Neutrophils, which are also known as polymorphonuclear
leukocytes (PMN), represent 50 to 60% of the total circulating
leukocytes and constitute the ''first line of defence'' against
infectious agents or ''nonself'' substances that penetrate the
body's physical barriers. Once an inflammatory response is
initiated, neutrophils are the first cells to be recruited to
sites of infection or injury. Their targets include bacteria,
fungi, protozoa, viruses, virally infected cells and tumour cells.
Their development in the bone marrow takes about two weeks; during
this period, they undergo proliferation and differentiation.
During maturation, they pass trough six morphological stages:
myeloblast, promyeloblast, myelocyte, metamyelocyte,
non-segmented (band) neutrophil, segmented neutrophil. The
segmented neutrophil is a fully functionally active cell. It
contains cytoplasmic granules (primary or azurophil and secondary
or specific) and a lobulated chromatin-dense nucleus with no
nucleolus. The bone marrow of a normal healthy adult produces more
than 10
neutrophils
per day and more than 10
per day in
settings of acute inflammation. Upon release from the bone marrow
to the circulation the cells are in a nonactivated state and have
a half-life of only 4 to 10 h before marginating and entering
tissue pools, where they survive for 1 to 2 days. Cells of the
circulating and marginated pools can exchange with each other.
Senescent neutrophils are thought undergo apoptosis
(programmed cell death) prior to removal by macrophages. The
viability is significantly shorter in individuals suffering from
infectious or acute inflammatory diseases when the tissue
requirement for newly recruited neutrophils increases
considerably.
Subpopulations of neutrophils have been identified by various
criteria. These cells exist not only in dormant ( resting) or
activated states but also in various intermediate stages.
For, example, priming is a mechanism whereby dormant
neutrophils acquire a state of preactivation that enable a more
powerful response to be generated once microbial activity is
initiated.