Granulocytes are a category of white blood cells in the innate immune system characterized by the presence of granules in their cytoplasm. They are also called polymorphonuclear leukocytes (PMN, PML, or PMNL) because of the varying shapes of the nucleus, which is usually lobed into three segments. This distinguishes them from the mononuclear agranulocytes. In common parlance, the term polymorphonuclear leukocyte often refers specifically to "neutrophil granulocytes", the most abundant of the granulocytes; the other types (eosinophils, basophils, and mast cells) have lower numbers of lobes. Granulocytes are produced via granulopoiesis in the bone marrow. Neutrophils are normally found in the bloodstream and are the most abundant type of phagocyte, constituting 60% to 65% of the total circulating white blood cells, and consisting of two subpopulations: neutrophil-killers and neutrophil-cagers. One litre of human blood contains about five billion (5x109) neutrophils, which are about 12–15 micrometres in diameter. Once neutrophils have received the appropriate signals, it takes them about thirty minutes to leave the blood and reach the site of an infection. Neutrophils do not return to the blood; they turn into pus cells and die. Mature neutrophils are smaller than monocytes, and have a segmented nucleus with several sections(two to five segments); each section is connected by chromatin filaments. Neutrophils do not normally exit the bone marrow until maturity, but during an infection neutrophil precursors called myelocytes and promyelocytes are released. Neutrophils have three strategies for directly attacking micro-organisms: phagocytosis (ingestion), release of soluble anti-microbials (including granule proteins), and generation of neutrophil extracellular traps (NETs). Neutrophils are professional phagocytes:they are ferocious eaters and rapidly engulf invaders coated with antibodies and complement, as well as damaged cells or cellular debris. The intracellular granules of the human neutrophil have long been recognized for their protein-destroying and bactericidal properties. Neutrophils can secrete products that stimulate monocytes and macrophages; these secretions increase phagocytosis and the formation of reactive oxygen compounds involved in intracellular killing.

Neutrophils have two types of granules; primary (azurophilic) granules (found in young cells) and secondary (specific) granules (which are found in more mature cells). Primary granules contain cationic proteins and defensins that are used to kill bacteria, proteolytic enzymes and cathepsin G to break down (bacterial) proteins, lysozyme to break down bacterial cell walls, and myeloperoxidase (used to generate toxic bacteria-killing substances). In addition, secretions from the primary granules of neutrophils stimulate the phagocytosis of IgG antibody-coated bacteria. The secondary granules contain compounds that are involved in the formation of toxic oxygen compounds, lysozyme, and lactoferrin (used to take essential iron from bacteria). Neutrophil extracellular traps (NETs) comprise a web of fibers composed of chromatin and serine proteases that trap and kill microbes extracellularly. Trapping of bacteria is a particularly important role for NETs in sepsis, where NET are formed within blood vessels

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