Neutrophil — The Immune System's First Responder

Neutrophils are granular polymorphonuclear leukocytes originating from myeloid stem cells within the bone marrow. Characterised morphologically by a multi-lobed nucleus and specialised cytoplasmic granules, they constitute between fifty and seventy percent of all circulating white blood cells in healthy adults. Their primary physiological responsibility is rapid chemotaxis—migrating swiftly toward biochemical signals generated by invading microorganisms or damaged tissue.

Upon arriving at an infection site, neutrophils neutralise pathogens through several coordinated mechanisms. These include phagocytosis, wherein microorganisms are engulfed and degraded by intracellular enzymes; the generation of reactive oxygen species via respiratory burst; and the release of neutrophil extracellular traps (NETs). NETs are web-like structures composed of chromatin and antimicrobial proteins that ensnare and neutralise extracellular pathogens before they can disseminate further.

In cancer therapy, neutrophils serve as a vital indicator of bone marrow function and immune defence. Many standard cancer treatments, including chemotherapy and targeted agents, temporarily suppress bone marrow activity, reducing neutrophil production. Keeping track of neutrophil levels helps clinicians determine whether it is safe to proceed with the next treatment cycle, when supportive therapies are needed, and how vulnerable a patient might be to potentially serious infections.

Neutrophil status is evaluated via a standard full blood count with differential. This test reports both the relative percentage of neutrophils among white blood cells and the Absolute Neutrophil Count (ANC). The ANC is calculated by multiplying the total white blood cell count by the percentage of mature (segmented) and immature (band) neutrophils, providing the definitive metric used in oncological decision-making.

Neutrophils are monitored across all cancer types involving systemic therapy. They are particularly pivotal in leukaemia, myelodysplastic syndromes, and lymphoma, where abnormal cell growth directly crowds out healthy neutrophil production in the bone marrow. They are also central to the clinical management of solid tumours—such as breast, colorectal, and lung cancers—treated with myelosuppressive drug regimens.

If neutrophil levels drop substantially, oncologists may administer synthetic growth factors known as granulocyte colony-stimulating factors (G-CSFs), such as filgrastim, to stimulate accelerated production in the bone marrow. Clinicians also use ANC thresholds to calibrate chemotherapy scheduling; if counts are inadequate, treatment is deferred temporarily to allow bone marrow recovery, shielding the patient from severe infectious complications.