primary lymphoid organ

Examples of primary lymphoid organ in the following topics:

• Lymphoid Tissue

  • The lymphoid tissue may be primary or secondary depending upon its stage of lymphocyte development and maturation.
  • Central or primary lymphoid organs generate lymphocytes from immature progenitor cells such as lymphoblasts.
  • The thymus gland and bone marrow contain primary lymphoid tissue where B and T cells are generated.
  • Besides generation, primary lymphoid tissue is the site where lymphocytes undergo the early stages of maturation.
  • The thymus and bone marrow are primary lymphoid tissue, while the lymph nodes, tonsils, and spleen are secondary lymphoid tissue.

• Cells and Organs of the Immune System

  • The immune system includes primary lymphoid organs, secondary lymphatic tissues and various cells in the innate and adaptive immune systems.
  • The key primary lymphoid organs of the immune system include the thymus and bone marrow, as well as secondary lymphatic tissues including spleen, tonsils, lymph vessels, lymph nodes, adenoids, skin, and liver.
  • The red bone marrow is a key element of the lymphatic system, being one of the primary lymphoid organs that generate lymphocytes from immature hematopoietic progenitor cells.
  • Bone marrow and thymus constitute the primary lymphoid tissues involved in the production and early selection of lymphocytes.
  • Lymphoid tissue is found in many organs, particularly the lymph nodes.

• Clonal Selection and Tolerance

  • Central tolerance is distinct from periphery tolerance in that it occurs while cells are still present in the primary lymphoid organs (thymus and bone-marrow), prior to export into the periphery.
  • Negative selection is the process that eliminates developing lymphocytes whose antigen receptors bind strongly to self antigens present in the lymphoid organs.

• Development of the Immune System

  • In fact, many of the infections acquired by neonates are caused by low-virulence organisms like staphylococcus and pseudomonas.
  • There is evidence that these steroids act directly not only on the primary and secondary sexual characteristics, but also affect the development and regulation of the immune system.
  • The lymphoid vs. myeloid model of lymphopoiesis has the virtue of relative simplicity and agreement with nomenclature and terminology, as well as validity for tests with mice. pHSC pluripotent, self-renewing, hematopoietic stem cells give rise to MPP multipotent progenitors (these give rise to ELP, or PRO, prolymphocytes); early lymphoid progenitors; and finally to the CLP common lymphoid progenitor, a cell type fully committed to the lymphoid lineage. pHSC, MPP and ELP cells are not fully committed to the lymphoid lineage because if one is removed to a different location, it may differentiate into non-lymphoid progeny.
  • However, CLP are committed to the lymphoid lineage.
  • This revised flowchart indicates multi-lymphoid progenitor, early thymic progenitor, lymphoid extrusion, CD4 and CD8, monocyte/macrophate, myeloid extrusion, and T-cells.

• Cardiovascular and Lymphatic System Defenses

  • Lymphatic organs play an important part in the immune system, having a considerable overlap with the lymphoid system.
  • Lymphoid tissue is found in many organs, particularly the lymph nodes, and in the lymphoid follicles associated with the digestive system such as the tonsils.
  • Lymphoid tissues contain lymphocytes, but they also contain other types of cells for support.
  • The system also includes all the structures dedicated to the circulation and production of lymphocytes (the primary cellular component of lymph), which includes the spleen, thymus, bone marrow, and the lymphoid tissue associated with the digestive system.
  • When micro-organisms invade the body, or the body encounters antigens (such as pollen), antigens are transported to the lymph.

• Structure of the Lymphatic System

  • The lymphatic system consists of lymphatic vessels and associated lymphoid organs.
  • Lymphoid tissue is found in many organs including the lymph nodes, as well as in the lymphoid follicles in the pharynx such as the tonsils.
  • Lymphoid tissues contain lymphocytes (a type of highly differentiated white blood cell), but they also contain other types of cells for structural and functional support, such as the dendritic cells, which play a key role in the immune system.

• Regulating Immune Tolerance

  • Mucosal immunity is formed by mucosa-associated lymphoid tissue, or MALT, which functions independently of the systemic immune system; it has its own innate and adaptive components.
  • M cells are located in the Peyer's patch, which is a lymphoid nodule.
  • The primary mechanism for developing immune tolerance to self-antigens occurs during the selection for weakly, self-binding cells during T and B lymphocyte maturation.
  • Antigen-presenting cells, T cells, and B cells aggregate within the Peyer's patch, forming organized lymphoid follicles.

• Development of the Dual Lymphocyte System

  • During this process, all lymphocytes originate from a common lymphoid progenitor before differentiating into their distinct lymphocyte types.
  • B cells mature into B lymphocytes in the bone marrow, while T cells migrate to, and mature in, a distinct organ called the thymus.
  • Following maturation, the lymphocytes enter the circulation and peripheral lymphoid organs (e.g. the spleen and lymph nodes) where they survey for invading pathogens and/or tumor cells.
  • All lymphocytes originate during this process from a common lymphoid progenitor before differentiating into their distinct lymphocyte types.

• WBC Formation

  • The daughters are the myeloid and lymphoid progenitor cells, which cannot self renew but differentiate into various myeloid leukocytes and lymphocytes respectively.
  • When bone marrow develops, it eventually assumes the task of forming most of the blood cells for the entire organism.
  • However, maturation, activation, and some proliferation of lymphoid cells occurs in lymphoid organs (spleen, thymus, and lymph nodes).
  • It may cause these organs to hypertrophy and increase in size substantially.
  • During fetal development, the liver functions as the main haematopoetic organ since bones and marrow develop later.

• Tonsils

  • The tonsils are small masses of secondary lymphoid tissue located in the pharynx.
  • They function similarly to other types of secondary lymphoid organs and also capture antigens from respiratory tract pathogens.
  • The tonsils are made of secondary lymphoid tissue and covered with an epithelium characteristic of the part of the body where they are located.
  • The secondary lymphoid tissue within the tonsils functions like the same type of the tissue in lymph nodes.