Neutrophils account for 40 to 75 per cent of the total white cell count in the peripheral blood and are essential components of the host defense system. They are characterized by a dense nucleus consisting of two to five lobes and pale cytoplasm packed with granules whose contents are essential for killing and degrading micro-organisms. Neutrophils are derived from pluripotent stem cells located in the bone marrow under the influence of growth factors, interleukins 1 and 3 (IL-1 and IL-3), granulocyte colony-stimulating factor, and granulocyte macrophage colony-stimulating factor over a period of 6 to 10 days. In the normal state, the bone marrow storage compartment contains 10 to 15 times the number of neutrophils found in peripheral blood. Following their release from the marrow, they spend 6 to 10 h in the circulation before moving in response to endotoxins, cytokines, and chemokines into tissues where their lifespan is of the order of 4 to 5 days.

Through their cell-surface receptors, neutrophils can respond to chemotactic stimuli that direct them to the site of infection. These stimuli include breakdown products of bacterial walls and the complement cascade (e.g. C5a). Following stimulation, the polarized neutrophil adheres to the endothelium, passes through it by diapedesis, and migrates towards the site of infection. Once there, it attacks bacteria by phagocytosis and kills them by multiple mechanisms, the most important of which are release of bactericidal enzymes from the granules and the respiratory burst pathway which produces bactericidal free radicals from molecular oxygen.

Apart from their major role in the defense against bacterial infections, neutrophils also contribute to defense against fungal and viral infections. Consequently, quantitative or qualitative abnormalities of neutrophils are associated with severe and occasionally fatal infections with these pathogens.

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