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==Types of Immune Response== ===Innate Immune System=== Prior to actually getting into the body, [[Pathogen|pathogens]] or toxicants have to pass the body’s most basic immune defense: the skin and mucous. Mucous especially plays an important role because it is present in all the places where infection is most likely to occur (e.g. the eyes, nose and mouth). Innate immunity is the nonspecific method of eliminating pathogens from the body. The main agents that carry out this mass, undiscriminating disposal of pathogens and dead or defective cells are called [[macrophage]]s and [[neutrophil]]s.<ref name="Walter2002">{{Cite book | last = Walter | first = Peter | last2 = Roberts | first2 = Keith | last3 = Raff | first3 = Martin | last4 = Lewis | first4 = Julian | last5 = Johnson | first5 = Alexander | last6 = Alberts | first6 = Bruce | date = 2002 | chapter = Innate Immunity | url = https://www.ncbi.nlm.nih.gov/books/NBK26846/ | title = Molecular Biology of the Cell |edition=4th | editor-last = Alberts | editor-first = B | editor-last2 = Johnson | editor-first2 = A | editor-last3 = Lewis | editor-first3 = J | location = New York | publisher = Garland Science |language=en}}</ref> Macrophages are cells derived from monocytes which are made in the bone marrow. Initially, monocytes do not have a specific purpose. However, when these cells travel to and populate different tissues, those tissues emit signals for the type of immune cell that is needed in that area. Monocytes, therefore, alter their shape and function to satisfy work more effectively in their new location. These functions can vary from generalized (e.g. macrophages) to specific such as work done by [[B cell|B-]] and [[T cell]]s.<ref>{{Cite journal | url = https://doi.org/10.1016/j.mattod.2015.01.019 | title = Biomaterial based modulation of macrophage polarization: a review and suggested design principles | journal = Materials Today | volume = 18 | issue = 6 | date = July–August 2015 | pages = 313-325 | first = Rukmani | last = Sridharan | first2 = Andrew R. | last2 = Cameron | first3 = Daniel J. | last3 = Kelly | first4 = Cathal J. | last4 = Kearney | first5 = Fergal J. | last5 = O'Brien |doi=10.1016/j.mattod.2015.01.019|access-date=2019-02-27}}</ref> Macrophages are in almost every type of tissue and they are more abundant in tissue types that are especially susceptible to infection (e.g. lungs, stomach).<ref name="macrophages">{{Cite web | url = https://www.immunology.org/public-information/bitesized-immunology/cells/macrophages | title = Macrophages|website = British Society for Immunology|access-date=2019-02-27}}</ref> Because macrophages float around the body until needed, they are nearby and able to quickly migrate to the sight of infection. Within a span of hours they may be done neutralizing the pathogen. Neutrophils are the most populous [[white blood cell]] and, like macrophages, they are phagocytic and come from the bone marrow. They differ from macrophages a couple of ways: they mature in the [[thymus]] despite being produced in the bone marrow<ref name="britannica">{{Cite web | url = https://www.britannica.com/science/neutrophil | title = Neutrophil {{!}} leukocyte|website=Encyclopedia Britannica|language=en|access-date=2019-02-27}}</ref>, and also contain sacs inside them called granules that aid in the breakdown process. While macrophages are migratory and generally close by and can sense sites of infection, neutrophils need to be recruited to the sites.<ref>{{Cite web | url = https://www.news-medical.net/life-sciences/What-is-the-difference-Between-a-Phagocyte-Macrophage-Neutrophil-and-Eosinophil.aspx | title = What is the difference Between a Phagocyte, Macrophage, Neutrophil and Eosinophil? | date = 2018-10-29 | website = News-Medical.net|language=en|access-date=2019-02-27}}</ref> If pathogens make it past the skin and mucous and the macrophages are unable to clear them, the body has uses its [[adaptive immune system]] to attack the germs. After the initial exposure, the body's adaptive immune system "remembers" a specific signature on the germs so that it can be identified and eliminated quickly during all subsequent exposures.<ref name="NBK279396">{{Cite book | url =https://www.ncbi.nlm.nih.gov/books/NBK279396/ | title = The innate and adaptive immune systems | last = National Center for Biotechnology Information|location=|url-status=dead|archive-date=|archive-url= | date = 2016-08-04|publisher=Institute for Quality and Efficiency in Health Care|language=en}}</ref> ===Acquired or adaptive immunity=== Adaptive immunity is so named because the body must first experience an initial infection for this type of immunity to form against that specific contagion. The first time the body experiences an infection is the worst because it does not yet know how to best eliminate the infiltrating substance. After initial exposure, however, the body has several mechanisms to remember and quickly and aggressively neutralize the pathogen. Many times this system does its job so well that a person may not even realize they are symptomatic or have an infection.<ref name="NBK279396" /> The body needs to identify a pathogen prior to eliminating it. There are several cells that the immune system uses to recognize pathogens. Dendritic cells are antigen-presenting cells that help begin this secondary immune response. Dendritic cells (Greek: dendron, meaning tree) received their name because of the arm-like branches that spread out and grab antigens released by infectious agents.<ref name="dendritic">{{Cite web | url = https://www.immunology.org/public-information/bitesized-immunology/cells/dendritic-cells | title = Dendritic Cells | website = British Society for Immunology|access-date=2019-02-27}}</ref> The dendritic cells place the antigens, like flags, onto their surface for [[T helper cell|helper T-cells]] to recognize. After palpating a dendritic cell, white blood cells call helper T-cells (also called CD4+ T-cells) which secrete lymphokines to direct other immune cells to target the infection. Helper T-cells also promote the production and release of proteins called antibodies.<ref name="NBK279397">{{Cite book | url =https://www.ncbi.nlm.nih.gov/books/NBK279397/ | title = The defense mechanisms of the adaptive immune system | last = National Center for Biotechnology Information|location=Cologne, Germany | date = 2016-08-04|publisher=Institute for Quality and Efficiency in Health Care|language=en}}</ref>{{citation needed | date = 2021}} Antibodies clump around pathogens neutralizing their infectious capabilities, perforate the surface of pathogens encouraging its deterioration, and signal to other cells to engulf and destroy the invader. [[B cell|B-cells]] are produced in the bone marrow and are the antibody factories of the body. They make specific antibodies for the specific pathogen(s) that the body is currently fighting. Another type of T-cell, [[Cytotoxic T cell|killer (cytotoxic) T-cells]] also must first be presented with antigens by one of the body’s antigen-presenting cells for it to activate. Once given a target, killer T-cells directly destroy infectious and defective material. Similar to antibodies, they punch holes in the membranes of bacteria, and infected or malfunctioning cells.<ref name="dendritic" /> The last kinds of T-cells are memory and regulatory. Memory T-cells engulfs the material from the infected or defective cells and store that information in case of a future infection by the same pathogen, or on locating a similar defect. This allows the immune system to react faster because it already knows what is needed to respond to that specific pathogen or defect. [[Regulatory T cell]]s keep helper and killer T-cells' activity at higher levels only when they are necessary. These T-cells will reduce helper and killer T-cells and decrease their activity after the pathogen has been eliminated.<ref name="dendritic" />
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