Chapter 20: The Lymphatic System and Lymphoid Organs and Tissues

The Lymphatic System: Overview

The lymphatic system is a network of vessels, nodes, and organs that helps maintain fluid balance, defend against pathogens, and absorb fats from the digestive tract. It is distributed throughout the body, with key organs including lymph nodes, the spleen, thymus, and MALT (mucosa-associated lymphoid tissue).

• Locations: Lymphatic vessels are found throughout most tissues, except the central nervous system, bones, and teeth. Major organs include lymph nodes (clusters throughout the body), spleen (upper left abdomen), thymus (anterior mediastinum), and MALT (mucosal linings).

• Organs: Lymph nodes, spleen, thymus, tonsils, Peyer's patches, and appendix.

• Functions:

  • Fluid balance: Returns excess interstitial fluid to the bloodstream.

  • Immune defense: Houses and transports lymphocytes, filters pathogens.

Structure and Function of Lymphatic Vessels

Lymphatic vessels transport lymph from tissues back to the circulatory system. Their structure is specialized to allow fluid entry and prevent backflow.

• Minivalves: Overlapping endothelial cells form one-way minivalves that open when interstitial fluid pressure increases, allowing fluid to enter lymphatic capillaries.

• Types of Lymphatic Vessels (by size):

  1. Lymphatic capillaries (smallest)

  2. Lymphatic collecting vessels (medium)

  3. Lymphatic trunks and ducts (largest)

• Ducts: There are two main lymphatic ducts:

  • Right lymphatic duct: Drains right upper limb, right side of head and thorax.

  • Thoracic duct: Drains rest of the body.

Lymphangitis vs. Lymphedema

• Lymphangitis: Inflammation of lymphatic vessels, often visible as red streaks under the skin, usually caused by infection.

• Lymphedema: Swelling due to blocked lymphatic drainage, leading to accumulation of lymph in tissues.

• Comparison: Lymphangitis is an inflammatory condition, while lymphedema is a mechanical blockage.

Lymph Nodes: Filtration and Immune Function

Lymph nodes are small, bean-shaped structures that filter lymph and house lymphocytes. They are key sites for immune surveillance and activation.

• Fluid Movement: Lymph enters via afferent vessels, passes through sinuses, and exits via fewer efferent vessels, slowing flow and maximizing filtration.

• Buboes: Swollen, tender lymph nodes due to infection (e.g., plague, bacterial infections).

• Secondary Cancer Sites: Lymph nodes can serve as sites for metastasis, often painless and firm.

• Comparison Table:

The Spleen: Blood Filtration and Immune Function

The spleen is the largest lymphoid organ, located in the upper left abdomen. It filters blood, removes aged red blood cells, and stores platelets and monocytes.

• Location: Left upper quadrant, beneath the diaphragm.

• Functions:

  • Removes bloodborne pathogens and old RBCs.

  • Stores platelets and monocytes.

  • Site of lymphocyte proliferation and immune response.

• Example: After splenic injury, patients are at increased risk for infections.

MALT: Mucosa-Associated Lymphoid Tissue

MALT protects mucosal surfaces throughout the body from pathogens. It is found in the digestive, respiratory, and genitourinary tracts.

• Three Large MALTs:

  1. Tonsils: Guard the pharynx and oral cavity.

  2. Peyer's patches: Located in the small intestine, monitor intestinal bacteria.

  3. Appendix: Contains lymphoid tissue, protects the gut.

• Function: Trap and destroy pathogens entering through mucosal surfaces.

The Thymus: T Lymphocyte Maturation

The thymus is a specialized lymphoid organ where T lymphocytes mature. It is most active during childhood and shrinks with age.

• Differences from Other Lymphatic Tissues:

  • Does not directly fight pathogens; functions in T cell maturation.

  • Contains epithelial cells (not reticular fibers).

  • Atrophies after puberty.

• Example: Removal of the thymus in early life impairs immune function.

Additional info: The lymphatic system is essential for both fluid homeostasis and immune defense. Disorders of the lymphatic system can lead to increased susceptibility to infection, swelling, or cancer metastasis.