Editing Lymph node (section)

== Function ==
{{Main|Lymphatic system}}In the lymphatic system, a lymph node is a secondary lymphoid organ.<ref name="Grays2016" /> [[File:Diagram of a lymph node CRUK 022.svg|thumb|Diagram of a lymph node showing [[lymphocyte]]s]]
The primary function of lymph nodes is the filtering of lymph to identify and fight infection. In order to do this, lymph nodes contain lymphocytes, a type of white blood cell, which includes B cells and T cells. These circulate through the bloodstream and enter and reside in lymph nodes.{{sfn|Hoffbrand's|2016|p=103,110}} B cells produce [[antibody|antibodies]]. Each antibody has a single predetermined target, an [[antigen]], that it can bind to. These circulate throughout the bloodstream and if they find this target, the antibodies bind to it and stimulate an immune response. Each B cell produces different antibodies, and this process is driven in lymph nodes. B cells enter the bloodstream as "naive" cells produced in [[bone marrow]]. After entering a lymph node, they then enter a lymphoid follicle, where they multiply and divide, each producing a different antibody. If a cell is stimulated, it will go on to produce more antibodies (a plasma cell) or act as a [[Memory T cell|memory cell]] to help the body fight future infection.{{sfn|Hoffbrand's|2016|p=111}} If a cell is not stimulated, it will undergo [[apoptosis]] and die.{{sfn|Hoffbrand's|2016|p=111}}

[[Antigens]] are molecules found on [[bacteria]]l [[cell wall]]s, chemical substances secreted from bacteria, or sometimes even molecules present in body tissue itself. These are taken up by cells throughout the body called [[antigen-presenting cell]]s, such as dendritic cells.{{sfn|Hoffbrand's|2016|p=109}} These antigen presenting cells enter the lymph system and then lymph nodes. They present the antigen to T cells and, if there is a T cell with the appropriate T cell receptor, it will be activated.{{sfn|Hoffbrand's|2016|p=111}}

B cells acquire antigen directly from the afferent lymph. If a B cell binds its cognate antigen it will be activated. Some B cells will immediately develop into antibody secreting plasma cells, and secrete IgM. Other B cells will internalize the antigen and present it to follicular helper T cells on the B and T cell zone interface. If a cognate FTh cell is found it will upregulate CD40L and promote somatic hypermutation and isotype class switching of the B cell, increasing its antigen binding affinity and changing its effector function. Proliferation of cells within a lymph node will make the node expand.

Lymph is present throughout the body, and circulates through [[lymphatic vessels]]. These drain into and from lymph nodes{{snd}}afferent vessels drain into nodes, and efferent vessels from nodes. When lymph fluid enters a node, it drains into the node just beneath the capsule in a space called the subcapsular sinus. The subcapsular sinus drains into trabecular sinuses and finally into medullary sinuses. The sinus space is criss-crossed by the [[pseudopod]]s of macrophages, which act to trap foreign particles and filter the lymph. The medullary sinuses converge at the hilum and lymph then leaves the lymph node via the ''[[efferent lymphatic vessel]]'' towards either a more central lymph node or ultimately for drainage into a central [[Subclavian vein|venous subclavian blood vessel]].
* The B cells migrate to the nodular cortex and medulla.
* The T cells migrate to the deep cortex. This is a region of a lymph node called the '''paracortex''' that immediately surrounds the medulla. Because both naive T cells and [[dendritic cell]]s express [[CCR7]], they are drawn into the paracortex by the same [[chemotactic factors]], increasing the chance of T cell activation. Both B and T lymphocytes enter lymph nodes from circulating blood through specialized [[high endothelial venules]] found in the paracortex.