead to increased neutrophil levels.
Conversely, a low neutrophil count, termed neutropenia, can increase susceptibility to infections. Common reasons for decreased neutrophil levels include certain viral infections like measles or HIV, some autoimmune conditions such as lupus, and problems with bone marrow function. Cancer treatments like chemotherapy and radiation are also frequent causes of neutropenia, as they can inhibit neutrophil production in the bone marrow.