View source for Immunology

{{short description|Branch of medicine studying  the immune system}}
{{Redirect|Immunobiology|the journal|Immunobiology (journal)}}
{{Infobox medical specialty
| title = Immunology
| image = [[File:MRSA, Ingestion by Neutrophil.jpg|180px]]
| caption =MRSA (yellow) engulfed by [[neutrophil]] (purple) Photo Source: [[National Institute of Allergy and Infectious Diseases]]
| system  = [[Immune system|Immune]]
| subdivisions = {{hlist|class=nowrap |[[Cellular immunology|Cellular]] |[[Clinical immunology|Clinical]]}} {{nowrap|[[Immunogenetics|Genetic {{smaller|(Immunogenetics)}}]]}} {{hlist|class=nowrap |[[Humoral immunology|Humoral]] |[[Molecular immunology|Molecular]]}}
| diseases = [[Rheumatoid arthritis]] [[Inflammation]] {{ublist|class=nowrap |[[Autoimmune disease]] |[[Hypersensitivity]] |[[Immune disorder]] |[[Immunodeficiency]]}}
| tests = {{ublist|class=nowrap |[[Agglutination (biology)|Agglutination]] |[[Immunoassay]] |[[Immunoprecipitation]] |[[Serology]]}}
| specialist = [[Immunologist]]
}}
{{TopicTOC-Immunology}}

'''Immunology''' is a branch of [[biology]] and [[medicine]]<ref>{{Cite web |last=Fossen |first=Christian |name-list-style=vanc |title=What is Biology? |url=https://www.ntnu.edu/biology/about-us/what-is-biology |access-date=2018-07-25 |website=www.ntnu.edu}}</ref> that covers the study of [[Immune system|immune systems]]<ref>{{cite journal |vauthors=Villani AC, Sarkizova S, Hacohen N |date=April 2018 |title=Systems Immunology: Learning the Rules of the Immune System |journal=Annual Review of Immunology |volume=36 |issue=1 |pages=813–42 |doi=10.1146/annurev-immunol-042617-053035 |pmc=6597491 |pmid=29677477}}</ref> in all [[Organism|organisms]].

Immunology charts, measures, and contextualizes the [[Physiology|physiological]] functioning of the immune system in states of both health and diseases; malfunctions of the immune system in immunological disorders (such as [[Autoimmune disease|autoimmune diseases]], [[Hypersensitivity|hypersensitivities]],<ref>{{Cite web |title=Hypersensitivities {{!}} Microbiology |url=https://courses.lumenlearning.com/microbiology/chapter/hypersensitivities/ |access-date=2018-07-25 |website=courses.lumenlearning.com}}</ref> [[immune deficiency]],<ref>{{Cite web |title=Specific Disease Types {{!}} Immune Deficiency Foundation |url=https://primaryimmune.org/about-primary-immunodeficiencies/specific-disease-types |access-date=2018-07-25 |website=primaryimmune.org}}</ref> and [[transplant rejection]]<ref>{{Cite web |title=Transplant rejection: MedlinePlus Medical Encyclopedia |url=https://medlineplus.gov/ency/article/000815.htm |access-date=2018-07-25 |website=medlineplus.gov}}</ref>); and the physical, chemical, and physiological characteristics of the components of the immune system ''[[in vitro]]'',<ref>{{cite journal |vauthors=Pierce CW, Solliday SM, Asofsky R |date=March 1972 |title=Immune responses in vitro. IV. Suppression of primary M, G, and A plaque-forming cell responses in mouse spleen cell cultures by class-specific antibody to mouse immunoglobulins |journal=The Journal of Experimental Medicine |volume=135 |issue=3 |pages=675–97 |doi=10.1084/jem.135.3.675 |pmc=2139142 |pmid=4536706}}</ref> ''[[In situ#Biology and biomedical engineering|in situ]]'', and ''[[in vivo]]''.<ref>{{cite journal |vauthors=Miyahara S, Yokomuro K, Takahashi H, Kimura Y |date=November 1983 |title=Regeneration and the immune system. I. In vitro and in vivo activation of lymphocytes by liver regeneration and the role of Kupffer cells in stimulation |journal=European Journal of Immunology |volume=13 |issue=11 |pages=878–83 |doi=10.1002/eji.1830131104 |pmid=6227489 |s2cid=22400759}}</ref> Immunology has applications in numerous disciplines of medicine, particularly in the fields of organ transplantation, oncology, rheumatology, virology, bacteriology, parasitology, psychiatry, and dermatology.

The term was coined by Russian biologist [[Ilya Ilyich Mechnikov]],<ref>{{Cite web|url=https://embryo.asu.edu/pages/ilya-ilyich-mechnikov-elie-metchnikoff-1845-1916|title= Ilya Ilyich Mechnikov (Elie Metchnikoff) (1845–1916) | work = The Embryo Project | publisher = Arizona State University}}</ref> who advanced studies on immunology and received the [[Nobel Prize]] for his work in 1908 with [[Paul Ehrlich]] "in recognition of their work on immunity". He pinned small thorns into starfish larvae and noticed unusual cells surrounding the thorns. This was the active response of the body trying to maintain its integrity. It was Mechnikov who first observed the phenomenon of [[phagocytosis]],<ref>{{Cite encyclopedia|url=https://www.britannica.com/science/phagocytosis| title = Phagocytosis: Definition, Process, & Examples|encyclopedia=Encyclopedia Britannica|access-date=2018-07-25 }}</ref> in which the body defends itself against a foreign body. Ehrlich accustomed mice to the poisonous ricin and abrin. After feeding them with small but increasing dosages of ricin he ascertained that they had become "ricin-proof". Ehrlich interpreted this as immunization and observed that it was abruptly initiated after a few days and was still in existence after several months.

Prior to the designation of [[Immunity (medical)|immunity]],<ref>{{Cite web | url = https://en.oxforddictionaries.com/definition/immunity  | archive-url = https://web.archive.org/web/20180725153512/https://en.oxforddictionaries.com/definition/immunity  | url-status = dead  | archive-date = July 25, 2018  | title = Definition of immunity in English | work = Oxford Dictionaries }}</ref> from the etymological root {{Lang|la|immunis}}, which is [[Latin]] for 'exempt', early physicians characterized organs that would later be proven as essential components of the immune system. The important lymphoid organs of the immune system are the [[thymus]],<ref>{{cite journal | vauthors = Lee DK, Hakim FT, Gress RE | title = The thymus and the immune system: layered levels of control | journal = Journal of Thoracic Oncology | volume = 5 | issue = 10 Suppl 4 | pages = S273–76 | date = October 2010 | pmid = 20859118 | pmc = 2951290 | doi = 10.1097/JTO.0b013e3181f20474 }}</ref> [[bone marrow]], and chief lymphatic tissues such as [[spleen]], [[tonsil]]s, [[Lymphatic system|lymph vessels]], [[lymph node]]s, [[adenoid]]s, and [[liver]]. However, many components of the immune system are [[Cell (biology)|cell]]ular in nature, and not associated with specific organs, but rather embedded or circulating in various [[Tissue (biology)|tissue]]s located throughout the body.

== Classical immunology ==
Classical immunology ties in with the fields of [[epidemiology]] and [[medicine]]. It studies the relationship between the body systems, [[pathogen]]s, and immunity. The earliest written mention of immunity can be traced back to the [[plague of Athens]] in 430 BCE. [[Thucydides]] noted that people who had recovered from a previous bout of the disease could [[nurse]] the sick without contracting the illness a second time.<ref>{{Cite web|url=http://nfs.unipv.it/nfs/minf/dispense/immunology/immun.html|title=The Concept of Immunity. History and Applications.|last=Gherardi|first=E|date=2007-01-02|website=Immunology Course Medical School, University of Pavia|archive-url=https://web.archive.org/web/20070102170207/http://nfs.unipv.it/nfs/minf/dispense/immunology/immun.html|archive-date=2007-01-02|access-date=2018-07-27}}</ref> Many other ancient societies have references to this phenomenon, but it was not until the 19th and 20th centuries before the concept developed into scientific theory.

The study of the molecular and cellular components that comprise the immune system, including their function and interaction, is the central science of immunology. The immune system has been divided into a more primitive [[Innate immunity|innate immune system]] and, in [[vertebrate]]s, an [[Adaptive immunity|acquired or adaptive immune system]]. The latter is further divided into [[Humoral immunity|humoral]] (or [[antibody]]) and [[Cell-mediated immunity|cell-mediated]] components.{{citation needed|date=June 2022}}

The immune system has the capability of self and non-self-recognition.<ref>{{cite book |first1=Robert R. |last1=Rich |first2=David D. |last2=Chaplin |year=2019 |chapter=The Human Immune Response |doi=10.1016/B978-0-7020-6896-6.00001-6|title=Clinical Immunology |edition=5th |series=Principles and Practice |pages=3–17.e1 |isbn=9780702068966 |s2cid=88829315 }}</ref> An antigen is a substance that ignites the immune response. The cells involved in recognizing the antigen are Lymphocytes. Once they recognize, they secrete antibodies. Antibodies are proteins that neutralize the disease-causing microorganisms. Antibodies do not directly kill pathogens, but instead, identify antigens as targets for destruction by other immune cells such as phagocytes or NK cells.

The  (antibody) response is defined as the interaction between antibodies and [[antigen]]s.<ref>{{cite book |last1=Janeway |first1=Charles A |last2=Travers |first2=Paul |last3=Walport |first3=Mark |last4=Shlomchik |first4=Mark J. |name-list-style=vanc |title=Immunobiology the immune system health & disease |date=2001 |publisher=Garland |location=New York |isbn=978-0-8153-3642-6 |edition=5th |chapter=Chapter 9: The Humoral Immune Response |chapter-url=https://www.ncbi.nlm.nih.gov/books/NBK10752/ |url=https://archive.org/details/immunobiology00char }}</ref> Antibodies are specific [[protein]]s released from a certain class of immune cells known as [[B cell|B&nbsp;lymphocyte]]s, while antigens are defined as anything that elicits the generation of antibodies ('''anti'''body '''gen'''erators). Immunology rests on an understanding of the properties of these two biological entities and the cellular response to both.<!-- which can not only kill infected cells in its own right, but is also crucial in controlling the antibody response. Put simply, both systems are highly interdependent-->

It is now getting clear that the immune responses contribute to the development of many common disorders not traditionally viewed as immunologic,<ref>{{Cite web|url=https://www.immunology.org/public-information/what-is-immunology|title=What is immunology? {{!}} British Society for Immunology|website=www.immunology.org|access-date=2018-07-21|archive-date=2018-07-21|archive-url=https://web.archive.org/web/20180721073046/https://www.immunology.org/public-information/what-is-immunology|url-status=dead}}</ref> including metabolic, cardiovascular, cancer, and neurodegenerative conditions like Alzheimer's disease. Besides, there are direct implications of the immune system in the infectious diseases (tuberculosis, malaria, hepatitis, pneumonia, dysentery, and helminth infestations) as well. Hence, research in the field of immunology is of prime importance for the advancements in the fields of modern medicine, biomedical research, and biotechnology.

Immunological research continues to become more specialized, pursuing non-classical models of immunity and functions of cells, organs and systems not previously associated with the immune system (Yemeserach 2010).

== Diagnostic immunology ==
{{main|Immunodiagnostics}}

The specificity of the bond between antibody and antigen has made the antibody an excellent tool for the detection of substances by a variety of diagnostic techniques. Antibodies specific for a desired antigen can be conjugated with an [[Isotopic labeling|isotopic (radio)]] or [[Fluorescent labelling|fluorescent label]] or with a color-forming enzyme in order to detect it. However, the similarity between some antigens can lead to false positives and other errors in such tests by antibodies cross-reacting with antigens that are not exact matches.<ref name="pmid1993317">{{cite journal | vauthors = Miller JJ, Valdes R | title = Approaches to minimizing interference by cross-reacting molecules in immunoassays | journal = Clinical Chemistry | volume = 37 | issue = 2 | pages = 144–53 | date = February 1991 | pmid = 1993317 | doi = 10.1093/clinchem/37.2.144 | doi-access = free }}</ref>

== Immunotherapy ==
{{Main|Immunotherapy}}

The use of immune system components or antigens to treat a disease or disorder is known as [[immunotherapy]]. Immunotherapy is most commonly used to treat allergies, autoimmune disorders such as [[Crohn's disease]], [[Hashimoto's thyroiditis]] and [[rheumatoid arthritis]], and certain [[cancer]]s. Immunotherapy is also often used for patients who are immunosuppressed (such as those with [[HIV]]) and people with other immune deficiencies.
This includes regulating factors such as IL-2, IL-10, GM-CSF B, IFN-α.

== Clinical immunology ==
<!--[[Clinical immunology]] redirects here-->

Clinical immunology is the study of [[disease]]s caused by disorders of the immune system (failure, aberrant action, and malignant growth of the cellular elements of the system). It also involves diseases of other systems, where immune reactions play a part in the pathology and clinical features.

The diseases caused by disorders of the immune system fall into two broad categories:
* [[immunodeficiency]], in which parts of the immune system fail to provide an adequate response (examples include [[chronic granulomatous disease]] and primary immune diseases);
* [[autoimmunity]], in which the immune system attacks its own host's body (examples include [[systemic lupus erythematosus]], [[rheumatoid arthritis]], [[Hashimoto's disease]] and [[myasthenia gravis]]).
Other immune system disorders include various hypersensitivities (such as in [[asthma]] and other [[Allergy|allergies]]) that respond inappropriately to otherwise harmless [[Chemical compound|compound]]s.

The most well-known disease that affects the immune system itself is [[Acquired Immunodeficiency Syndrome|AIDS]], an immunodeficiency characterized by the suppression of CD4+ ("helper") [[T cell]]s, [[dendritic cells]] and [[macrophages]] by the [[HIV|human immunodeficiency virus]] (HIV).

Clinical immunologists also study ways to prevent the immune system's attempts to destroy [[allograft]]s ([[transplant rejection]]).<ref>{{Cite web|url=https://www.immunology.org/public-information/bitesized-immunology/organs-and-tissues/transplant-rejection-t-helper-cell|archive-url=https://web.archive.org/web/20190423151148/https://www.immunology.org/public-information/bitesized-immunology/organs-and-tissues/transplant-rejection-t-helper-cell|url-status=dead|archive-date=April 23, 2019|title=Transplant rejection: T-helper cell paradigm {{!}} British Society for Immunology|website=www.immunology.org|access-date=2019-04-23}}</ref>

Clinical immunology and allergy is usually a subspecialty of [[internal medicine]] or [[pediatrics]]. Fellows in Clinical Immunology are typically exposed to many of the different aspects of the specialty and treat allergic conditions, primary immunodeficiencies and systemic autoimmune and autoinflammatory conditions. As part of their training fellows may do additional rotations in [[rheumatology]], [[pulmonology]], [[otorhinolaryngology]], [[dermatology]] and the immunologic lab.<ref>{{cite web |title=Clinical Immunology and Allergy Competencies |url=https://www.royalcollege.ca/rcsite/documents/ibd/clinical-immunology-and-allergy-competencies-e.pdf |publisher=The Royal College of Physicians and Surgeons of Canada |access-date=2021-09-26 |archive-date=2021-09-26 |archive-url=https://web.archive.org/web/20210926191240/https://www.royalcollege.ca/rcsite/documents/ibd/clinical-immunology-and-allergy-competencies-e.pdf |url-status=dead }}</ref>

===Clinical and pathology immunology===

When health conditions worsen to emergency status, portions of immune system organs, including the thymus, spleen, bone marrow, lymph nodes, and other lymphatic tissues, can be [[Surgery|surgically]] excised for examination while patients are still alive.

== Theoretical immunology ==

Immunology is strongly experimental in everyday practice but is also characterized by an ongoing theoretical attitude. Many theories have been suggested in immunology from the end of the nineteenth century up to the present time. The end of the 19th century and the beginning of the 20th century saw a battle between "cellular" and "humoral" theories of immunity. According to the cellular theory of immunity, represented in particular by [[Elie Metchnikoff]], it was cells&nbsp;– more precisely, phagocytes&nbsp;– that were responsible for immune responses. In contrast, the humoral theory of immunity, held by [[Robert Koch]]<ref>{{Cite encyclopedia|url=https://www.britannica.com/biography/Robert-Koch|title=Robert Koch {{!}} German bacteriologist|encyclopedia=Encyclopedia Britannica|access-date=2018-07-25 }}</ref> and [[Emil von Behring]],<ref>{{Cite web|url=https://www.nobelprize.org/nobel_prizes/medicine/laureates/1901/behring-article.html|title=Emil von Behring: The Founder of Serum Therapy|website=www.nobelprize.org|access-date=2018-07-25}}</ref> among others, stated that the active immune agents were soluble components (molecules) found in the organism's "humors" rather than its cells.<ref>{{cite book |last1=Silverstein |first1=Arthur M. | name-list-style = vanc |title=A history of immunology |date=1989 |publisher=Academic Press |location=San Diego |isbn=978-0-12-643770-6 | oclc = 909269335 }}</ref><!--
     --><ref>{{cite book | vauthors = Tauber AI, Chernyak L |title=Metchnikoff and the Origins of Immunology |publisher=Oxford University Press |location=New York |date=1991 | isbn = 978-0-19-506447-6 | oclc = 22906314 }}</ref><!--
     --><ref name="Tauber AI 1994">{{cite journal | vauthors = Tauber AI |title=The Immune Self: Theory or Metaphor? |journal=Immunology Today |publisher=Cambridge University Press |location=Cambridge |date=1994 |volume=15 |issue=3 |pages=134–6 |doi=10.1016/0167-5699(94)90157-0 |pmid=8172646 | oclc = 4930079483 }}</ref>

In the mid-1950s, [[Frank Macfarlane Burnet|Macfarlane Burnet]], inspired by a suggestion made by [[Niels Jerne]],<ref>{{cite journal | vauthors = Jerne NK | title = The natural-selection theory of antibody selection | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 41 | issue = 11 | pages = 849–57 | date = November 1955 | pmid = 16589759 | pmc = 534292 | doi = 10.1073/pnas.41.11.849 | bibcode = 1955PNAS...41..849J | doi-access = free }}</ref> <!--
 -->formulated the [[clonal selection theory]] (CST) of immunity.<ref>{{cite book | vauthors = Burnet FM |title=The Clonal Selection Theory of Acquired Immunity | url = https://archive.org/details/clonalselectiont00burn |publisher=Cambridge University Press |location=Cambridge |date=1959}}</ref> <!--
 -->On the basis of CST, Burnet developed a theory of how an immune response is triggered according to the self/nonself distinction: "self" constituents (constituents of the body) do not trigger destructive immune responses, while "nonself" entities (e.g., pathogens, an allograft) trigger a destructive immune response.<ref>{{cite book | vauthors = Burnet FM |title=Cellular Immunology: Self and Notself |publisher=Cambridge University Press |location=Cambridge |date=1969}}</ref> <!--
 -->The theory was later modified to reflect new discoveries regarding [[histocompatibility]] or the complex "two-signal" activation of T cells.<ref>{{cite journal | vauthors = Bretscher P, Cohn M | title = A theory of self-nonself discrimination | journal = Science | volume = 169 | issue = 3950 | pages = 1042–49 | date = September 1970 | pmid = 4194660 | doi = 10.1126/science.169.3950.1042 | bibcode = 1970Sci...169.1042B | s2cid = 26916828 }}</ref> <!--
 -->The self/nonself theory of immunity and the self/nonself vocabulary have been criticized,<ref name="Tauber AI 1994"/><ref name="Matzinger P 2002 301–5">{{cite journal | vauthors = Matzinger P | title = The danger model: a renewed sense of self | journal = Science | volume = 296 | issue = 5566 | pages = 301–05 | date = April 2002 | pmid = 11951032 | doi = 10.1126/science.1071059 | url = http://www.scs.carleton.ca/~soma/biosec/readings/matzinger-science.pdf | citeseerx = 10.1.1.127.558 | bibcode = 2002Sci...296..301M | s2cid = 13615808 }}</ref><ref>{{cite book |last1=Pradeu |first1=Thomas |last2=Vitanza |first2=Elizabeth | name-list-style = vanc |title=The limits of the self: immunology and biological identity |date=2012 |publisher=Oxford University Press |location=Oxford |isbn=978-0-19-977528-6 | oclc = 793571104 }}</ref> but remain very influential.<ref>{{cite journal | vauthors = Langman RE, Cohn M | title = A minimal model for the self-nonself discrimination: a return to the basics | journal = Seminars in Immunology | volume = 12 | issue = 3 | pages = 189–95; discussion 257–344 | date = June 2000 | pmid = 10910739 | doi = 10.1006/smim.2000.0231 }}</ref><ref>{{cite book | vauthors = Clark WR |title=In defense of self: how the immune system really works | url = https://archive.org/details/indefenseofselfh0000clar | url-access = registration |date=2008 |publisher=Oxford University Press |location=New York |isbn=978-0-19-533663-4 | oclc = 917294223 }}</ref>

More recently, several theoretical frameworks have been suggested in immunology, including "[[Autopoiesis|autopoietic]]" views,<ref>{{cite journal | vauthors = Coutinho A, Forni L, Holmberg D, Ivars F, Vaz N |title=From an antigen-centered, clonal perspective of immune responses to an organism-centered network perspective of autonomous reactivity of self-referential immune systems |journal=Immunological Reviews |volume=79 |pages=151–68 |date=1984 |doi=10.1111/j.1600-065x.1984.tb00492.x |pmid=6235170 |s2cid=46481630 }}</ref> <!--
 -->"cognitive immune" views,<ref>{{cite book | vauthors = Irun C |title=Tending Adam's garden: Evolving the cognitive immune self |publisher=Academic Press |location=San Diego |date=2000}}</ref> <!--
 -->the "[[danger model]]" (or "danger theory"),<ref name="Matzinger P 2002 301–5"/> <!--
 --> and the "discontinuity" theory.<ref>{{cite journal | vauthors = Pradeu T, Carosella ED | title = On the definition of a criterion of immunogenicity | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 103 | issue = 47 | pages = 17858–61 | date = November 2006 | pmid = 17101995 | pmc = 1693837 | doi = 10.1073/pnas.0608683103 | bibcode = 2006PNAS..10317858P | doi-access = free }}</ref><ref>{{cite journal | vauthors = Pradeu T, Jaeger S, Vivier E | title = The speed of change: towards a discontinuity theory of immunity? | journal = Nature Reviews. Immunology | volume = 13 | issue = 10 | pages = 764–69 | date = October 2013 | pmid = 23995627 | doi = 10.1038/nri3521 | s2cid = 11366176 | url = http://philsci-archive.pitt.edu/10046/1/Pradeu-Jaeger-Vivier_Discontinuity_theory_Final_draft.pdf }}</ref> <!--
 -->The danger model, suggested by [[Polly Matzinger]] and colleagues, has been very influential, arousing many comments and discussions.<ref>{{cite journal | vauthors = Janeway CA, Goodnow CC, Medzhitov R | title = Danger – pathogen on the premises! Immunological tolerance | journal = Current Biology | volume = 6 | issue = 5 | pages = 519–22 | date = May 1996 | pmid = 8805259 | doi = 10.1016/S0960-9822(02)00531-6 | s2cid = 14347980 | doi-access = free }}</ref><ref>{{cite journal | vauthors =Vance RE |title=Cutting edge commentary: a Copernican revolution? Doubts about the danger theory  |journal=Journal of Immunology |volume=165 | issue = 4  |pages=1725–28 |date=2000 |doi=10.4049/jimmunol.165.4.1725|pmid=10925247 |doi-access=free }}</ref><ref>{{cite journal | vauthors = Matzinger P | title = The evolution of the danger theory. Interview by Lauren Constable, Commissioning Editor | journal = Expert Review of Clinical Immunology | volume = 8 | issue = 4 | pages = 311–17 | date = May 2012 | pmid = 22607177 | pmc = 4803042 | doi = 10.1586/eci.12.21 }}</ref><ref>{{cite journal | vauthors = Pradeu T, Cooper EL | title = The danger theory: 20 years later | journal = Frontiers in Immunology | volume = 3 | pages = 287 | date = 2012 | pmid = 23060876 | pmc = 3443751 | doi = 10.3389/fimmu.2012.00287 | doi-access = free }}</ref>

== Developmental immunology ==

The body's capability to react to antigens depends on a person's age, antigen type, maternal factors and the area where the antigen is presented.<ref name="isbn0-7167-4947-5">{{cite book |vauthors=Goldsby RA, Kindt TK |author3=Osborne BA |author4=Kuby J |title=Immunology |edition=5th |publisher=W.H. Freeman |location=San Francisco |date=2003 |isbn=978-0-7167-4947-9 |url=https://archive.org/details/immunology00gold_0 }}</ref> [[Neonate]]s are said to be in a state of physiological immunodeficiency, because both their innate and adaptive immunological responses are greatly suppressed.  Once born, a child's immune system responds favorably to protein antigens while not as well to [[glycoprotein]]s and [[polysaccharide]]s. In fact, many of the infections acquired by neonates are caused by low virulence organisms like ''[[Staphylococcus]]'' and ''[[Pseudomonas]]''.  In neonates, [[Opsonin|opsonic]] activity and the ability to activate the [[complement cascade]] is very limited.  For example, the mean level of [[Apolipoprotein C3<!--Is this correct?-->|C3]] in a newborn is approximately 65% of that found in the adult. [[Phagocytosis|Phagocytic]] activity is also greatly impaired in newborns. This is due to lower opsonic activity, as well as diminished [[Downregulation and upregulation|up-regulation]] of [[integrin]] and [[selectin]] receptors, which limit the ability of [[neutrophil]]s to interact with [[Cell adhesion molecule|adhesion molecule]]s in the [[endothelium]]. Their [[monocyte]]s are slow and have a reduced [[Adenosine triphosphate|ATP]] production, which also limits the newborn's phagocytic activity.  Although, the number of total [[lymphocyte]]s is significantly higher than in adults, the cellular and humoral immunity is also impaired. [[Antigen-presenting cell]]s in newborns have a reduced capability to activate T&nbsp;cells.  Also, T&nbsp;cells of a newborn proliferate poorly and produce very small amounts of [[cytokine]]s like IL-2, IL-4, IL-5, IL-12, and IFN-g which limits their capacity to activate the humoral response as well as the phagocitic activity of macrophage.  B&nbsp;cells develop early during [[gestation]] but are not fully active.<ref name="Jaspan et al">{{cite journal | vauthors = Jaspan HB, Lawn SD, Safrit JT, Bekker LG | title = The maturing immune system: implications for development and testing HIV-1 vaccines for children and adolescents | journal = AIDS | volume = 20 | issue = 4 | pages = 483–94 | date = February 2006 | pmid = 16470112 | doi = 10.1097/01.aids.0000210602.40267.60 | s2cid = 20277590 | doi-access = free }}</ref>

[[File:Monocyte.svg|thumb|Artist's impression of [[monocyte]]s]]

Maternal factors also play a role in the body's immune response.  At birth, most of the [[immunoglobulin]] present is maternal IgG. These antibodies are transferred from the placenta to the fetus using the FcRn (neonatal Fc receptor).<ref>{{Cite web|url=https://www.immunology.org/public-information/bitesized-immunology/immune-development/neonatal-immunology|title=Neonatal Immunology &#124; British Society for Immunology|website=www.immunology.org}}</ref> Because IgM, IgD, IgE and IgA do not cross the placenta, they are almost undetectable at birth. Some IgA is provided by [[breast milk]]. These passively-acquired antibodies can protect the newborn for up to 18 months, but their response is usually short-lived and of low [[Affinity (pharmacology)<!--Is this correct?-->|affinity]].<ref name="Jaspan et al"/>  These antibodies can also produce a negative response.  If a child is exposed to the antibody for a particular antigen before being exposed to the antigen itself then the child will produce a dampened response. [[Passive immunity|Passively acquired maternal antibodies]] can suppress the antibody response to active immunization. Similarly, the response of T-cells to vaccination differs in children compared to adults, and vaccines that induce Th1 responses in adults do not readily elicit these same responses in neonates.<ref name="Jaspan et al"/> Between six and nine months after birth, a child's immune system begins to respond more strongly to [[glycoprotein]]s, but there is usually no marked improvement in their response to [[polysaccharide]]s until they are at least one year old. This can be the reason for distinct time frames found in [[vaccination schedule]]s.<!--
     --><ref name="pmid11739030">{{cite journal | vauthors = Glezen WP | title = Maternal vaccines | journal = Primary Care | volume = 28 | issue = 4 | pages = 791–806, vi–vii | date = December 2001 | pmid = 11739030 | doi = 10.1016/S0095-4543(05)70041-5 }}</ref><!--
     --><ref name="pmid9286377">{{cite book | vauthors = Holt PG, Macaubas C, Cooper D, Nelson DJ, McWilliam AS | chapter = Th-1/Th-2 Switch Regulation in Immune Responses to Inhaled Antigens | title = Dendritic Cells in Fundamental and Clinical Immunology | volume = 417 | pages = 301–06 | date = 1997 | pmid = 9286377 | doi = 10.1007/978-1-4757-9966-8_49 | isbn = 978-1-4757-9968-2 | series = Advances in Experimental Medicine and Biology }}</ref>

During adolescence, the human body undergoes various physical, physiological and immunological changes triggered and mediated by [[hormone]]s, of which the most significant in females is [[Estradiol|17-β-estradiol]] (an [[estrogen]]) and, in males, is [[testosterone]]. Estradiol usually begins to act around the age of 10 and testosterone some months later.<ref name="pmid127002">{{cite journal | vauthors = Sizonenko PC, Paunier L | title = Hormonal changes in puberty III: Correlation of plasma dehydroepiandrosterone, testosterone, FSH, and LH with stages of puberty and bone age in normal boys and girls and in patients with Addison's disease or hypogonadism or with premature or late adrenarche | journal = The Journal of Clinical Endocrinology and Metabolism | volume = 41 | issue = 5 | pages = 894–904 | date = November 1975 | pmid = 127002 | doi = 10.1210/jcem-41-5-894 }}</ref> <!--
 -->There is evidence that these [[steroid]]s not only act directly on the [[Primary sexual characteristics|primary]] and [[secondary sexual characteristics]] but also have an effect on the development and regulation of the immune system,<ref name="pmid11407317">{{cite journal | vauthors = Verthelyi D | title = Sex hormones as immunomodulators in health and disease | journal = International Immunopharmacology | volume = 1 | issue = 6 | pages = 983–93 | date = June 2001 | pmid = 11407317 | doi = 10.1016/S1567-5769(01)00044-3 }}</ref> <!--
 -->including an increased risk in developing [[Puberty|pubescent]] and post-pubescent autoimmunity.<ref name="pmid2973658">{{cite journal | vauthors = Stimson WH | title = Oestrogen and human T lymphocytes: presence of specific receptors in the T-suppressor/cytotoxic subset | journal = Scandinavian Journal of Immunology | volume = 28 | issue = 3 | pages = 345–50 | date = September 1988 | pmid = 2973658 | doi = 10.1111/j.1365-3083.1988.tb01459.x | s2cid = 38920551 }}</ref> <!--
 -->There is also some evidence that cell surface receptors on B cells and macrophages may detect sex hormones in the system.<ref name="pmid11996938">{{cite journal | vauthors = Benten WP, Stephan C, Wunderlich F | title = B cells express intracellular but not surface receptors for testosterone and estradiol | journal = Steroids | volume = 67 | issue = 7 | pages = 647–54 | date = June 2002 | pmid = 11996938 | doi = 10.1016/S0039-128X(02)00013-2 | s2cid = 1056135 }}</ref>

The female sex hormone 17-β-estradiol has been shown to regulate the level of immunological response,<ref name="pmid12900050">{{cite journal | vauthors = Beagley KW, Gockel CM | title = Regulation of innate and adaptive immunity by the female sex hormones estradiol and progesterone | journal = FEMS Immunology and Medical Microbiology | volume = 38 | issue = 1 | pages = 13–22 | date = August 2003 | pmid = 12900050 | doi = 10.1016/S0928-8244(03)00202-5 | doi-access = free }}</ref> <!--
 -->while some male [[androgen]]s such as testosterone seem to suppress the stress response to infection. Other androgens, however, such as [[DHEA]], increase immune response.<ref name="pmid9949320">{{cite journal | vauthors = Kanda N, Tamaki K | title = Estrogen enhances immunoglobulin production by human PBMCs | journal = The Journal of Allergy and Clinical Immunology | volume = 103 | issue = 2 Pt 1 | pages = 282–88| date = February 1999 | pmid = 9949320 | doi = 10.1016/S0091-6749(99)70503-8 }}</ref> <!--
 -->As in females, the male sex hormones seem to have more control of the immune system during puberty and post-puberty than during the rest of a male's adult life.

Physical changes during puberty such as [[thymic involution]] also affect immunological response.<ref name="pmid10737767">{{cite journal | vauthors = McFarland RD, Douek DC, Koup RA, Picker LJ | title = Identification of a human recent thymic emigrant phenotype | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 97 | issue = 8 | pages = 4215–20 | date = April 2000 | pmid = 10737767 | pmc = 18202 | doi = 10.1073/pnas.070061597 | bibcode = 2000PNAS...97.4215M | doi-access = free }}</ref>

== Ecoimmunology and behavioural immunity ==
{{main|Ecoimmunology|Behavioral immune system}}
Ecoimmunology, or ecological immunology, explores the relationship between the immune system of an organism and its social, biotic and abiotic environment.

More recent ecoimmunological research has focused on host pathogen defences traditionally considered "non-immunological", such as [[pathogen avoidance]], self-medication, [[Symbiosis|symbiont]]-mediated defenses, and fecundity trade-offs.<ref name=":0">{{cite journal | vauthors = Parker BJ, Barribeau SM, Laughton AM, de Roode JC, Gerardo NM | title = Non-immunological defense in an evolutionary framework | journal = Trends in Ecology & Evolution | volume = 26 | issue = 5 | pages = 242–48 | date = May 2011 | pmid = 21435735 | doi = 10.1016/j.tree.2011.02.005 }}</ref> Behavioural immunity, a phrase coined by [[Mark Schaller]], specifically refers to psychological pathogen avoidance drivers, such as [[disgust]] aroused by stimuli encountered around pathogen-infected individuals, such as the smell of [[Vomiting|vomit]].<ref>{{Cite journal|title=Commentaries on Evolutionary Foundations of Cultural Variation: Evoked Culture and Mate Preferences|journal=Psychological Inquiry|volume=17|issue=2|pages=96–137|doi=10.1207/s15327965pli1702_2|year=2006|s2cid=219729311}}</ref> More broadly, "behavioural" ecological immunity has been demonstrated in multiple species. For example, the [[Monarch butterfly]] often lays its eggs on certain toxic [[Asclepias|milkweed]] species when infected with parasites. These toxins reduce parasite growth in the offspring of the infected Monarch. However, when uninfected Monarch butterflies are forced to feed only on these toxic plants, they suffer a fitness cost as reduced lifespan relative to other uninfected Monarch butterflies.<ref>{{cite journal | vauthors = Lefèvre T, Oliver L, Hunter MD, De Roode JC | title = Evidence for trans-generational medication in nature | journal = Ecology Letters | volume = 13 | issue = 12 | pages = 1485–93 | date = December 2010 | pmid = 21040353 | doi = 10.1111/j.1461-0248.2010.01537.x | hdl = 2027.42/79381 | url = https://deepblue.lib.umich.edu/bitstream/2027.42/79381/1/j.1461-0248.2010.01537.x.pdf | hdl-access = free }}</ref> This indicates that laying eggs on toxic plants is a costly behaviour in Monarchs which has probably evolved to reduce the severity of parasite infection.<ref name=":0" />

Symbiont-mediated defenses are also [[Heritability|heritable]] across host generations, despite a non-genetic direct basis for the transmission. [[Aphid]]s, for example, rely on several different symbionts for defense from key parasites, and can vertically transmit their symbionts from parent to offspring.<ref>{{cite journal | vauthors = Koga R, Meng XY, Tsuchida T, Fukatsu T | title = Cellular mechanism for selective vertical transmission of an obligate insect symbiont at the bacteriocyte-embryo interface | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 109 | issue = 20 | pages = E1230–37 | date = May 2012 | pmid = 22517738 | pmc = 3356617 | doi = 10.1073/pnas.1119212109 | doi-access = free }}</ref> Therefore, a symbiont that successfully confers protection from a parasite is more likely to be passed to the host offspring, allowing coevolution with parasites attacking the host in a way similar to traditional immunity.

The preserved immune tissues of extinct species, such as the thylacine (''Thylacine cynocephalus''), can also provide insights into their biology.<ref>{{cite journal | vauthors = Old J | title = Immunological insights into the life and times of the extinct Tasmanian tiger (''Thylacinus cynocephalus'') | journal = PLOS ONE | volume = 10 | pages = e0144091 | date = 2015 | issue = 12 | doi = 10.1371/journal.pone.0144091 | pmid = 26655868 | pmc = 4684372 | bibcode = 2015PLoSO..1044091O | doi-access = free }}</ref>

== Cancer immunology ==
{{main|Cancer immunology}}

The study of the interaction of the immune system with cancer cells can lead to diagnostic tests and therapies with which to find and fight cancer. The immunology concerned with physiological reaction characteristic of the immune state. Inflammation is an immune response that has been observed in many types of cancers.<ref>{{Cite journal |last=Taniguchi |first=Koji |last2=Karin |first2=Michael |date=January 2018 |title=NF-κB, inflammation, immunity and cancer: coming of age |url=https://www.nature.com/articles/nri.2017.142 |journal=Nature Reviews Immunology |language=en |volume=18 |issue=5 |pages=309–324 |doi=10.1038/nri.2017.142 |issn=1474-1741}}</ref>

== Reproductive immunology ==
{{main|Reproductive immunology}}

This area of the immunology is devoted to the study of immunological aspects of the reproductive process including [[fetus]] acceptance. The term has also been used by fertility clinics to address fertility problems, recurrent miscarriages, premature deliveries and dangerous complications such as [[pre-eclampsia]].

== See also ==
* [[List of immunologists]]
* [[Immunomics]]
* ''[[International Reviews of Immunology]]''
* [[Outline of immunology]]
* [[History of immunology]]
* [[Osteoimmunology]]

== References ==
{{Reflist|30em}}

==External links==
{{Wikiversity department}}
*{{Commons category-inline|Immunology}}
*[https://www.aai.org/ American Association of Immunologists]
*[https://www.immunology.org/ British Society for Immunology]
*[http://www.focisnet.org Federation of Clinical Immunology Societies]

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