Editing Immune system (section)

=== Immunodeficiencies ===
{{further|Immunodeficiency}}
[[Immunodeficiency|Immunodeficiencies]] occur when one or more of the components of the immune system are inactive. The ability of the immune system to respond to pathogens is diminished in both the young and the [[old age|elderly]], with immune responses beginning to decline at around 50 years of age due to [[immunosenescence]].<ref>{{cite journal | vauthors = Aw D, Silva AB, Palmer DB | title = Immunosenescence: emerging challenges for an ageing population | journal = Immunology | volume = 120 | issue = 4 | pages = 435–46 | date = Apr 2007 | pmid = 17313487 | pmc = 2265901 | doi = 10.1111/j.1365-2567.2007.02555.x }}</ref><ref name="nutrition">{{cite journal | vauthors = Chandra RK | title = Nutrition and the immune system: an introduction | journal = The American Journal of Clinical Nutrition | volume = 66 | issue = 2 | pages = 460S–63S | date = Aug 1997 | pmid = 9250133 | doi = 10.1093/ajcn/66.2.460S | doi-access = free }}</ref> In [[developed country|developed countries]], [[obesity]], [[alcohol abuse|alcoholism]], and drug use are common causes of poor immune function, while [[malnutrition]] is the most common cause of immunodeficiency in [[developing country|developing countries]].<ref name="nutrition" /> Diets lacking sufficient protein are associated with impaired cell-mediated immunity, complement activity, phagocyte function, [[immunoglobulin A|IgA]] antibody concentrations, and cytokine production. Additionally, the loss of the [[thymus]] at an early age through [[Mutation|genetic mutation]] or surgical removal results in severe immunodeficiency and a high susceptibility to infection.<ref>{{cite journal | vauthors = Miller JF | title = The discovery of thymus function and of thymus-derived lymphocytes | journal = Immunological Reviews | volume = 185 | issue = 1 | pages = 7–14 | date = Jul 2002 | pmid = 12190917 | doi = 10.1034/j.1600-065X.2002.18502.x | s2cid = 12108587 }}</ref> Immunodeficiencies can also be inherited or '[[Immunodeficiency#Acquired immune deficiency|acquired]]'.{{sfn | Reece | 2011 | p=967}} [[Severe combined immunodeficiency]] is a rare [[genetic disorder]] characterized by the disturbed development of functional T cells and B cells caused by numerous genetic mutations.<ref name="five">{{cite journal |vauthors=Burg M, Gennery AR |title=Educational paper: The expanding clinical and immunological spectrum of severe combined immunodeficiency |journal=Eur J Pediatr |volume= 170 |issue=5 |pages=561–571 |year=2011 |doi=10.1007/s00431-011-1452-3 |pmid=21479529 |pmc=3078321 }}</ref> [[Chronic granulomatous disease]], where [[phagocyte]]s have a reduced ability to destroy pathogens, is an example of an inherited, or [[Primary immunodeficiency|congenital, immunodeficiency]]. [[AIDS]] and some types of [[cancer]] cause acquired immunodeficiency.<ref>{{cite journal | vauthors = Joos L, Tamm M | title = Breakdown of pulmonary host defense in the immunocompromised host: cancer chemotherapy | journal = Proceedings of the American Thoracic Society | volume = 2 | issue = 5 | pages = 445–48 | year = 2005 | pmid = 16322598 | doi = 10.1513/pats.200508-097JS }}</ref><ref>{{cite journal | vauthors = Copeland KF, Heeney JL | title = T helper cell activation and human retroviral pathogenesis | journal = Microbiological Reviews | volume = 60 | issue = 4 | pages = 722–42 | date = Dec 1996 | pmid = 8987361 | pmc = 239461 | doi = 10.1128/MMBR.60.4.722-742.1996 }}</ref>