Neutrophils are the most abundant type of white blood cell, accounting for 50–70% of circulating white cells. They are the body's primary first-line responders to bacterial infection and acute inflammation — rapidly migrating to sites of infection, where they engulf and destroy pathogens through phagocytosis and by releasing antimicrobial enzymes and reactive oxygen species.

Elevated neutrophils (neutrophilia) typically indicate bacterial infection, inflammation, physiological stress (including exercise, pregnancy, surgery), or corticosteroid use. Low neutrophils (neutropenia) increase the risk of serious bacterial infection and can result from chemotherapy, bone marrow suppression, viral infections (including COVID-19 in some cases), autoimmune disorders, or medications.