Lymphatic System

• Stroma

  Supporting tissue of the organ
• Components of stroma

  • Capsule
  • Trabeculae: portion capsule; extend into the center
  • Reticular fibers
• Parenchyma


  Active cells within the organ supported by the stroma
• Parenchymal cells

  • Lymphocytes
• Lymphatic system


  Consists of the organs that monitor body surfaces and internal fluid compartments and react to presence of potentially harmful substances and microorganisms (Antigens)
• Main cells

  • Lymphocytes
• Lymph node


  Organs that filter microbe; where bacteria collected from initial site are collected and killed
• Reticular fibers


  Support lymph node
• Functions of lymphatic system

  • Collects excess interstitial fluid into the lymphatic capillaries
  • Transports absorbed lipids from the small intestines
  • Protect the body against invading pathogens or antigens
• Components of lymphatic system

  • Cells
  • Lymphatic tissue
  • Lymphatic organ
  • Lymphatic vessels
• Types of lymphoid cells

  • T cells
  • B cells
  • Natural killer/NK cells
• Circulating pool of lymphocytes

  70% of total lymphocytes, majority are T cells, enter and exit the systemic and lymphatic circulation, provide immunologic surveillance
• Remaining lymphocytes

  30% do not circulate and stay in specific lymphatic tissue
• Lymphoid organs

  • Spleen
  • Thymus
  • Lymph nodes
  • Lymphatic capillaries
• Lymphoid tissues as part of other organs

  • Bone marrow
  • Peyers patches of Ileum
  • Tonsils
  • Appendix
• Primary lymphoid organs


  Site of production and maturation of T and B cells
• Production and maturation of lymphoid cells

  • T-Cells: Bone marrow (production), Thymus (maturation)
  • B-Cells: Bone marrow (production and maturation)
• Secondary lymphoid organs


  Site of storage, differentiation, and proliferation upon encounter with specific antigens
• Secondary lymphoid organs

  • Lymph nodes
  • Spleen
  • MALT
• Lymphatic vessels


  Network of blind-ending capillaries in loose connective tissue, remove substances and fluid from the extracellular spaces resulting in formation of lymph fluid, walls are more permeable than blood capillaries so large substances including bacteria can gain entry
• Circulation of lymphocytes

  1. Lymphocytes in the lymph enter the lymph nodes through the afferent lymphatic vessels

  2. Some T and B lymphocytes remain in the lymph nodes

  3. Some leave via the efferent lymphatic vessels

  4. Efferent lymphatic vessels drain into the right lymphatic trunk which drains into the right internal jugular vein and right subclavian vein

  5. Lymphocytes from the blood enter through the high endothelial venule
• Patterns of lymphatic tissue arrangement

  • Diffuse
  • Nodular
• Diffuse lymphatic tissue (MALT)

  Accumulation of lymphatic tissue not enclosed by capsule, found within the GIT, GUT and respiratory passages, these cells interact with antigen and then migrate to the regional lymph nodes
• Nodule


  Accumulation of lymphatic tissue enclosed within capsule
• Types of lymphatic nodules

  • Primary - contains small lymphocytes
  • Secondary - have distinctive features like germinal centers and mantle zone/corona
• Primary lymphatic nodule


  Dark-staining, more euchromatin (active chromatin), have not encountered antigen
• Secondary lymphatic nodule

  Light-staining, more euchromatin (active chromatin), encountered an antigen, has germinal center and mantle zone/corona
• Locations of lymphatic nodules

  • Tonsils
  • Peyers patches of the ileum
  • Appendix
• Pharyngeal tonsils


  Lymphatic nodules at the entrance of the oropharynx, lined with pseudostratified columnar epithelium with lymphatic nodule
• Lingual tonsils


  Lined with stratified squamous non-keratinizing epithelium, have mucus glands
• Palatine tonsils


  Lined with stratified squamous non-keratinizing epithelium
• Peyer's patches of ileum


  Example of nodular arrangement, have villi and short crypts
• Appendix


  Example of nodular arrangement, has longer crypts
• Lymph nodes


  Small encapsulated kidney bean-shaped organ located along the pathways of the lymphatic vessels, filters lymph before it is returned to the blood circulation, have afferent and efferent lymphatic vessels

  
• Components of lymph nodes

  • Stroma - Supporting structures
  • Parenchyma - T cells, B cells, macrophages
• Deep cortex/Paracortex of lymph node


  Between cortex and medulla, composed mainly of T cells
• Filtration of lymph in lymph nodes

  1. Afferent lymphatic vessel drains into the subcapsular sinuses then into the trabecular sinuses

  2. From the trabecular sinuses, lymph goes into medullary sinuses

  3. In the medulla, macrophages form a meshwork to trap antigens and cancer cells
• High endothelial venule 


  Situated in the deep cortex, lined by columnar or cuboidal cells, allows entry of lymphocytes from blood
• Thymus


  Broad, flat, pyramid-shaped, bilobed organ located in the superior anterior mediastinum, site of maturation of lymphocytes, production of lymphocytes during fetal life
• Palatine Tonsils


  Covered by a non-keratinizing stratified squamous epithelium. Dense connective tissue underlies the lymphoid tissue, forming a partial capsule. have multiple deep invaginations (tonsillar crypts) to increase surface area that is in contact with the external environment.