Editing Thymus (section)

===Immunodeficiency===
As the thymus is where T cells develop, congenital problems with the development of the thymus can lead to [[immunodeficiency]], whether because of a problem with the development of the thymus gland, or a problem specific to thymocyte development. Immunodeficiency can be profound.{{sfn|Davidson's|2018|p=67}} Loss of the thymus at an early age through genetic mutation (as in [[DiGeorge syndrome]], [[CHARGE syndrome]], or a very rare "nude" thymus causing absence of hair and the thymus{{sfn|Harrison's|2015|pp=2493}}) results in severe immunodeficiency and subsequent high susceptibility to infection by viruses, [[protozoa]], and [[fungi]].{{sfn|Davidson's|2018|pp=79-80}} [[Nude mouse|Nude mice]] with the very rare "nude" deficiency as a result of [[FOXN1]] mutation are a strain of research mice as a model of T cell deficiency.<ref>{{cite book |last1=Fox |first1=James G. | name-list-style = vanc |title=The Mouse in Biomedical Research: Immunology |date=2006 |publisher=Elsevier |isbn=978-0-08-046908-9 |pages=277 |url=https://books.google.com/books?id=63K0IifQIrgC&pg=PA277 |language=en}}</ref>

The most common congenital cause of thymus-related immune deficiency results from the deletion of the [[Chromosome 22 (human)|22nd chromosome]], called [[DiGeorge syndrome]].{{sfn|Harrison's|2015|pp=2493}}{{sfn|Davidson's|2018|pp=79-80}} This results in a failure of development of the third and fourth pharyngeal pouches, resulting in failure of development of the thymus, and variable other associated problems, such as [[congenital heart disease]], and abnormalities of mouth (such as [[cleft palate]] and [[cleft lip]]), failure of development of the [[parathyroid glands]], and the presence of a fistula between the [[trachea]] and the [[oesophagus]].{{sfn|Davidson's|2018|pp=79-80}} Very low numbers of circulating T cells are seen.{{sfn|Davidson's|2018|pp=79-80}} The condition is diagnosed by [[fluorescence in situ hybridization|fluorescent in situ hybridization]] and treated with [[thymus transplantation]].{{sfn|Harrison's|2015|pp=2493}}

[[Severe combined immunodeficiency]] (SCID) are group of rare congenital genetic diseases that can result in combined T, B, and [[NK cell]] deficiencies.{{sfn|Davidson's|2018|pp=79-80}} These syndromes are caused by mutations that affect the maturation of the [[hematopoietic progenitor cell]]s, which are the precursors of both B and T cells.{{sfn|Davidson's|2018|pp=79-80}} A number of genetic defects can cause SCID, including [[IL-2 receptor]] gene loss of function, and mutation resulting in deficiency of the [[enzyme]] [[adenine deaminase]].{{sfn|Davidson's|2018|pp=79-80}}