Neutrophils in the innate immune response

Polymorphonuclear leukocytes (PMNs or neutrophils) are an essential component of the human innate immune system. Circulating neutrophils are rapidly recruited to sites of infection by host- and/or pathogen-derived components, which also prime these host cells for enhanced microbicidal activity. PMNs bind and ingest microorganisms by a process known as phagocytosis, which typically triggers production of reactive oxygen species and the fusion of cytoplasmic granules with pathogen-containing vacuoles. The combination of neutrophil reactive oxygen species and granule components is highly effective in killing most bacteria and fungi. Inasmuch as PMNs are the most abundant type of leukocyte in humans and contain an arsenal of cytotoxic compounds that are non-specific, neutrophil homeostasis must be highly regulated. To that end, constitutive PMN turnover is regulated by apoptosis, a process whereby these cells shut down and are removed safely by macrophages. Notably, apoptosis is accelerated following phagocytosis of bacteria, a process that appears important for the resolution of infection and inflammation. This review provides a general overview of the role of human neutrophils in the innate host response to infection and summarizes some of the recent advances in neutrophil biology.