Neutrophils

What is Neutrophils?

Neutrophils are the most numerous white blood cells and the front line of the innate immune system. They typically make up 50–70% of the total leukocyte count in adults, with an absolute reference range of 1.8–7.5 ×10⁹/L. Neutrophils are produced in the bone marrow, circulate for 6–12 hours in the blood, and then migrate into tissues where they live for 1–5 days. They phagocytose and kill bacteria, fungi, and damaged tissue using an arsenal of enzymes (myeloperoxidase, elastase) and reactive oxygen species. Neutrophils can also release NETs (Neutrophil Extracellular Traps) — webs of chromatin and antimicrobial proteins that capture bacteria. Neutrophilia (elevated absolute neutrophil count, > 7.5 ×10⁹/L) is the most common cause of leucocytosis. Physiological causes include intense exercise and acute psychological stress (catecholamines mobilise neutrophils from marginal pooling along the vessel wall), corticosteroids (which cause demargination and delayed apoptosis), and pregnancy. Pathological causes are bacterial infections, acute inflammation, myocardial infarction, tissue damage, and — rarely — chronic myeloid leukaemia (CML) at extremely high counts. The proportion of band forms (immature neutrophils, also called a 'left shift') rises in acute bacterial infection and sepsis. Neutropaenia (low absolute neutrophil count, ANC < 1.8 ×10⁹/L) is clinically more urgent than neutrophilia. Causes include bone-marrow suppression from chemotherapy, immunosuppressants, or aplasia; drug-induced neutropaenia (carbimazole, clozapine, some NSAIDs, sulphonamides); viral infections (HIV, EBV, hepatitis); autoimmune neutropaenia; and lifestyle-related cyclic neutropaenia. Infection risk rises sharply at ANC < 1.0 ×10⁹/L and is severe at ANC < 0.5 ×10⁹/L (severe neutropaenia or agranulocytosis).

Why is Neutrophils relevant?

Neutrophils are the most direct reflection of active bacterial immune defence and are therefore the most monitored subpopulation in the setting of infection, immunosuppression, and haematological surveillance. An elevated neutrophil count in someone with fever and rising CRP is a strong pointer to a bacterial cause; with viral infection, neutrophils are typically normal or low while lymphocytes rise. That combination guides the first diagnostic step. For people using corticosteroids (prednisone, dexamethasone), the link between medication and neutrophilia is essential: with standard steroid doses, a neutrophil count of 10–15 ×10⁹/L is normal and not alarming. The same value in someone without steroid use does warrant context. This is an example of why the medication list must always be part of laboratory interpretation. For athletes and people under chronic stress, the neutrophil count moves with physical load: it rises acutely from catecholamine-mediated demargination (sometimes to 12–15 ×10⁹/L immediately after intensive training) and falls again quickly. After very intensive and prolonged training (overreaching), neutropaenia can occur — a sign of bone-marrow stress that calls for rest. Trends help recognise this pattern.

Neutrophils high or low — what it means

Neutrophils are always interpreted as an absolute count in ×10⁹/L (ANC), not only as a percentage of total leukocytes. The ANC is the clinically relevant measure for infection risk and bone-marrow function. Adding CRP provides infection context: at CRP > 50–100 mg/L a bacterial infection is plausible; high neutrophilic leucocytosis with normal CRP points more toward physiological causes (stress, training, medication). With neutrophilia: always ask about corticosteroid use, recent intense exercise, and psychological stress before considering infectious or haematological causes. A differential including a peripheral blood smear is appropriate with extreme neutrophilia (> 20 ×10⁹/L) or when a myeloid disorder is suspected. A left shift (band forms > 0.7 ×10⁹/L) points to active bacterial infection or sepsis. With neutropaenia, urgency is the norm: ANC < 1.0 ×10⁹/L in someone with fever is a medical emergency (febrile neutropaenia) where prompt antibiotic treatment is indicated. With asymptomatic neutropaenia, the first step is medication review (carbimazole, clozapine, trimethoprim-sulfamethoxazole), repeat testing after stopping the suspected agent, and with persistent neutropaenia a bone-marrow biopsy or aspirate.

Educational information only — not medical advice. Consult a healthcare professional for clinical decisions.