Lymphatic System and Innate Immunity Notes

The Lymphatic System and Innate Immunity

Overview of the Lymphatic System

  • Functions of the Lymphatic System:
    • Fluid recovery
    • Blood capillaries filter fluid into tissue spaces (15% of blood fluid).
    • Lymphatic system reabsorbs this fluid (2 to 4 L/day) and returns it to the bloodstream.
    • Immunity:
    • Lymph fluid captures foreign cells and chemicals from tissues.
    • Passes through lymph nodes where immune cells (like T and B lymphocytes) help activate an immune response.
    • Lipid Absorption:
    • Lacteals in the small intestine absorb dietary lipids not absorbed by blood capillaries.

Composition of the Lymphatic System

  • Lymph:
    • The recovered fluid from tissues is called lymph once it enters lymphatic vessels.
  • Lymphatic Vessels:
    • Lymphatic Capillaries:
    • Penetrate nearly all body tissues except cartilage, cornea, and bone.
    • Comprised of overlapping endothelial cells acting like roof shingles.
    • Closed at one end and allow bacteria and cells to enter due to large gaps between cells.
    • Larger Lymphatic Vessels:
    • Have three layers: inner (endothelium and valves), middle (elastic fibers and smooth muscle), and outer (thin layer).
    • Collect lymph from capillaries and transport it through lymph nodes.

Major Cells in the Lymphatic System

  • Neutrophils:
    • Antibacterial properties and first responders to infection.
  • Natural Killer (NK) Cells:
    • Large lymphocytes that attack infected and cancerous cells.
  • T Lymphocytes (T Cells):
    • Mature in the thymus; play major roles in the immune response.
  • B Lymphocytes (B Cells):
    • Activated B cells proliferate and differentiate into plasma cells which produce antibodies.
  • Macrophages:
    • Develop from monocytes; phagocytize debris and display antigens to T cells.
  • Dendritic Cells:
    • Mobile antigen-presenting cells alerting the immune system of pathogens.
  • Reticular Cells:
    • Stationary cells contributing to the stroma of lymphatic organs.

Lymph Nodes

  • Characteristics:
    • Most numerous lymphatic organs (~450 in adults).
    • Functions:
    • Cleanse lymph and act as activation sites for T and B lymphocytes.
    • Bean-shaped structures with a fibrous capsule containing compartments separated by trabeculae.

Innate Immunity

  • Definition:
    • Innate immunity serves as the body’s first line of defense against pathogens, with a rapid and nonspecific response.
  • Three Lines of Defense Against Pathogens:
    1. First Line of Defense: Skin and mucous membranes physical barriers.
    2. Second Line of Defense: Innate defense mechanisms such as leukocytes, macrophages, antimicrobial proteins, inflammation, and fever.
    3. Third Line of Defense: Adaptive immunity, which is specific and provides memory against pathogens.

Anatomical and Physiological Responses

  • Chemotaxis and Inflammation:
    • In response to injury and pathogens, cytokines are released to recruit immune cells such as neutrophils and macrophages.
    • Purpose of Inflammation: Limits pathogen spread, destroys pathogens, and initiates tissue repair.
    • Four cardinal signs: redness, swelling, heat, and pain.
  • Processes of Inflammation:
    1. Mobilization of body defenses through signal proteins (cytokines).
    2. Containment and destruction of pathogens.
    3. Cleanup and repair of affected tissue.

Natural Killer Cells Mechanism

  • Function:
    • Attack and destroy infected or cancerous cells.
  • Killing Mechanism:
    • Release of perforins which create holes in target cell membranes for entry of granzymes, leading to apoptosis.