Editing Lymphatic system (section)

===Secondary lymphoid organs===
{{Further|Clonal selection}}
The secondary (or peripheral) lymphoid organs, which include [[lymph node]]s and the [[spleen]], maintain mature naive lymphocytes and initiate an [[adaptive immune system|adaptive immune response]].<ref>{{cite journal | vauthors = Ruddle NH, Akirav EM | title = Secondary lymphoid organs: responding to genetic and environmental cues in ontogeny and the immune response | journal = Journal of Immunology | volume = 183 | issue = 4 | pages = 2205–12 | date = August 2009 | pmid = 19661265 | pmc = 2766168 | doi = 10.4049/jimmunol.0804324 }}</ref> The secondary lymphoid organs are the sites of lymphocyte activation by [[antigen]]s.<ref>{{cite journal |title=Secondary Lymphoid Organs: Responding to Genetic and Environmental Cues in Ontogeny and the Immune Response1 |year=2009 |pmc=2766168 |last1=Ruddle |first1=N. H. |last2=Akirav |first2=E. M. |journal=Journal of Immunology |volume=183 |issue=4 |pages=2205–2212 |doi=10.4049/jimmunol.0804324 |pmid=19661265 }}</ref> Activation leads to [[Clonal selection|clonal expansion]], and affinity maturation. Mature lymphocytes recirculate between the blood and the secondary lymphoid organs until they encounter their specific antigen.

====Spleen====
{{Main|Spleen}}
The main functions of the spleen are:
# to produce [[immune cell]]s to fight [[antigen]]s
# to remove [[particulate matter]] and aged blood cells, mainly [[red blood cell]]s
# to produce blood cells during fetal life.

The spleen synthesizes [[antibodies]] in its [[white pulp]] and removes antibody-coated bacteria and antibody-coated blood cells by way of blood and [[lymph node]] circulation. The white pulp of the spleen provides immune function due to the lymphocytes that are housed there. The spleen also consists of red pulp which is responsible for getting rid of aged red blood cells, as well as pathogens. This is carried out by macrophages present in the red pulp. A study published in 2009 using mice found that the spleen contains, in its reserve, half of the body's [[monocyte]]s within the [[red pulp]].<ref name="Swirski">{{cite journal | vauthors = Swirski FK, Nahrendorf M, Etzrodt M, Wildgruber M, Cortez-Retamozo V, Panizzi P, Figueiredo JL, Kohler RH, Chudnovskiy A, Waterman P, Aikawa E, Mempel TR, Libby P, Weissleder R, Pittet MJ | display-authors = 6 | title = Identification of splenic reservoir monocytes and their deployment to inflammatory sites | journal = Science | volume = 325 | issue = 5940 | pages = 612–6 | date = July 2009 | pmid = 19644120 | pmc = 2803111 | doi = 10.1126/science.1175202 | bibcode = 2009Sci...325..612S }}</ref> These monocytes, upon moving to injured tissue (such as the heart), turn into [[dendritic cell]]s and [[macrophage]]s while promoting tissue healing.<ref name="Swirski" /><ref name="Jia">{{cite journal | vauthors = Jia T, Pamer EG | title = Immunology. Dispensable but not irrelevant | journal = Science | volume = 325 | issue = 5940 | pages = 549–50 | date = July 2009 | pmid = 19644100 | pmc = 2917045 | doi = 10.1126/science.1178329 | bibcode = 2009Sci...325..549J }}</ref><ref>{{cite web |url=https://www.nytimes.com/2009/08/04/science/04angier.html |title=Finally, the Spleen Gets Some Respect |first=Natalie |last=Angier | name-list-style = vanc |work=The New York Times |date=August 3, 2009 |url-status=live |archive-url=https://web.archive.org/web/20180127113051/https://www.nytimes.com/2009/08/04/science/04angier.html |archive-date=2018-01-27}}</ref> The spleen is a center of activity of the [[mononuclear phagocyte system]] and can be considered analogous to a large lymph node, as its absence causes a predisposition to certain [[infection]]s. Notably, the spleen is important for a multitude of functions. The spleen removes pathogens and old erythrocytes from the blood (red pulp) and produces lymphocytes for immune response (white pulp). The spleen also is responsible for recycling some erythrocytes components and discarding others. For example, hemoglobin is broken down into amino acids that are reused.

Research on [[bony fish]] has shown that a high concentration of T cells are found in the white pulp of the spleen.<ref name="Koppang"/>

Like the [[thymus]], the spleen has only [[efferent lymphatic vessels]]. Both the [[short gastric arteries]] and the [[splenic artery]] supply it with blood.<ref>{{Cite book | publisher = Lippincott Williams & Wilkins | isbn = 978-0-7817-7076-7 | last = Blackbourne | first = Lorne H | name-list-style = vanc | title = Surgical recall | date = 2008-04-01 | page = [https://archive.org/details/surgicalrecall00blac/page/259 259] | url = https://archive.org/details/surgicalrecall00blac/page/259 }}</ref> The [[germinal centers]] are supplied by [[arteriole]]s called ''penicilliary radicles''.<ref>{{cite web |url=http://medical-dictionary.thefreedictionary.com/penicilliary+radicles |title=Penicilliary radicles |work=Saunders Comprehensive Veterinary Dictionary |edition=3rd |year=2007 |publisher=Elsevier, Inc. |via=The Free Dictionary by Farlex |access-date=2011-04-03 |archive-url=https://web.archive.org/web/20160304110148/http://medical-dictionary.thefreedictionary.com/penicilliary+radicles |archive-date=2016-03-04 |url-status=dead }}</ref>

In the human until the fifth month of [[prenatal development]], the spleen creates [[red blood cells]]; after birth, the [[bone marrow]] is solely responsible for [[hematopoiesis]]. As a major lymphoid organ and a central player in the reticuloendothelial system, the spleen retains the ability to produce lymphocytes. The spleen stores [[red blood cells]] and lymphocytes. It can store enough blood cells to help in an emergency. Up to 25% of lymphocytes can be stored at any one time.<ref name="Spleen: Information, Surgery and Functions">{{cite web |url=https://chp.edu/CHP/organs+spleen+functions |archive-url=https://web.archive.org/web/20110926085253/http://www.chp.edu/CHP/organs+spleen+functions |url-status=dead |archive-date=2011-09-26 |title=Spleen: Information, Surgery and Functions |publisher=Children's Hospital of Pittsburgh - Chp.edu |date=2010-11-17 |access-date=2011-04-03 }}</ref>

====Lymph nodes====
{{Main|Lymph node}}{{Further|List of lymph nodes of the human body}}
[[File:Illu lymph node structure.png|thumb|350px|right|A lymph node showing [[Afferent lymph vessel|afferent]] and [[Efferent lymph vessel|efferent]] [[lymphatic vessel]]s]]
[[Image:Lymph node regions.svg|thumb|240px|Regional lymph nodes]]
A [[lymph node]] is an organized collection of lymphoid tissue, through which the lymph passes on its way back to the blood. Lymph nodes are located at intervals along the lymphatic system. Several [[afferent lymph vessel]]s bring in lymph, which percolates through the substance of the lymph node, and is then drained out by an [[efferent lymph vessel]]. Of the nearly 800 lymph nodes in the human body, about 300 are located in the head and neck.<ref name="SinghV">{{cite book |last1=Singh |first1=Vishram | name-list-style = vanc |title=Textbook of Anatomy Head, Neck, and Brain; Volume III |date=2017 |isbn=9788131237274 |pages=247–249 |publisher=Elsevier India |edition=2nd}}</ref> Many are grouped in clusters in different regions, as in the underarm and abdominal areas. Lymph node clusters are commonly found at the proximal ends of limbs (groin, armpits) and in the neck, where lymph is collected from regions of the body likely to sustain pathogen contamination from injuries. Lymph nodes are particularly numerous in the [[mediastinum]] in the chest, neck, pelvis, [[axilla]], [[groin|inguinal region]], and in association with the blood vessels of the intestines.<ref name=grays/>

The substance of a lymph node consists of lymphoid follicles in an outer portion called the [[Cortex (anatomy)|cortex]]. The inner portion of the node is called the [[medulla of lymph node|medulla]], which is surrounded by the cortex on all sides except for a portion known as the [[Hilum of lymph node|hilum]]. The hilum presents as a depression on the surface of the lymph node, causing the otherwise spherical lymph node to be bean-shaped or ovoid. The efferent lymph vessel directly emerges from the lymph node at the hilum. The arteries and veins supplying the lymph node with blood enter and exit through the hilum. The region of the lymph node called the paracortex immediately surrounds the medulla. Unlike the cortex, which has mostly immature T cells, or [[thymocytes]], the paracortex has a mixture of immature and mature T cells. Lymphocytes enter the lymph nodes through specialised [[high endothelial venule]]s found in the paracortex.

A lymph follicle is a dense collection of lymphocytes, the number, size, and configuration of which change in accordance with the functional state of the lymph node. For example, the follicles expand significantly when encountering a foreign antigen. The selection of [[B cell]]s, or ''B lymphocytes'', occurs in the [[germinal centre]] of the lymph nodes.

Secondary lymphoid tissue provides the environment for the foreign or altered native molecules (antigens) to interact with the lymphocytes. It is exemplified by the [[lymph node]]s, and the lymphoid follicles in [[tonsil]]s, [[Peyer's patch]]es, [[spleen]], [[adenoid]]s, [[skin]], etc. that are associated with the [[mucosa-associated lymphoid tissue]] (MALT).

In the [[gastrointestinal wall]], the [[vermiform appendix|appendix]] has mucosa resembling that of the colon, but here it is heavily infiltrated with lymphocytes.