Summary
Neutrophils are the most abundant type of white blood cell and the primary defenders against bacterial and fungal infections. They rise dramatically within hours of bacterial infection (neutrophilia) and fall below safe levels in neutropenia — where even minor infections can become life-threatening. The neutrophil count is reported as part of the white cell differential in a full blood count.
Neutrophils are short-lived (6–8 hours in circulation) but are produced in enormous numbers — approximately 100 billion per day. They detect bacteria via pattern recognition receptors, migrate to infection sites, and destroy pathogens through phagocytosis, degranulation, and neutrophil extracellular traps (NETs).
Neutrophilia (elevated neutrophils) is the most common cause of a raised total white cell count and most commonly reflects bacterial infection, steroid therapy, or physiological stress. Neutropenia (low neutrophils) is a serious condition that dramatically increases infection susceptibility — most commonly caused by chemotherapy, viral infections, or certain medications.
The absolute neutrophil count (ANC) is the key metric: ANC < 1.5 × 10⁹/L = mild neutropenia; < 0.5 × 10⁹/L = severe neutropenia (life-threatening infection risk).
What It Is
Neutrophils are granulocytes (containing cytoplasmic granules) derived from granulocyte-monocyte progenitors in the bone marrow under the control of G-CSF (granulocyte colony-stimulating factor). They contain two types of granules: primary (azurophilic — myeloperoxidase, elastase, defensins) and secondary (specific — lactoferrin, collagenase).
On microscopy, neutrophils have a characteristic multi-lobed (polymorphonuclear) nucleus — hence PMN. Immature forms (band cells, metamyelocytes) seen in severe infection or leukaemia are reported as a ‘left shift’ on the blood film.
Reference range: 2.0–7.5 × 10⁹/L (adults). Absolute neutrophil count (ANC) = total WBC × neutrophil percentage. The ANC, not the percentage, is the clinically relevant metric for infection risk.
Functions
First-line bacterial defence
Neutrophils are the first immune cells to arrive at infection sites, phagocytosing and killing bacteria within minutes.
Bacterial infection indicator
Neutrophilia is the hallmark of bacterial infection — a key guide for decisions about antibiotic therapy.
Infection risk in neutropenia
ANC < 0.5 × 10⁹/L causes febrile neutropenia — a medical emergency with 10–20% mortality without prompt antibiotic treatment.
Treatment monitoring
Absolute neutrophil count is the critical metric monitored after chemotherapy to determine when it is safe to give the next cycle.
Reference Ranges
Absolute Neutrophil Count
Measured in ×10⁹/L| Status | Range (×10⁹/L) | Range (cells/μL (×10³)) | What it means |
|---|---|---|---|
| Severe neutropenia | < 0.5 | Life-threatening infection risk — haematological emergency. Empirical antibiotics if febrile. | |
| Mild neutropenia | 0.5–2.0 | Neutropenia — investigate cause. Significant infection risk below 1.0. | |
| Normal | 2.0–7.5 | Normal neutrophil count — adequate bacterial defence. | |
| Elevated | > 7.5 | Neutrophilia — bacterial infection, steroids, physiological stress, or myeloproliferative disorder. |
Absolute neutrophil count (ANC) = total WBC × neutrophil fraction. ANC < 0.5 × 10⁹/L with fever = febrile neutropenia — urgent treatment required. Physiological causes (exercise, stress, steroids, smoking) commonly raise neutrophils without infection.
Symptoms of Imbalance
Neutrophil abnormalities manifest as either increased infection susceptibility (low) or signs of bacterial infection (high).
- Recurrent bacterial infections
- Mouth ulcers and gum disease
- Skin abscesses
- Fever without obvious focus
- Slow wound healing
- Perianal infections
- Fever, sweats, and rigors (bacterial infection)
- Localised infection signs: pain, redness, swelling, warmth
- Physiological neutrophilia (exercise, stress): no symptoms
- In leukaemia: fatigue, weight loss, bone pain, splenomegaly
Causes of Imbalance
- Chemotherapy (most common cause in hospitals)
- Viral infections (HIV, EBV, influenza suppress bone marrow)
- Medications: NSAIDs, carbimazole, clozapine, antibiotics (rare)
- Autoimmune neutropenia
- Bone marrow failure (aplastic anaemia, leukaemia)
- Severe sepsis (consumption outpacing production)
- Felty's syndrome (rheumatoid arthritis with neutropenia)
- Bacterial infection — the most common clinical cause
- Corticosteroids (mobilise marginated neutrophils)
- Physical stress, exercise, emotional stress
- Smoking (chronic neutrophilia)
- Chronic myeloid leukaemia (very high neutrophilia with left shift)
- Other myeloproliferative disorders
- Post-splenectomy
FAQs
Febrile neutropenia (FN) is defined as a temperature > 38°C with an absolute neutrophil count < 0.5 × 10⁹/L — most commonly occurring after chemotherapy. It is a medical emergency because neutropenic patients cannot mount a normal inflammatory response to infection, and untreated bacterial sepsis can be fatal within hours. Patients with suspected FN require immediate hospital assessment, blood cultures, and empirical broad-spectrum antibiotics before culture results return.
Yes. Psychological stress and physical exertion both trigger adrenaline release, which mobilises neutrophils from the ‘marginated pool’ (cells rolling along vessel walls) into the circulating pool. This can raise the absolute neutrophil count by 3–5 × 10⁹/L within minutes, with no infection or inflammation involved. Corticosteroids cause a more prolonged neutrophilia by both mobilising marginated cells and blocking their migration into tissues.
A left shift means immature neutrophil forms — band cells, metamyelocytes, or even myelocytes — are present in the circulation. In normal circumstances, only mature segmented neutrophils circulate. A left shift occurs when the bone marrow is under extreme demand (usually severe bacterial infection or sepsis) and releases immature cells early. A very marked left shift with blasts (the most primitive cell type) suggests acute leukaemia rather than infection.
Important causes include: carbimazole and propylthiouracil (thyroid medications — require regular FBC monitoring); clozapine (antipsychotic — mandatory WBC monitoring); NSAIDs (rare idiosyncratic reaction); certain antibiotics (trimethoprim, chloramphenicol); anticonvulsants (carbamazepine, phenytoin); and disease-modifying antirheumatic drugs (methotrexate, sulfasalazine). Any new medication causing unexplained neutropenia should be stopped and the prescriber informed.
References
- Freifeld AG, et al. Clinical practice guideline for the use of antimicrobial agents in neutropenic patients with cancer: 2010 update by the IDSA. Clin Infect Dis. 2011;52(4):e56–e93. View source
- Berliner N. The complete blood count and leukocyte differential count. N Engl J Med. 2020;383(18):1774–1783. View source