Emergency granulopoiesis

Neutrophils are a key cell type of the innate immune system. They are short-lived and need to be continuously generated in steady-state conditions from haematopoietic stem and progenitor cells in the bone marrow to ensure their immediate availability for the containment of invading pathogens. However, if microbial infection cannot be controlled locally, and consequently develops into a life-threatening condition, neutrophils are used up in large quantities and the haematopoietic system has to rapidly adapt to the increased demand by switching from steady-state to emergency granulopoiesis. This involves the markedly increased de novo production of neutrophils, which results from enhanced myeloid precursor cell proliferation in the bone marrow. In this Review, we discuss the molecular and cellular events that regulate emergency granulopoiesis, a process that is crucial for host survival.