| Granulocyte | |
|---|---|
 Types of granulocytes | |
| Details | |
| System | Immune system |
| Identifiers | |
| MeSH | D006098 |
| FMA | 62854 |
| Anatomical terms of microanatomy | |
Granulocytes are cells in theinnate immune system characterized by the presence ofspecific granules in theircytoplasm.[1] Such granules distinguish them from the variousagranulocytes. Allmyeloblastic granulocytes are polymorphonuclear, that is, they have varying shapes (morphology) of thenucleus (segmented, irregular; often lobed into three segments); and are referred to aspolymorphonuclear leukocytes (PMN,PML, orPMNL). In common terms,polymorphonuclear granulocyte refers specifically to "neutrophil granulocytes",[2] the most abundant of the granulocytes; the other types (eosinophils,basophils, andmast cells) have varying morphology. Granulocytes are produced viagranulopoiesis in thebone marrow.
There are four types of granulocytes (full name polymorphonuclear granulocytes):[3]
Except for the mast cells, their names are derived from theirstaining characteristics; for example, the most abundant granulocyte is theneutrophil granulocyte, which has neutrally stainingcytoplasmic granules.[4]
Neutrophils are normally found in thebloodstream and are the most abundant type ofphagocyte, constituting 60% to 65% of the total circulating white blood cells,[5] and consisting of twosubpopulations: neutrophil-killers and neutrophil-cagers. One litre of human blood contains about five billion neutrophils,[6] which are about 12–15micrometres in diameter.[7] Once neutrophils have received the appropriate signals, it takes them about thirty minutes to leave the blood and reach the site of an infection.[8] Neutrophils do not return to the blood; they turn intopus cells and die.[8] Mature neutrophils are smaller than monocytes, and have a segmentednucleus with several sections(two to five segments); each section is connected bychromatin filaments. Neutrophils do not normally exit the bone marrow until maturity, but during an infection neutrophil precursors calledmyelocytes andpromyelocytes are released.[9]
Neutrophils have three strategies for directly attacking micro-organisms:phagocytosis (ingestion), release of soluble anti-microbials (including granule proteins), and generation ofneutrophil extracellular traps (NETs).[10]Neutrophils are professionalphagocytes:[11] they are ferocious eaters and rapidly engulf invaders coated withantibodies andcomplement, 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.[12] Neutrophils can secrete products that stimulate monocytes andmacrophages; these secretions increase phagocytosis and the formation of reactive oxygen compounds involved in intracellular killing.[13]
Neutrophils have two types of granules; primary (azurophilic) granules (found in young cells) andsecondary (specific) granules (which are found in more mature cells). Primary granules contain cationic proteins anddefensins that are used to kill bacteria, proteolytic enzymes and cathepsin G to break down (bacterial) proteins, lysozyme to break down bacterialcell walls, andmyeloperoxidase (used to generate toxic bacteria-killing substances).[14] In addition, secretions from theprimary granules of neutrophils stimulate the phagocytosis ofIgG antibody-coated bacteria.[15] The secondary granules contain compounds that are involved in the formation of toxicoxygen compounds, lysozyme, andlactoferrin (used to take essentialiron from bacteria).[14]Neutrophil extracellular traps (NETs) comprise a web of fibers composed ofchromatin andserine 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.[16]
Eosinophils also have kidney-shapedlobed nuclei (two to four lobes). The number of granules in an eosinophil can vary because they have a tendency todegranulate while in the blood stream.[17] Eosinophils play a crucial part in the killing of parasites (e.g., enteric nematodes) because their granules contain a unique, toxic basic protein and cationic protein (e.g.,cathepsin[14]);[18] receptors that bind toIgG andIgA are used to help with this task.[19] These cells also have a limited ability to participate in phagocytosis,[20] they are professional antigen-presenting cells, they regulate other immune cell functions (e.g.,CD4+ T cell,dendritic cell,B cell,mast cell,neutrophil, andbasophil functions),[21] they are involved in the destruction of tumor cells,[17] and they promote the repair of damaged tissue.[22] A polypeptide calledinterleukin-5 interacts with eosinophils and causes them to grow and differentiate; this polypeptide is produced by basophils and by T-helper 2 cells (TH2).[18]
Basophils are one of the least abundant cells inbone marrow andblood (occurring at less than two percent of all cells). Like neutrophils and eosinophils, they have lobednuclei; however, they have only two lobes, and thechromatin filaments that connect them are not very visible. Basophils have receptors that can bind toIgE,IgG,complement, andhistamine. Thecytoplasm of basophils contains a varied amount of granules; these granules are usually numerous enough to partially conceal the nucleus.Granule contents of basophils are abundant with histamine,heparin,chondroitin sulfate,peroxidase,platelet-activating factor, and other substances.[23]
When an infection occurs, mature basophils will be released from the bone marrow and travel to the site of infection.[24] When basophils are injured, they will release histamine, which contributes to theinflammatory response that helps fight invading organisms. Histamine causes dilation and increased permeability ofcapillaries close to the basophil. Injured basophils and otherleukocytes will release another substance calledprostaglandins that contributes to an increased blood flow to the site of infection. Both of these mechanisms allow blood-clotting elements to be delivered to the infected area (this begins the recovery process and blocks the travel ofmicrobes to other parts of the body). Increased permeability of the inflamed tissue also allows for morephagocyte migration to the site of infection so that they can consume microbes.[20]
Mast cells are a type of granulocyte that are present in tissues;[3] they mediate host defense againstpathogens (e.g.,parasites) andallergic reactions, particularlyanaphylaxis.[3] Mast cells are also involved in mediatinginflammation andautoimmunity as well as mediating and regulatingneuroimmune system responses.[3][25][26]
Granulocytes are derived from stem cells residing in the bone marrow. The differentiation of these stem cells from multipotenthematopoietic stem cell into granulocytes is termedgranulopoiesis. Multiple intermediate cell types exist in this differentiation process, includingmyeloblasts andpromyelocytes.[27]
Examples of toxic materials produced or released bydegranulation by granulocytes on the ingestion of microorganisms are:
Granulocytopenia is an abnormally low concentration of granulocytes in the blood. This condition reduces the body's resistance to many infections. Closely related terms includeagranulocytosis (etymologically, "no granulocytes at all"; clinically, granulocyte levels less than 5% of normal) andneutropenia (deficiency ofneutrophil granulocytes). Granulocytes live only one to two days in circulation (four days inspleen or other tissue), sotransfusion of granulocytes as a therapeutic strategy would confer a very short-lasting benefit. In addition, there are many complications associated with such a procedure.
There is usually a granulocytechemotactic defect in individuals suffering fromtype 1 diabetes mellitus.
Research suggests giving granulocyte transfusions to prevent infections decreased the number of people who had a bacterial or fungal infection in the blood.[28] Further research suggests participants receiving therapeuticgranulocyte transfusions show no difference in clinical reversal of concurrent infection.[29]
_diagram_en.svg)
MCs originate from a bone marrow progenitor and subsequently develop different phenotype characteristics locally in tissues. Their range of functions is wide and includes participation in allergic reactions, innate and adaptive immunity, inflammation, and autoimmunity [34]. In the human brain, MCs can be located in various areas, such as the pituitary stalk, the pineal gland, the area postrema, the choroid plexus, thalamus, hypothalamus, and the median eminence [35]. In the meninges, they are found within the dural layer in association with vessels and terminals of meningeal nociceptors [36]. MCs have a distinct feature compared to other hematopoietic cells in that they reside in the brain [37]. MCs contain numerous granules and secrete an abundance of prestored mediators such as corticotropin-releasing hormone (CRH), neurotensin (NT), substance P (SP), tryptase, chymase, vasoactive intestinal peptide (VIP), vascular endothelial growth factor (VEGF), TNF, prostaglandins, leukotrienes, and varieties of chemokines and cytokines some of which are known to disrupt the integrity of the blood-brain barrier (BBB) [38–40].
They key role of MCs in inflammation [34] and in the disruption of the BBB [41–43] suggests areas of importance for novel therapy research. Increasing evidence also indicates that MCs participate in neuroinflammation directly [44–46] and through microglia stimulation [47], contributing to the pathogenesis of such conditions such as headaches, [48] autism [49], and chronic fatigue syndrome [50]. In fact, a recent review indicated that peripheral inflammatory stimuli can cause microglia activation [51], thus possibly involving MCs outside the brain.
Media related toGranulozyt at Wikimedia Commons| International | |
|---|---|
| National | |
| Other | |
