Lymphatic system

• Immunosurveillance


  Monitoring the body for harmful substances
• Lymphatic system

  Groups of cells, tissues, vessels and organs that monitor body surfaces and internal fluid compartments and react to the presence of potentially harmful substances
• Immune system

  A collection of lymphatic tissues and organs that interact and communicate with each other in extraordinarily complex ways
• Lymphatic vessels


  1. Begin as blind-ended capillaries in connective tissue and interstitial spaces

  2. Have thin permeable walls lined by endothelium and contain one way valves to prevent back flow of lymph

  3. Join to form larger vessels and run next to the blood vessels

  4. Finally empties lymph into the blood circulation at the junction of the internal jugular and subclavian veins

  5. Before lymph is reintroduced to the blood it passes through the lymph nodes for immune surveillance
• Collection of lymph due to: Increased hydrostatic and osmotic pressure in arterioles


  Fluid leaks out of the capillaries into the surrounding connective tissue
• Collection of lymph due to: Decreased hydrostatic pressure in the venous system


  Allows the collection of some fluid from the surrounding connective tissue but there may be an accumulation of fluid in the interstitial space
• Lymphatic system

  Drains the interstitial fluids (lymph) and returns them to the bloodstream
• Collection of lymph

  Relates to oedema
• Non-specific (Innate) immune response


  • Physical barriers (e.g., the skin and mucous membranes)
  • Chemical defences (e.g., low pH ) that destroy
  • Secretory substances (e.g., thiocyanate in saliva, lysozymes, interferons, fibronectin, and complement in serum)
  • Phagocytic cells (e.g., macrophages, neutrophils, and monocytes) and natural killer (NK) cells
• Specific (Adaptive) immune response


  • Humoral – production of antibodies by B lymphocytes and plasma cells
  • Cell-mediated – attack and destroy virus-infected host cells foreign cells mediated by specific T lymphocytes
• Cells of the immune system

  • Lymphocytes and Antigen-presenting cells (APCs) which specifically coordinate the activities of adaptive immunity
  • Supporting cells: Other granulocytes and agranulocytes
• Types of lymphocytes


  • T Cells
  • B cells
  • Natural Killer cells
• T Cells


  • Develop in the thymus
  • Adaptive immunity: cell-mediated
  • Different functions: cytotoxic T cells kill transformed and infected cells; helper T cells mediate cytotoxic T cell and B cell responses
• B cells


  • Develop in the bone marrow and GALT
  • Adaptive immunity: Humoral
  • Once activated develops into a plasma cell which produces antibodies (immunoglobulins)
• Natural Killer cells


  • Innate immunity
  • Kill transformed and infected cells that they are genetically programmed to identify
• Support cells are organized in loose meshwork

  • Reticular cells & fibres form mesh works in lymph nodules, lymph nodes & spleen
  • Macrophages, dendritic and follicular dendritic cells reside in the loose meshwork of lymphatic organs
  • Langerhans cells are found in the epidermis
• The blood cells and immune cells arise in the bone marrow via various differentiation pathways from hematopoietic stem cells which are present throughout adulthood and allow for the continuous differentiation of blood cells to replace those lost to age or function
• Antigens may consist of soluble molecules or molecules that are still components of intact cells (bacteria, protozoa, or tumor cells)

  
  

  The immune response to antigens can be:

  ❖cellular (in which lymphocytes are primarily in charge of eliminating the antigen), or

  ❖humoral (in which antibodies are primarily responsible for the response), or

  ❖both.
• Antibody

  A glycoprotein of the immunoglobulin family that interacts specifically with an antigen
• The interaction between most antigens and antibodies is insufficient to stimulate immune responses
• Antigen presentation


  1. The APC first processes the antigen intra-cellularly and then displays antigen peptides on its surface

  2. Antigen can also be processed as a part of the B-cell activation pathway

  3. Antigens must be broken into small peptides and presented in conjunction with MHC II molecules by specialized APCs to the appropriate helper CD4 T lymphocytes
• Antigen Presenting Cells (APCs)


  • Macrophages
  • Perisinusoidal macrophages (Kupffer cells) of the liver
  • Langerhans' cells in the epidermis
  • Dendritic cells of spleen and lymph nodes
  • Type II and III of Epithelioreticular cells of the thymus
• Types of lymphocytes


  • Lymphocytes are the primary cells of adaptive immune responses.
  • Three major types of lymphocytes are present in the body: T lymphocytes, B lymphocytes, and NK cells.
  • T cells: Helper, Cytotoxic & Regulatory (Suppressor) T cells
  • B cells: Plasma cells, Memory B cells
  • Natural killer cells
• How the human immunodeficiency virus HIV bypasses the cell mediated response of T lymphocytes and compromises the immune response of an infected individual
• B cells


  • Mature in red bone marrow and are involved in the production and secretion of the various circulating antibodies (immunoglobulins Ig)
  • Express membrane bound Ig (immunoglobins) called B-cell receptors (BCRs) which serve as the antigen-specific binding site
  • Once a B cell is activated by binding to antigen, and to helper T cells, it differentiates into plasma cells that secrete a soluble form of their surface antibodies and memory B cells that responds quickly to re-encounter with the same antigen
  • The specific antibody produced by the plasma cell binds to the stimulating antigen, forming an antigen–antibody complex which are eliminated in a variety of ways, including destruction by NK cells and phagocytosis by macrophages and eosinophils
• Diffuse lymphatic tissue


  • Guard the body against pathogenic substances and are the site of initial immune response – lamina propria (CT underlying epithelia)
  • Not enclosed by a capsule, when associated with mucous membranes are called mucosa-associated lymphatic tissue (MALT)
  • After initial antigen exposure, cells of diffuse lymphatic tissue, travel to lymph nodes for proliferation and differentiation
  • Increased plasma cells in the lamina propria indicate antibody secretion
  • Increases eosinophils in the lamina propria indicates chronic inflammation and hypersensitivity reaction
• Lymphatic tissue cells

  • Antigen presenting cells
  • B lymphocytes
  • T lymphocytes
  • Natural killer cells
• Antigen presenting cells


  Process the antigen intra-cellularly and then displays antigen peptides on its surface
• Location of lymphocytes

  • Blood
  • Lamina propria
  • Lymphatic nodule
  • Lymph node
  • Spleen
  • Connective tissue
• Lymphocyte development

  • Antigen-independent proliferation and differentiation in primary lymphatic organs (thymus, bone marrow)
  • Antigen-dependent activation and become effector lymphocytes and memory cells in secondary lymphatic organs (Lymphatic nodules, lymph nodes, tonsils, spleen)
• Lymphatic organs


  • Primary lymphoid organs: Bone marrow, Thymus
  • Secondary lymphoid organs: Lymphoid nodules, Lymph nodes, Spleen
• Lymphatic nodules


  • Discrete concentrations of lymphocytes contained in a meshwork of reticular cells
  • No afferent lymphatic vessels but lymph drains from lymphatic tissue through efferent lymphatic vessels
  • Un-encapsulated (not enclosed by a capsule) but sharply delineated
  • Consist of mostly small lymphocytes in a primary nodule
  • A secondary nodule has a germinal centre
• Classification of lymphatic nodules

  • Primary lymphatic nodule: Mainly small lymphocytes
  • Secondary lymphatic nodule (with germinal Centre):
  • Germinal center - large lymphocytes, mainly B cells/ macrophages/ APC's/ plasma cells &
  • Mantle zone - small lymphocytes chiefly T cells
• Germinal center

  Represents a cascade of events that includes activation and proliferation of lymphocytes, differentiation of plasma cells, and antibody production
• Distribution of lymphatic nodules in the alimentary canal

  • Tonsil
  • Peyer's patches (ileum)
  • Vermiform appendix
• Tonsils

  • Ring of lymphatic tissue
  • 3 types: Pharyngeal tonsils (roof of pharynx), Palatine tonsils (either side of pharynx), Lingual tonsils (base of tongue)
  • Palatine tonsils consist of dense accumulations of lymphatic tissue located in the mucous membrane
  • The squamous epithelium that forms the surface of the tonsil dips into the underlying connective tissue in numerous places, forming tonsillar crypts
• Vermiform appendix

  • Significant part of the MALT with its lamina propria and submucosa filled with lymphocytes and lymphoid follicles
  • The lamina propria is heavily infiltrated with lymphocytes and contains numerous lymphatic nodules
  • The small lumen contains a sample of the microbial flora of the intestine, along with undigested material
• Peyer's patches


  • Located in the ileum (distal portion of the small intestine)
  • Aggregations of lymphatic nodules containing T and B lymphocytes
  • In addition to numerous isolated single (solitary) lymph nodules along intestines
• Lymph node

  • Bacteria infected interstitial fluid is taken up by lymphatic capillaries and transported to a regional lymph node
  • Dendritic cells and macrophages within this organ internalize and kill many of the pathogens that pass through
  • The lymph node is also the site of adaptive immune responses mediated by T cells, B cells, and accessory cells of the adaptive immune system
• Lymph node structure


  • Capsulated (surrounded by dense connective tissue capsule)
  • Divided into compartments by trabeculae
  • Supported by reticular meshwork
  • Contains reticular cells, dendritic cells, macrophages, follicular dendritic cells