Editing Lymphatic system (section)

====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.