LEC 15: lymphatic system

• Immunity

  Prevent or reduce damage or disease
• Innate immunity

  • Defences present from birth
  • Cannot learn from past immune responses
• Adaptive immunity

  • T & B lymphocytes help immune responses learn, adapt, and remember previous pathogens
• Lymphatic capillaries

  • Found between tissue cells and blood capillaries in loose connective tissue
  • Large molecules can pass into lymph capillaries
• Lymphatic vessels

  • Empties lymph back into circulatory system via right lymphatic duct or thoracic duct
  • Low pressure vessels
  • Uses same mechanisms as veins to return lymph to circulatory system
• Lymphoid cells

  • Lymphocytes arise in red bone marrow and mature into either T cells or B cells
  • T cells (thymus): manage immune response & attack and destroy infected cells
  • B cells (bone marrow): when encountering an antigen, they activate into plasma cells which secrete antibodies
  • Natural killer cells: innate immune cells, cytotoxic (cell-killing)
  • Macrophages: act as phagocytes, help activate T lymphocytes
  • Dendritic cells: found in lymphoid tissue, play a role in T lymphocyte activation
  • Reticular cells: produce fibers that make up the stroma
• Lymph nodes

  Filters to remove and destroy micro-organisms and other debris from lymph before transporting back to bloodstream
• Buboes


  • Inflamed, swollen, tender lymph nodes
  • Occurs when nodes are overwhelmed by what they are trying to destroy
• Spleen


  • Removes old and defective red blood cells and platelets
  • Provides a site for proliferation and immune surveillance
• Lymphoid tissues

  • May be diffuse lymphoid tissue found in every organ or lymphoid follicles which form part of larger organs
  • Common theme is predominance of lymphocytes and macrophages
• Mucosal-associated lymphoid tissues

  Located in mucous membranes
• Tonsils


  • Gather and remove pathogens entering pharynx in food or inhaled air
  • Tonsillar crypts catch bacteria and food particles to train immune cells
• Peyer's patches


  Found in wall of ileum
• Appendix


  Maintains normal flora of intestines
• Thymus

  • Matures T lymphocytes
  • Secretes hormones that cause lymphocytes to become immunocompetent
  • Most active during childhood
  • No B cells, no follicles
  • Not involved in fighting antigens
  • Made up of thymic lobules with an outer cortex containing rapidly dividing T cells and an inner medulla where self-reactive T cells are removed
• Class 1 MHC proteins

  T lymphocytes can see if a cell is infected
• Class 2 MHC proteins

  • Only dendritic cells, macrophages and B cells
  • Pathogens engulfed by these cells are digested into smaller pieces
• T cell selection

  1. Positive selection in cortex of thymus allows T lymphocytes that recognize MHC proteins to survive

  2. Negative selection in inner medulla of thymus allows T cells that don't recognize self antigen proteins to survive