Core Framework & Diagram Neutrophils: The First Responders

Elevated white cell count (leukocytosis) typically reflects increased neutrophil numbers — the result of bone marrow accelerating production in response to infection signals. Mild elevation (10–15×10⁹/L) usually suggests bacterial infection; marked elevation (>25×10⁹/L) indicates severe infection or intense bone marrow stress response. Extreme elevation (leukemoid reaction) can be seen in severe infection but also certain drugs or leukemia — professional assessment is needed.

Bacterial infection typically causes a pronounced rise in neutrophils, since they're the primary anti-bacterial force. Viral infection typically causes a relative rise in lymphocytes (or relative fall in neutrophils), since T cells and NK cells are the primary anti-viral force. This difference makes the CBC a useful rapid initial tool for distinguishing bacterial from viral infection — and an important factor in deciding whether to use antibiotics.

NETs play a protective role but also carry side effects. NETs forming within blood vessels can promote platelet aggregation and clotting factor activation, sometimes causing thrombosis — particularly in severe COVID-19 and sepsis, leading to microvessel obstruction and organ damage. The disseminated intravascular coagulation (DIC) in severe COVID-19 is partly driven by excessive NET activation. A classic example of the immune system's powerful weapons causing 'friendly fire' when dysregulated.

Granulocyte colony-stimulating factor (G-CSF) is a cytokine that promotes bone marrow neutrophil production. Used during chemotherapy-related bone marrow suppression (such as filgrastim or pegfilgrastim), G-CSF can accelerate neutrophil recovery, shorten the duration of severe neutropenia, and reduce infection risk. This is now a standard component of chemotherapy supportive care, helping patients rebuild their first immune defense line faster after chemotherapy.