Editing Immune system (section)

=== Innate immune cells ===
[[File:SEM blood cells.jpg|thumb|right |alt=See caption |A [[scanning electron microscope]] image of normal circulating human [[blood]]. One can see [[red blood cell]]s, several knobby [[white blood cell]]s, including [[lymphocyte]]s, a [[monocyte]], and a [[neutrophil]], and many small disc-shaped [[platelet]]s.]]
Some [[leukocytes]] (white blood cells) act like independent, single-celled organisms and are the second arm of the innate immune system. The innate leukocytes include the [[phagocyte#Professional phagocytes|"professional" phagocytes]] ([[macrophage]]s, [[neutrophil]]s, and [[dendritic cell]]s). These cells identify and eliminate pathogens, either by attacking larger pathogens through contact or by engulfing and then killing microorganisms. The other cells involved in the innate response include [[innate lymphoid cell]]s, [[mast cell]]s, [[eosinophil granulocyte|eosinophils]], [[basophil granulocyte|basophils]], and [[natural killer cell]]s.{{sfn| Sompayrac |2019 |pp= 1–4}}

[[Phagocytosis]] is an important feature of cellular innate immunity performed by cells called phagocytes that engulf pathogens or particles. Phagocytes generally patrol the body searching for pathogens, but can be called to specific locations by cytokines.{{sfn|Alberts|Johnson|Lewis|Raff|2002|loc=sec. [https://www.ncbi.nlm.nih.gov/books/NBK26846/#A4686 "Phagocytic Cells Seek, Engulf, and Destroy Pathogens"]}} Once a pathogen has been engulfed by a phagocyte, it becomes trapped in an intracellular [[vesicle (biology)|vesicle]] called a [[phagosome]], which subsequently fuses with another vesicle called a [[lysosome]] to form a [[phagolysosome]]. The pathogen is killed by the activity of digestive enzymes or following a [[respiratory burst]] that releases [[radical (chemistry)|free radicals]] into the phagolysosome.<ref>{{cite journal | vauthors = Ryter A | title = Relationship between ultrastructure and specific functions of macrophages | journal = Comparative Immunology, Microbiology and Infectious Diseases | volume = 8 | issue = 2 | pages = 119–33 | year = 1985 | pmid = 3910340 | doi = 10.1016/0147-9571(85)90039-6 }}</ref><ref>{{cite journal | vauthors = Langermans JA, Hazenbos WL, van Furth R | title = Antimicrobial functions of mononuclear phagocytes | journal = Journal of Immunological Methods | volume = 174 | issue = 1–2 | pages = 185–94 | date = Sep 1994 | pmid = 8083520 | doi = 10.1016/0022-1759(94)90021-3 }}</ref> Phagocytosis evolved as a means of acquiring [[nutrient]]s, but this role was extended in phagocytes to include engulfment of pathogens as a defense mechanism.<ref>{{cite journal | vauthors = May RC, Machesky LM | title = Phagocytosis and the actin cytoskeleton | journal = Journal of Cell Science | volume = 114 | issue = Pt 6 | pages = 1061–77 | date = Mar 2001 | doi = 10.1242/jcs.114.6.1061 | pmid = 11228151 | url = http://jcs.biologists.org/cgi/pmidlookup?view=long&pmid=11228151 | access-date = 6 November 2009 | archive-date = 31 March 2020 | archive-url = https://web.archive.org/web/20200331165444/https://jcs.biologists.org/content/114/6/1061.long | url-status = dead | url-access = subscription }}</ref> Phagocytosis probably represents the oldest form of host defense, as phagocytes have been identified in both vertebrate and invertebrate animals.<ref>{{cite journal | vauthors = Salzet M, Tasiemski A, Cooper E | s2cid = 28520695 | title = Innate immunity in lophotrochozoans: the annelids | journal = Current Pharmaceutical Design | volume = 12 | issue = 24 | pages = 3043–50 | year = 2006 | pmid = 16918433 | doi = 10.2174/138161206777947551 | url = https://pdfs.semanticscholar.org/da11/601b7ba0121f210136e0317729e3f367dd8c.pdf | archive-url = https://web.archive.org/web/20200331165454/https://pdfs.semanticscholar.org/da11/601b7ba0121f210136e0317729e3f367dd8c.pdf | url-status = dead | archive-date = 2020-03-31 }}</ref>

Neutrophils and macrophages are phagocytes that travel throughout the body in pursuit of invading pathogens.<ref>{{cite journal | vauthors = Zen K, Parkos CA | title = Leukocyte-epithelial interactions | journal = Current Opinion in Cell Biology | volume = 15 | issue = 5 | pages = 557–64 | date = Oct 2003 | pmid = 14519390 | doi = 10.1016/S0955-0674(03)00103-0 }}</ref> Neutrophils are normally found in the [[circulatory system|bloodstream]] and are the most abundant type of phagocyte, representing 50% to 60% of total circulating leukocytes.{{sfn|Stvrtinová | Jakubovský | Hulín |1995 |loc= Chapter: [https://web.archive.org/web/20010711220523/http://nic.savba.sk/logos/books/scientific/Inffever.html Inflammation and Fever]}} During the acute phase of [[inflammation]], neutrophils migrate toward the site of inflammation in a process called [[chemotaxis]] and are usually the first cells to arrive at the scene of infection. Macrophages are versatile cells that reside within tissues and produce an array of chemicals including enzymes, [[complement system|complement proteins]], and cytokines. They can also act as scavengers that rid the body of worn-out cells and other debris and as [[antigen-presenting cell]]s (APCs) that activate the adaptive immune system.<ref name="Rua_2015">{{cite journal | vauthors = Rua R, McGavern DB | title = Elucidation of monocyte/macrophage dynamics and function by intravital imaging | journal = Journal of Leukocyte Biology | volume = 98 | issue = 3 | pages = 319–32 | date = September 2015 | pmid = 26162402 | doi = 10.1189/jlb.4RI0115-006RR | pmc=4763596}}</ref>

Dendritic cells are phagocytes in tissues that are in contact with the external environment; therefore, they are located mainly in the skin, nose, lungs, stomach, and intestines.<ref name=Guermonprez>{{cite journal | vauthors = Guermonprez P, Valladeau J, Zitvogel L, Théry C, Amigorena S | title = Antigen presentation and T cell stimulation by dendritic cells | journal = Annual Review of Immunology | volume = 20 | issue = 1 | pages = 621–67 | year = 2002 | pmid = 11861614 | doi = 10.1146/annurev.immunol.20.100301.064828 }}</ref> They are named for their resemblance to [[neuron]]al [[dendrite]]s, as both have many spine-like projections. Dendritic cells serve as a link between the bodily tissues and the innate and adaptive immune systems, as they [[antigen presentation|present antigens]] to [[T cell]]s, one of the key cell types of the adaptive immune system.<ref name=Guermonprez />

[[Granulocyte]]s are leukocytes that have granules in their cytoplasm. In this category are neutrophils, mast cells, basophils, and eosinophils. Mast cells reside in [[connective tissue]]s and [[mucous membrane]]s and regulate the inflammatory response.{{sfn| Krishnaswamy | Ajitawi | Chi | 2006 | pp = 13–34}} They are most often associated with [[allergy]] and [[anaphylaxis]].{{sfn|Stvrtinová | Jakubovský | Hulín |1995 |loc= Chapter: [https://web.archive.org/web/20010711220523/http://nic.savba.sk/logos/books/scientific/Inffever.html   Inflammation and Fever]}} [[Basophil]]s and [[eosinophil]]s are related to neutrophils. They secrete chemical mediators that are involved in defending against [[parasitism|parasites]] and play a role in allergic reactions, such as [[asthma]].<ref>{{cite journal | vauthors = Kariyawasam HH, Robinson DS | title = The eosinophil: the cell and its weapons, the cytokines, its locations | journal = Seminars in Respiratory and Critical Care Medicine | volume = 27 | issue = 2 | pages = 117–27 | date = Apr 2006 | pmid = 16612762 | doi = 10.1055/s-2006-939514 | s2cid = 260317790 }}</ref>

Innate lymphoid cells (ILCs) are a group of [[Innate immune system|innate immune]] cells that are derived from [[common lymphoid progenitor]] and belong to the [[Lymphopoiesis|lymphoid lineage]]. These cells are defined by the absence of antigen-specific [[B-cell receptor|B-]] or [[T-cell receptor]] (TCR) because of the lack of [[recombination activating gene]]. ILCs do not express myeloid or dendritic cell markers.<ref>{{cite journal | vauthors = Spits H, Cupedo T | title = Innate lymphoid cells: emerging insights in development, lineage relationships, and function | journal = Annual Review of Immunology | volume = 30 | pages = 647–75 | year = 2012 | pmid = 22224763 | doi = 10.1146/annurev-immunol-020711-075053 }}</ref>

[[Natural killer cells]] (NK cells) are lymphocytes and a component of the innate immune system that does not directly attack invading microbes.<ref name="pmid28078307">{{cite journal |vauthors=Gabrielli S, Ortolani C, Del Zotto G, Luchetti F, Canonico B, Buccella F, Artico M, Papa S, Zamai L |title=The Memories of NK Cells: Innate-Adaptive Immune Intrinsic Crosstalk |journal=Journal of Immunology Research |volume=2016 |pages=1376595 |year=2016 |pmid=28078307 |pmc=5204097 |doi=10.1155/2016/1376595 |doi-access=free }}</ref> Rather, NK cells destroy compromised host cells, such as tumor cells or virus-infected cells, recognizing such cells by a condition known as "missing self". This term describes cells with low levels of a cell-surface marker called MHC I ([[major histocompatibility complex]])—a situation that can arise in viral infections of host cells.{{sfn|Bertok|Chow|2005|p= [https://books.google.com/books?id=DW3V2Uc-m8EC&q=%22missing+self%22+immunity+immune&pg=PA17 17]}} Normal body cells are not recognized and attacked by NK cells because they express intact self MHC antigens. Those MHC antigens are recognized by killer cell immunoglobulin receptors, which essentially put the brakes on NK cells.{{sfn| Rajalingam |2012|loc= Chapter: Overview of the killer cell immunoglobulin-like receptor system}}