Neutrophils are the most abundant type of white blood cell and a primary component of the body’s natural defense system. These specialized immune cells are the first responders of the innate immune system, providing immediate, non-specific protection against invading microorganisms. Constituting approximately 40% to 70% of all white blood cells, neutrophils continuously patrol the bloodstream for signs of infection or injury.
The Immune System’s First Responders
Neutrophils play a primary role in fighting infections, particularly those caused by bacteria and fungi. They are among the first immune cells to rapidly migrate to a site of inflammation or infection, a process known as chemotaxis. This directed movement occurs in response to chemical signals, such as interleukin-8 (IL-8), produced by damaged tissues or other immune cells.
Once at the site, neutrophils engage in phagocytosis, where they engulf and internalize pathogens into specialized vacuoles called phagosomes. Within these phagosomes, neutrophils unleash an arsenal of antimicrobial molecules and enzymes, including myeloperoxidase and elastase, which destroy the ingested microbes. This process also involves the rapid production of reactive oxygen species, like superoxide, aiding in pathogen elimination.
Beyond engulfing invaders, neutrophils employ another mechanism known as Neutrophil Extracellular Traps (NETs). These web-like structures are composed primarily of decondensed chromatin (DNA) mixed with antimicrobial proteins and enzymes, extruded from the cell. NETs are effective at trapping and immobilizing larger pathogens too large for phagocytosis, preventing their spread throughout the body.
Neutrophil Production and Lifespan
Neutrophils are continuously produced in the bone marrow from hematopoietic stem cells. This process, called granulopoiesis, involves the differentiation of these stem cells through several precursor stages, including myeloblasts, promyelocytes, myelocytes, and metamyelocytes, before maturing into segmented neutrophils. It takes approximately 4 to 6 days for a neutrophil to mature into a functional cell ready for circulation.
After their release from the bone marrow, neutrophils circulate in the bloodstream for a relatively short period, often less than 24 hours. From the bloodstream, they then migrate into tissues, where they perform their immune functions and survive for an additional one to two days. To maintain a sufficient defense force, the bone marrow of a healthy adult produces around 100 billion neutrophils each day.
Understanding High and Low Neutrophil Counts
Variations in neutrophil counts can indicate underlying health issues, and these levels are assessed as part of a complete blood count (CBC) with differential. An elevated neutrophil count, termed neutrophilia, signifies that the body is actively combating an infection or inflammation. Common causes include bacterial infections like pneumonia or appendicitis, acute and chronic inflammatory conditions such as rheumatoid arthritis, or physical stressors like surgery or trauma.
Certain medications, such as corticosteroids, can lead to temporary increases in neutrophil levels, as can chronic conditions like some cancers or pregnancy. Conversely, a low neutrophil count, known as neutropenia, can increase susceptibility to infections. This can result from decreased production in the bone marrow, accelerated destruction, or increased usage of neutrophils.
Causes of neutropenia include certain viral infections, such as HIV, hepatitis, or measles, which can temporarily suppress bone marrow activity. Chemotherapy and radiation therapy for cancer are frequent causes, as these treatments can harm neutrophil-producing cells in the bone marrow. Autoimmune diseases like lupus or rheumatoid arthritis, some genetic conditions, and nutritional deficiencies in vitamins like B12 or folate can contribute to low neutrophil levels.