Neutrophils (also known as polymorphonuclear leukocytes [PMNs] or granulocytes)
are the most abundant white cell in humans. Granulocytes and/or granulocyte
precursors normally comprise approx 60% of the nucleated cells in bone
marrow and the bloodstream. Mature neutrophils have a typical circulating halflife
of 6–8 h in the blood and then migrate through tissues for approx 2-3 d.
Their relatively short life-span is devoted largely to surveillance for invading
microorganisms. During infection, the neutrophil life-span is extended, granulopoiesis
increases, and large numbers of neutrophils are rapidly recruited to
the site(s) of infection. Following recognition (binding) and phagocytosis of
microorganisms, neutrophils utilize an extraordinary array of oxygen-dependent
and oxygen-independent microbicidal weapons to destroy infectious
agents. Oxygen-dependent mechanisms involve the production of reactive oxygen
species (ROS), while oxygen-independent mechanisms include degranulation
and release of lytic enzymes and bactericidal peptides. Inasmuch as these
processes are highly effective at killing most ingested microbes, neutrophils
serve as the primary cellular defense against infection.