Neutrophils are produced in bone marrow, released into blood after completing their maturation in marrow, circulate for less than a day (10-15 hours), and migrate out of the vessels into tissues or into alveoli and gut lumen. While circulating, neutrophils are distributed between large vessels (neutrophils here form the circulating neutrophil pool or CNP; these are the neutrophils in the usual blood sample) and small vessels (neutrophils here form the marginating neutrophil pool or MNP). The total body neutrophil pool or TNP comprises the CNP, MNP, and the pool of post-mitotic neutrophils in marrow.
Humoral factors produced at sites of inflammation stimulate increased production of neutrophils in marrow and increased release of neutrophils from marrow. Chemotactic factors produced at sites of inflammation direct migration of neutrophils from blood vessels into the tissue at those sites.
Normally, neutrophils are released from marrow in an age-dependent manner, i.e., the most mature cells are released before less mature cells. Corticosteroids tend to cause movement of neutrophils from the MNP to the CNP and release of some cells from the marrow pool, thereby raising the neutrophil count in a blood sample. Endotoxin tends to cause sequestration of neutrophils in the spleen, liver, and lung, thereby lowering the neutrophil count in a blood sample; endotoxin also stimulates release of neutrophils from marrow and increases granulocytopoiesis resulting in a rebound rise in neutrophil count with presence of immature neutrophils (bands, metamyelocytes).
Common causes of neutrophilia include inflammation (either infectious or non-infectious [trauma, surgery, burns, etc.]), exogenous or endogenous corticosteroids, and epinephrine. Immune-mediated diseases such as immune-mediated hemolytic anemia are also commonly associated with neutrophilia due to actions of inflammatory cytokines.
Common causes of neutropenia include overwhelming inflammation/infection in which tissue demand for neutrophils outpaces bone marrow production, endotoxemia (acute), primary bone marrow disease (e.g. aplasia, hypoplasia, or myelophthisis1), or drugs/infections that suppress bone marrow production (e.g. estrogens, chemotherapeutics, chronic ehrlichiosis).
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