thymus

Examples of thymus in the following topics:

• Thymus

  • Surprisingly, removal of the thymus does not result in a T cell immunodeficiency.
  • The thymus is a specialized organ of the immune system.
  • The thymus is of a pinkish-gray color, soft, and lobulated on its surfaces.
  • Though removal of the thymus in childhood causes severe immunodeficiency, later in life this is not an issue because of the proliferation of thymus activity early in life.
  • Central tolerance is another function of the thymus.

• Maturation of T Cells

  • T cells are produced in the bone marrow but travel to the thymus to mature.
  • Immature T cells that migrate to the thymus are called thymocytes.
  • The thymus contributes fewer cells as a person ages.
  • The thymus is thus thought to be important in building a large stock of naive T cells soon after birth that can later function without thymus support.
  • The remaining cells exit the thymus as mature naive T cells.

• Organs with Secondary Endocrine Functions

  • These include the heart, kidneys, intestines, thymus, and adipose tissue.
  • The thymus is found behind the sternum.
  • The thymus produces hormones referred to as thymosins, which contribute to the development of the immune response .

• Lymphoid Tissue

  • The thymus gland and bone marrow contain primary lymphoid tissue where B and T cells are generated.
  • T cells mature in the thymus, while B cells mature in the bone marrow.
  • T cells born in bone marrow travel to the thymus gland to mature.
  • More specialized primary lymph tissue, such as the thymus, develops from pharyngeal pouches (embryonic structures that differentiate into organs near the pharynx and throat) by the eighth week of gestation.
  • The thymus and bone marrow are primary lymphoid tissue, while the lymph nodes, tonsils, and spleen are secondary lymphoid tissue.

• 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.
  • Regulatory T cells can be considered both central tolerance and peripheral tolerance mechanisms, as they can be generated from self (or foreign)-reactive T cells in the thymus during T cell differentiation.

• Lymphocytes

  • T cells mature in the thymus and contain T cell receptors (TCRs) that allow them to bind to antigens on MHC complexes.
  • They have an immunosuppressive effect that inhibits cell-mediated immunity at the end of a response and destroys autoimmune T cells that aren't filtered out by negative selection in the thymus.
  • While in the bone marrow, B cells are sorted through positive and negative selection in a manner somewhat similiar to T cell maturation in the thymus, with the same process of killing B cells that are nonreactive to antigens or reactive to self-antigens.
  • Mature B cells leave the thymus and travel to secondary lymphoid tissue such as the lymph nodes.

• Cells and Organs of the Immune System

  • 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 thymus "educates" T cells and provides an inductive environment for the development of T cells from hematopoietic progenitor cells.
  • The thymus is largest and most active during the neonatal and pre-adolescent periods of development.
  • By the early teens, the thymus begins to atrophy and thymic stroma is replaced by adipose tissue.
  • Bone marrow and thymus constitute the primary lymphoid tissues involved in the production and early selection of lymphocytes.

• Cytotoxic T Lymphocytes and Mucosal Surfaces

  • Lymph moves about the body through the lymphatic system, which is made up of vessels, lymph ducts, lymph glands, and organs such as tonsils, adenoids, thymus, and spleen.
  • B cell maturation occurs in the bone marrow, whereas progenitor cells migrate from the bone marrow and develop and mature into naïve T cells in the organ called the thymus.

• WBC Formation

  • However, maturation, activation, and some proliferation of lymphoid cells occurs in lymphoid organs (spleen, thymus, and lymph nodes).
  • In some cases, the liver, thymus, and spleen may resume their haematopoietic function if necessary.

• Tonsils

  • Like the thymus, the tonsils reach their largest size near puberty and gradually atrophy thereafter.
  • Additionally, evidence exists that suggests that tonsils may play a role in  T cell maturation and development like the thymus does, but more research is needed.