lymph

• Lymphoid System


  Tissues and organs concerned with immune responses
• Lymphoid tissue


  Tissue where the parenchyma consists mainly of lymphocytes
• Lymphoid organs


  Organs that are primarily made up of lymphoid tissue
• Components of the Lymphoid System

  • Central/Primary lymphoid organs
  • Peripheral/Secondary lymphoid organs
  • Lymphoid tissue
• Central/Primary lymphoid organs

  • Where the stem cells (B and T) transform into mature and immunocompetent naïve lymphocytes
• Peripheral/Secondary lymphoid organs

  • Lymph nodes, spleen and the mucosa-associated lymphoid tissue (MALT)
• Mucosa-associated lymphoid tissue (MALT)


  Tonsils and other non-encapsulated lymphoid tissues in the gastrointestinal, respiratory, and genitourinary tracts
• Diffuse lymphoid tissue


  Part of lymphoid organs and most connective tissues in the body, prominent in the lamina propria and submucosa of the gastrointestinal, respiratory, and genitourinary tracts
• Types of diffuse lymphoid tissue

  • Loose lymphoid tissue
  • Dense lymphoid tissue
• Nodular lymphoid tissue/Lymphoid nodules

  Lymphoid tissue where clustered lymphocytes form lymphoid nodules, interspersed in areas of dense, diffuse lymphoid tissue
• Types of lymphoid nodules

  • Primary
  • Secondary
• Regions of lymphoid nodules

  • Germinal/Reaction Center - pale central area, mainly young, activated B-cells
  • Corona - darker, peripheral region for more mature lymphocytes
• Thymus


  Organ located just above the heart in the superior mediastinum, reaches maximum weight at puberty
• Functions of the thymus

  • Transform T stem cells into naïve T-cells
  • Produce cytokines that regulate T-cell proliferation, maturation, and function in the thymus and other lymphoid tissues and organs
• Composition of the thymus

  Almost exclusively T-lymphocytes (called thymocytes)
• Histologic organization of the thymus

  • Lobes incompletely divided into lobules by trabeculae coming from the capsule
  • Thin strands of connective tissue from the trabeculae form secondary septa within the gland
• Components of the thymic lobule

  • Stroma
  • Cortex
  • Medulla
• Stroma of the thymic lobules


  • Not made up of reticular cells and reticular fibers, but mainly of stellate cells called epitheloid cells (epithelial reticular cells)
• Cortex of the thymus


  • Contains epitheloid cells called nurse cells, enveloping young T-lymphocytes to promote proliferation and differentiation
• Medulla of the thymus


  • Contains fewer cells such as smaller, mature lymphocytes called small lymphocytes and macrophages, most distinctive feature is the presence of thymic corpuscles
• Stroma


  Connective tissue framework of an organ
• Cortex


  Outer region of an organ
• Medulla


  Inner region of an organ
• Components of the lymphoid system

  • Epithelioid cells
  • Nurse cells
  • Small lymphocytes
• Thymic lobule


  • Structural unit of the thymus
• Trabeculae


  • Connective tissue septa that subdivide the thymus
• The stroma of the thymic lobules is not made up of reticular cells and reticular fibers
• Epithelioid cells


  Stellate cells that make up the main component of the thymic stroma
• Nurse cells

  Epithelioid cells in the thymic cortex that envelop and promote proliferation and differentiation of young T-lymphocytes
• Small lymphocytes


  Mature lymphocytes found in the thymic medulla
• Hassall's corpuscles


  Distinctive feature of the thymic medulla
• Blood-thymus barrier


  • Located in the thymic cortex, made up of capillary endothelial cells, epithelioid cells, and perivascular space
• Lymph node


  Bean-shaped, encapsulated collection of lymphoid tissue interposed along the course of lymphatic vessels
• There are ~500 to 600 lymph nodes in the body
• Hilus


  Indented area of a lymph node where efferent lymphatic vessels leave and blood vessels enter and exit
• Afferent lymphatic vessels


  Vessels that enter the lymph node on its convex surface
• Lymph node capsule


  • Dense irregular connective tissue that encloses the lymph node
• Trabeculae


  Extensions of the lymph node capsule that incompletely subdivide the organ into compartments
• Cortex


  Peripherally-located region of the lymph node parenchyma
• Medulla


  Centrally-located region of the lymph node parenchyma, enclosed by the cortex