Lymphatic and Immune Systems

• Lymphatic system is a network of vessels and tissues that returns lymph to the blood supply.
• Immune system works in close association with the lymphatic system to carry out immune response.
• Lymphatic tissue is reticular connective tissue with many B/T lymphocytes for adaptive immunity.
• Lymphatic organs have masses of lymphatic tissues surrounded by a capsule.
• Primary lymphatic organs are responsible for immune cell production.
• Secondary lymphatic organs are responsible for immune cell activity.
• Lymph is fluid taken from interstitial spaces.
• Interstitial fluid is the plasma filtrate from the blood supply that builds up in interstitial spaces.
• Lymphatic vessels transport lymph.
• Lymph flows from interstitial space, to lymphatic capillaries, to lymphatic vessels, to lymph nodes, to lymph trunks, to lymphatic ducts, and finally to the subclavian veins.
• Afferent lymphatic vessels carry lymph from capillaries to nodes.
• Lymph nodes filter lymph and remove foreign substances through filtering, phagocytosis, and immune reactions.
• Efferent lymphatic vessels carry lymph away from nodes.

Function

• Lymphatic system drains excess interstitial fluid into the blood.
• Pressure differences between blood vessels and interstitial space mean not all fluid dropped off from capillaries returns.
• Capillaries filter 20 L/day, with only 17 L/day returned through the circulatory system.
• Permeable capillary walls allow transport of dietary fats and lipid-soluble vitamins (A, D, E, K).
• Lymphatic system targets microbes and abnormal cells to produce an immune response.

Lymphatic Capillaries

• Lymphatic capillaries are often in the same place as blood capillaries.
• Lymphatic capillaries are larger and more permeable than blood capillaries to allow diffusion of larger molecules such as fat.
• Endothelium cells are overlapped to allow only one-way flow of lymph, preventing backflow into interstitial space.
• Anchoring filaments attach endothelial cells to neighboring cells.
• Filaments pull endothelium apart if interstitial pressure is high, allowing entry.
• Capillary pressure closes off open gaps if capillary pressure is high.

Lymphatic Vessels

• Superficial lymphatic vessels run in subcutaneous region, following pathway of superficial veins.
• Deep lymphatic vessels follow pathway of arteries.
• Lymphatic vessels are like veins, but with thinner walls and more valves.
• Valves ensure one-way movement.

Lymph Trunks

• Lymph trunks form from merged vessels.
• Lymph trunks include:
  • Lumbar: Drains lower limbs, kidneys, adrenal glands, and part of the pelvic and abdominal walls.
  • Intestinal: Drains stomach, intestine, pancreas, spleen, and some of the liver.
  • Bronchomediastinal: Drains the thoracic walls, lungs, and heart.
  • Subclavian: Drains the upper limbs.
  • Jugular: Drains the head and neck.

Lymphatic Ducts

• Thoracic duct (left lymphatic duct) drains majority of the body into the left subclavian vein.
• Cisterna chyli is the beginning of the thoracic duct, formed by joining of lumbar trunks and intestinal trunk.
• Left jugular and subclavian trunks merge into the left bronchomediastinal trunk, which attaches independently to the left subclavian vein.
• Right lymphatic duct drains the right upper body into the right subclavian or internal jugular vein.
• Trunks usually do not merge, instead referred collectively as a "duct"
• Right lymphatic ducts include; right bronchomediastinal, subclavian, and jugular.

Pump Mechanisms

• External forces and pressures, along with valves, move fluid.
• Skeletal muscle pump uses three steps and 2 valves to return heart-wards:
  • Rest: Both valves open.
  • Contraction: Muscle contraction compresses fluid; proximal valve open, distal valve closed.
  • Relaxation: Muscle relaxes; proximal valve closed, distal valve open, allowing fluid to flow into empty decompressed vessel.
• Respiratory pump uses alternate compressions and decompressions.
  • Inhalation: Abdominal cavity compressed, fluid is forced into thoracic vessels and towards the heart.
  • Exhalation: Abdominal cavity is decompressed.

Primary Organs/Tissues

• Primary organs/tissues are areas where immune cells become immunocompetent.
• Red bone marrow contains stem cells that differentiate into all blood cells.
• B-cells are already immunocompetent when released.
• Pre-T-cells are immature cells released from bone marrow.
• Thymus converts pre-T-cells into active T-cells.
  • Location: Mediastinum, anterior and superior to the heart.
  • Activity: Occurs mostly during childhood, replaced by fat & connective tissue during adolescence.
  • Structure: Bi-lobed, with each lobe surrounded by a capsule.
  • Trabeculae: Extensions of the capsule divide the lobes into lobules.
  • Cortex: Outer region of lobule takes up pre-T-cells and allow them to mature, failures are digested by macrophages.
  • Medulla: Inner region of lobule, where mature T-cells collect.

Lymph Nodes

• Lymph nodes act to filter lymph.
• Location: ~600 nodes located along lymphatic vessels throughout the body, grouped in specific regions (especially axillary, mammary, cervical, and inguinal).
• Character: bean-shaped organs.
• Stroma: Outer supporting framework containing reticular fibers and fibroblasts.
  • Capsule: Outer layer of dense connective tissue has trabeculae that divide the nodes into compartments.
• Parenchyma: Inner functional component that facilitates immune response and filtering.
  • Outer cortex: Contains lymphatic nodules, with primary (no invaders) and secondary types (active, with germinal center)
  • Inner cortex
  • Medulla: Middle region
• Flow: Afferent vessels (several, concave side) > sinuses (subscapular > trabecular > medullary) > efferent vessels (1-2, hilum).
• Immune response: Lymph passes various cell types.
  • Germinal center of outer cortex: Follicular dendritic cells collect antigens to show B-cells, which respond by forming plasma cells (produce antibodies) and memory B cells. There are also macrophages surrounding germinal center: B-cells.
  • Inner cortex: Dendritic cells present antigens to T-cells, which form cytotoxic (attack antigens) and helper (aid cytotoxic and B cells) cells.
  • Medulla: B-cells, plasma cells, macrophages.

Spleen

• Location: Between stomach, diaphragm, left kidney, and left colic flexure of kidney.
• Blood supply: Splenic artery and vein enter at the hilum.
• Character: Organ, largest mass of lymphatic tissue in the body.
• Stroma: Similar to lymph nodes; surrounded by visceral peritoneum.
• Parenchyma: Divided into 2 areas:
  • White pulp: Series of lymphatic cells (B, T, macrophages) surrounding central arteries.
  • Red pulp: Venous sinuses and splenic cords closely associated with veins; contain macrophages to digest old RBCs and platelets; stores 1/3 of body's total platelets.

Lymphatic Nodules

• Lymphatic nodules (follicles) are masses of lymphatic tissues without a capsule, often found in other organs throughout the body.
• Mucosa-associated lymphatic tissues (MALT): Found in mucous membranes of GI, urinary, reproductive, and respiratory systems.
• Tonsils: pharyngeal, palatine, and lingual
  • Pharyngeal: Single mass on the posterior wall of the nasopharynx.
  • Palatine: Bilateral masses posterior to oral cavity.
    • Known as "the tonsils".
  • Lingual: Bilateral masses at base of tongue.
• Appendix: Contains lymphatic nodules.
• Peyer's patches are aggregated lymphatic follicles in the ileum.