2. Thymus, Lymph Nodes, Spleen, and Lymphatic Nodules Structure and Functions

Thymus Structure and Function

  • Thymus Overview
    • A bilobed organ that is largest at puberty and atrophies with age.
    • Composed of multiple parts:
    • Capsule
    • Lobule
      • Cortex
      • Medulla
      • Trabecula
  • Medulla Composition
    • Contains more mature T cells, epithelial cells, dendritic cells, and macrophages.
    • Key Structures:
    • Thymic (Hassall's) corpuscles
      • Concentric layers of epithelial cells that degenerate to form keratin-filled granules.
      • Uncertain role, possibly sites for T cell death.
    • T cells that mature in the thymus migrate to lymph nodes, spleen, and other lymphatic tissues.
  • Age-Related Changes
    • The thymus has a reddish appearance due to high lymphoid tissue and blood supply.
    • Fatty infiltrations with age replace lymphoid tissue, leading to a yellowish appearance but the actual size remains unchanged, defined by its connective tissue capsule.
    • Thymus mass in infants is about 70 g (2.3 oz); by adulthood, it reduces to about 3 g (0.1 oz).
  • T Cell Production and Lifespan
    • T cells continue to proliferate in the thymus throughout life, but this process decreases with age.

Lymph Nodes

  • General Information
    • Approximately 600 bean-shaped lymph nodes are present throughout the body, often clustered.
    • Located superficially and deep, with dense groups near mammary glands, axillae, and groin.
  • Size and Structure
    • Size ranges from 1-25 mm (0.04-1 in.).
    • Covered by a capsule of dense connective tissue; the capsule extends into the node as trabeculae, providing structural support and routes for blood vessels.
  • Stroma and Parenchyma
    • Stroma: Supporting framework of reticular fibers and fibroblasts.
    • Parenchyma: Divided into superficial cortex and deep medulla.
    • Cortex:
      • Consists of outer cortex (containing B-cell aggregates) and inner cortex (mainly T cells).
    • Medulla:
      • Contains B cells, plasma cells, and macrophages.
  • Lymphatic Nodules
    • Egg-shaped aggregates of B cells (lymphatic nodules) located mainly in the cortex.
    • Primary Lymphatic Nodules: Primarily contain naïve B cells.
    • Secondary Lymphatic Nodules: Form in response to antigens, sites for plasma cell and memory B cell formation, with a germinal center containing B cells.
  • T Cells in Lymph Nodes
    • T cells enter lymph nodes from other tissues and undergo proliferation upon antigen presentation by dendritic cells.
  • Lymph Flow in Nodes
    • Flow direction: Afferent lymphatic vessels → Subcapsular sinus → Trabecular sinuses → Medullary sinuses → Efferent lymphatic vessels.
    • Efferent lymphatics drained at the hilum, bringing filtered lymph and immune cells out.
  • Functionality
    • Lymph nodes function as filters, trapping foreign substances by reticular fibers. Macrophages and lymphocytes respond to and destroy these substances.
    • The structure facilitates slower lymph flow, maximizing filtering action.

Clinical Connection: Metastasis

  • Definition
    • Metastasis refers to the spread of disease (e.g., cancer) from one part of the body to another, often through lymphatic vessels.
  • Implications
    • Enlarged lymph nodes can indicate metastasis; they feel firm and fixed to underlying structures.
    • Contrast with infected nodes that are generally softer and movable.

Spleen Structure and Function

  • General Overview
    • The spleen is the largest lymphatic organ in the body, measuring about 12 cm (5 in.).
    • Located in the left hypochondriac region, between the stomach and diaphragm.
  • Structural Composition
    • Surrounded by a dense connective tissue capsule, with trabeculae extending into the organ.
    • Covered by a visceral peritoneum.
  • Parenchyma
    • Divided into white pulp (lymphatic tissue) and red pulp (blood-filled venous sinuses and splenic cords).
  • Functions of the Spleen
    1. Immunity: Similar to lymph nodes, with B cells and T cells conducting immune functions; macrophages destroy pathogens by phagocytosis.
    2. Blood Cell Management:
    • Removal of defective or worn-out red blood cells and platelets by macrophages.
    • Storage of platelets (up to 1/3 of the body's supply).
    • Production of blood cells (hemopoiesis) during fetal development.

Clinical Connection: Ruptured Spleen

  • Considerations
    • Most frequently injured organ during abdominal trauma.
    • Injuries to the spleen can lead to significant hemorrhage and shock.
    • Splenectomy may be necessary; however, absence of the spleen results in reduced immune functions, increasing susceptibility to infections.
  • Recommendations
    • Patients without a spleen often receive prophylactic antibiotics prior to invasive procedures to prevent sepsis.

Lymphatic Nodules

  • Definition
    • Egg-shaped masses of lymphatic tissue scattered throughout mucosa, lacking a surrounding capsule.
  • MALT (Mucosa-associated lymphatic tissue)
    • Found in mucous membranes of the gastrointestinal, urinary, reproductive tracts, and respiratory airways.
  • Aggregation Examples
    • Tonsils: Form a ring at pharyngeal junctions, participating in immune responses against foreign materials.
    • Pharyngeal tonsil (adenoid): Embedded in the nasopharynx wall.
    • Palatine tonsils: Located in the oral cavity, commonly removed during tonsillectomy.
    • Lingual tonsils: Located at the tongue base, may also be removed during surgery.
  • Clinical Connection: Tonsillitis
    • Inflammation or infection of the tonsils, often caused by viruses or bacteria.
    • Symptoms: Sore throat, fever, swollen lymph nodes, nasal congestion, difficulty swallowing, headache.
    • Treatment varies: Viral tonsillitis is self-resolving, while bacterial tonsillitis usually requires antibiotics, with tonsillectomy considered for recurrent cases or complications.