Editing Lymphatic system (section)

===Primary lymphoid organs===
The primary (or central) lymphoid organs, including the thymus, bone marrow, [[fetal liver]] and [[yolk sac]], are responsible for generating [[Lymphocyte|lymphocytes]] from immature [[Progenitor cell|progenitor cells]] in the absence of antigens.<ref>{{Cite book |title=Paul's fundamental immunology |date=2023 |publisher=Wolters Kluwer/Lippincott Williams & Wilkins |isbn=978-1-9751-4253-7 |editor-last=Flajnik |editor-first=Martin F. |edition=8th |location=Philadelphia Baltimore New York London Buenos Aires Hong Kong Sydney Tokyo |page=228 |language=en |chapter=CHAPTER 8 Lymphoid tissues and organs |editor-last2=Singh |editor-first2=Nevil J. |editor-last3=Holland |editor-first3=Steven M.}}</ref> The [[thymus]] and the [[bone marrow]] constitute the primary lymphoid organs involved in the production and early [[clonal selection]] of lymphocyte tissues.

[[Bird|Avian]] species's primary lymphoid organs include the bone marrow, thymus, [[bursa of Fabricius]], and yolk sac.<ref>{{Citation |last1=Pendl • |first1=Helene |title=Immunology |date=2016 |work=Current Therapy in Avian Medicine and Surgery |pages=400–432 |url=https://linkinghub.elsevier.com/retrieve/pii/B9781455746712000203 |access-date=2024-08-28 |publisher=Elsevier |language=en |doi=10.1016/b978-1-4557-4671-2.00020-3 |isbn=978-1-4557-4671-2 |last2=Tizard |first2=Ian}}</ref>

====Bone marrow====
{{Main|Bone marrow}}
Bone marrow is responsible for both the creation of [[T cell]] precursors and the production and maturation of [[B cell]]s, which are important cell types of the immune system. From the bone marrow, B cells immediately join the circulatory system and travel to secondary lymphoid organs in search of pathogens. T cells, on the other hand, travel from the bone marrow to the thymus, where they develop further and mature. Mature T cells then join B cells in search of pathogens. The other 95% of T cells begin a process of [[apoptosis]], a form of [[programmed cell death]]: T cells which cannot interact strongly enough with self-antigens are eliminated during {{section link|T_cell#Positive_selection}} while T cells that attack the body's own proteins are eliminated during {{section link|T_cell#Negative_selection}}.

====Thymus====
{{Main|Thymus}}
The thymus increases in size from birth in response to postnatal antigen stimulation. It is most active during the neonatal and pre-adolescent periods. The thymus is located between the inferior neck and the superior thorax. At puberty, by the early teens, the thymus begins to atrophy and regress, with adipose tissue mostly replacing the thymic stroma. However, residual T cell lymphopoiesis continues throughout adult life, providing some immune response. The thymus is where the T lymphocytes mature and become immunocompetent. The loss or lack of the thymus results in severe immunodeficiency and subsequent high susceptibility to infection. In most species, the thymus consists of lobules divided by septa which are made up of epithelium which is often considered an epithelial organ. T cells mature from thymocytes, proliferate, and undergo a selection process in the thymic cortex before entering the medulla to interact with epithelial cells.

Research on [[bony fish]] showed a buildup of T cells in the thymus and spleen of lymphoid tissues in [[salmon]] and showed that there are not many T cells in non-lymphoid tissues.<ref name="Koppang">{{cite journal |vauthors=Koppang EO, Fischer U, Moore L, Tranulis MA, Dijkstra JM, Köllner B, Aune L, Jirillo E, Hordvik I |title=Salmonid T cells assemble in the thymus, spleen and in novel interbranchial lymphoid tissue |journal=J Anat |volume=217 |issue=6 |pages=728–39 |date=December 2010 |pmid=20880086 |pmc=3039185 |doi=10.1111/j.1469-7580.2010.01305.x |url=}}</ref>

The thymus provides an inductive environment for the development of T cells from hematopoietic progenitor cells. In addition, thymic stromal cells allow for the selection of a functional and self-tolerant T cell repertoire. Therefore, one of the most important roles of the thymus is the induction of central tolerance. However, the thymus is not where the infection is fought, as the T cells have yet to become immunocompetent.