Neutrophils are the most numerous type of white blood cell, making up over half of the body’s total circulating white blood cell count. Originating in the bone marrow, they are a component of the innate immune system. These cells act as the body’s first line of defense, rapidly responding to signals of infection or injury. Measuring their quantity in the bloodstream is a routine part of a Complete Blood Count, providing insight into a person’s overall immune status and health.
What Neutrophils Do in the Body
Neutrophils are continuously produced in the bone marrow and possess a short lifespan, often lasting less than a day once they enter the bloodstream. They are highly mobile cells that circulate until they detect chemical signals released by invading pathogens or damaged tissue. This directed movement toward a source of inflammation or infection is a process known as chemotaxis.
Once at the site of concern, neutrophils employ a process called phagocytosis, where they physically engulf and destroy foreign invaders, particularly bacteria and fungi. They are equipped with powerful enzymes and antimicrobial substances stored in granules that are released into the phagosome, the vesicle containing the trapped microbe. Beyond engulfing threats, they can also release their nuclear contents to form web-like structures called Neutrophil Extracellular Traps (NETs) to ensnare and neutralize pathogens.
Interpreting the Absolute Neutrophil Count
The clinically relevant measure for assessing the immune system’s capacity to fight infection is the Absolute Neutrophil Count (ANC). This number is not merely the percentage of neutrophils reported on a blood differential but rather a calculation of the total number of circulating neutrophils per volume of blood. The ANC is derived by multiplying the total white blood cell count by the percentage of mature neutrophils and the percentage of immature neutrophils, often called “bands,” and then dividing by one hundred.
For healthy adults, the standard normal range for the Absolute Neutrophil Count is between 1,500 and 8,000 cells per microliter of blood (or 1.5 to 8.0 x 10\(^9\)/L). This range represents the count necessary to maintain an adequate defense against common infections. The specific normal reference range can vary slightly between different laboratories.
Clinicians rely on the ANC because a count outside of the normal range signals a potential issue with the production, destruction, or utilization of these cells. The measurement is an indicator used to assess a patient’s risk of developing a serious infection.
Understanding Low Neutrophil Levels
A low neutrophil count, medically termed neutropenia, is diagnosed when the ANC falls below 1,500 cells/µL. This condition directly compromises the immune system’s ability to mount a defense, leading to an increased susceptibility to bacterial and fungal infections. The severity of neutropenia is categorized, as the risk of serious infection correlates with how low the count drops.
Mild neutropenia is defined as an ANC between 1,000 and 1,500/µL, while a count between 500 and 1,000/µL is considered moderate. The most concerning state is severe neutropenia, which is an ANC below 500 cells/µL, as this level carries a high risk of life-threatening infection. Chemotherapy and radiation are common causes, as they suppress the bone marrow where neutrophils are produced.
Other factors contributing to low levels include certain medications, such as some antibiotics or anti-inflammatory drugs, that can interfere with neutrophil production or increase their destruction. Autoimmune disorders, like lupus and rheumatoid arthritis, may cause the body to mistakenly attack its own neutrophils. Severe viral infections, including HIV or hepatitis, can also suppress bone marrow function, leading to a reduction in the ANC.
Understanding High Neutrophil Levels
An elevated neutrophil count, known as neutrophilia, is an ANC exceeding 8,000 cells/µL. This finding indicates that the body is actively mobilizing its immune resources in response to an acute physical challenge. The most frequent cause of neutrophilia is an acute bacterial infection, which triggers a massive release of reserve neutrophils from the bone marrow into the bloodstream.
Inflammatory conditions, even those unrelated to infection, can also stimulate a high neutrophil response, including tissue damage following trauma, surgery, or burns, and chronic inflammatory diseases like gout or rheumatoid arthritis. Certain medications, particularly corticosteroids like prednisone, can artificially raise the count by causing neutrophils lining blood vessel walls to move into the main circulation. While often a temporary reaction to illness or stress, a persistently high neutrophil count warrants further medical investigation to rule out underlying chronic issues or blood disorders.