lymphatic

• Pathogen


  Disease-producing microbe such as bacteria and viruses
• Immunity


  Ability to ward off damage or disease through defenses
• Susceptibility


  Lack of resistance
• Types of immunity


  • Innate (nonspecific)
  • Adaptive (specific)
• Innate immunity


  • Defenses present at birth
  • Does not involve specific recognition of a microbe
  • Acts against all microbes in the same way
• Components of innate immunity

  • First line of defense (physical and chemical barriers of skin and mucous membranes)
  • Second line of defense (antimicrobial substances, natural killer cells, phagocytes, inflammation, fever)
• Adaptive immunity


  • Involves specific recognition of a microbe
  • Involves lymphocytes (T cells and B cells)
• Components of the lymphatic system

  • Lymph
  • Lymphatic vessels
  • Lymphatic tissue
  • Red bone marrow
• Functions of the lymphatic system

  • Drains excess interstitial fluid
  • Transports dietary lipids
  • Carries out immune responses
• Formation and flow of lymph

  1. Lymphatic capillaries absorb interstitial fluid

  2. Lymph flows through lymphatic vessels

  3. Lymph passes through lymph nodes

  4. Lymph drains into lymph trunks

  5. Lymph flows into thoracic duct or right lymphatic duct

  6. Lymph returns to bloodstream
• Lymphatic capillaries


  • Greater permeability than blood capillaries
  • Absorb large molecules like proteins and lipids
  • Slightly larger diameter than blood capillaries
  • One-way structure allows fluid to enter but not exit
• Tissues that lack lymphatic capillaries include avascular tissues, portions of the spleen, and red bone marrow
• Principal lymph trunks


  • Lumbar
  • Intestinal
  • Bronchomediastinal
  • Subclavian
  • Jugular

  
• Cisterna chyli


  Dilation at the beginning of the thoracic duct where lymph from the lumbar and intestinal trunks is collected
• All lymph returns to the bloodstream through the thoracic (left) lymphatic duct and right lymphatic duct
• Respiratory pump


  Pressure changes during inhalation maintain lymph flow
• Skeletal muscle pump


  Contraction of skeletal muscles helps move lymph through vessels
• Sequence of fluid flow

  Blood capillaries (blood) → Interstitial spaces (interstitial fluid) → Lymphatic capillaries (lymph) → Lymphatic vessels (lymph) → Lymphatic trunks or ducts (lymph) → Junction of the internal jugular and subclavian veins (blood)

  
• Inhalation


  Promotes the flow of lymph
• Exhalation


  Hinders the flow of lymph
• Lymphatic vessels


  • Contain valves that prevent backflow of lymph
  • Contain smooth muscle that contracts to help move lymph
• Skeletal muscle contractions


  • Compress lymphatic vessels and force lymph toward the junction of the internal jugular and subclavian veins
• Primary lymphatic organs


  • Red bone marrow
  • Thymus
• Secondary lymphatic organs and tissues

  • Lymph nodes
  • Spleen
  • Lymphatic nodules (follicles)

  
• Thymus


  Bilobed organ located in the mediastinum between the sternum and the aorta
• Maturation of T cells in the thymus

  1. Pre-T cells migrate from red bone marrow to the cortex of the thymus

  2. Proliferate and undergo positive selection in the cortex

  3. Surviving T cells enter the medulla

  4. T cells leave the thymus and migrate to lymph nodes, spleen, and other lymphatic tissues
• Thymus


  • Largest at puberty, then functional portion atrophies with age
  • Populated with T cells before atrophy, but some T cells continue to proliferate throughout life
• Lymph nodes


  Bean-shaped organs located along lymphatic vessels, usually in groups

  
• Structure of a lymph node

  1. Capsule of dense connective tissue

  2. Trabeculae divide the node into compartments

  3. Reticular fibers and fibroblasts form the stroma

  4. Parenchyma divided into cortex and medulla

  
• Outer cortex of lymph node

  • Contains lymphatic nodules (follicles) with germinal centers
  • Sites of plasma cell and memory B cell formation
• Inner cortex of lymph node

  • Contains T cells and dendritic cells
• Medulla of lymph node

  • Contains B cells, antibody-producing plasma cells, and macrophages

  
• Lymph flow through a lymph node

  1. Enters through afferent lymphatic vessels

  2. Flows through subcapsular, trabecular, and medullary sinuses

  3. Exits through efferent lymphatic vessels

  
• Function of lymph nodes


  • Filter lymph, trapping foreign substances
  • Macrophages destroy some foreign substances by phagocytosis
  • Lymphocytes destroy others by immune responses
• Lymph flow through a lymph node

  1. Lymph enters through afferent lymphatic vessels

  2. Lymph flows into subcapsular sinus

  3. Lymph flows through trabecular sinuses

  4. Lymph flows into medullary sinuses

  5. Lymph exits through efferent lymphatic vessels
• Lymph nodes


  • Slow flow of lymph allows additional time for lymph to be filtered
  • All lymph flows through multiple lymph nodes, exposing it to multiple filtering events before returning to the blood
• Afferent lymphatic vessels


  Vessels that transport lymph into a lymph node

  
• Efferent lymphatic vessels


  Vessels that transport lymph out of a lymph node
• Foreign substances in lymph that enter a lymph node are filtered
• Medulla of a lymph node

  • Contains B cells, antibody-producing plasma cells, and macrophages
  • Cells are embedded in a network of reticular fibers and reticular cells