Neutrophil kinetics in health and disease

Neutrophils play a key role in the elimination of pathogens. They are remarkably short-lived with a circulating half life of 6–8 h and hence are produced at a rate of 5 × 10(10)–10 × 10(10) cells/day. Tight regulation of these cells is vital because they have significant histotoxic capacity and are widely implicated in tissue injury. This review outlines our current understanding of how neutrophils are released from the bone marrow; in particular, the role of the CXC chemokine receptor 4/stromal-derived factor 1 axis, the relative size and role of the freely circulating and marginated (i.e. slowly transiting) pools within the vascular compartment, and the events that result in the uptake and removal of circulating neutrophils. We also review current understanding of how systemic stress and inflammation affect this finely balanced system.