Neutrophils and Neutrophil Kinetics

Neutrophils develop under the influence of interleukin-3 (IL-3), granulocyte/monocyte-colony stimulating factor (GM-CSF), and G-CSF. Following differentiation from the common myeloid progenitor, proliferation within the mitotic pool, and maturation in the post-mitotic pool, neutrophils are released into the peripheral blood. Neutrophils remain in circulation for about 12 hours before entering tissues and undergoing programmed cell death (apoptosis) or being recruited to combat infections, invaders, or other inflammatory stimuli (Fig. 2.1). Senescent or aged neutrophils are generally lost through the mucosal surfaces of the intestinal and respiratory tracts, although phagocytosis by macrophages within tissues also occurs. Neutrophils are generally viable in tissues for up to 12 hours, however, inflammation can greatly affect neutrophil survival time, as well as transit times in the bone marrow and peripheral blood.

The peripheral blood contains a freely circulating neutrophil pool and a marginating pool of neutrophils which roll along the vascular endothelium. From the marginating pool, neutrophils may migrate into tissues or re-join the circulating pool. Venipuncture draws from the circulating pool, thus only neutrophils within the circulating pool are evaluated on the leukogram. Circulating and marginating pools are interchangeable and about equal in size (with some species variation) but their relative proportions can be altered under certain circumstances.