Organs of the Lymphatic System

The lymphatic system consists of a network of lymphatic vessels, lymphoid tissues, and specialized organs that play key roles in immune defense, fluid balance, and absorption of dietary fats. The main organs of the lymphatic system are the lymph nodes, thymus, spleen, tonsils, and bone marrow, along with other diffuse lymphoid tissues like Peyer's patches and MALT (mucosa-associated lymphoid tissue).

Primary Lymphoid Organs

These are responsible for the production and maturation of lymphocytes.

1. Bone Marrow

• Location: Found in the medullary cavities of long bones and flat bones (e.g., sternum, iliac crest)
• Function:
  • Site of hematopoiesis (formation of all blood cells including lymphocytes)
  • B cell maturation site

2. Thymus

• Location: Upper anterior mediastinum, posterior to the sternum
• Function:
  • Site of T lymphocyte maturation and differentiation
  • Produces thymosins that influence T cell development
• Structure: Bi-lobed organ with a cortex (immature T cells) and medulla (mature T cells)
• Note: Active during childhood, involutes with age

Secondary Lymphoid Organs

These organs filter lymph and blood and are sites of immune cell activation and antigen presentation.

3. Lymph Nodes

• Location: Scattered throughout the body along lymphatic vessels (e.g., cervical, axillary, inguinal regions)
• Function:
  • Filter lymph to trap pathogens and foreign particles
  • Provide an environment for interaction between antigen-presenting cells and lymphocytes
• Structure: Encapsulated with a cortex (B cells in follicles) and paracortex (T cells)

4. Spleen

• Location: Left upper quadrant of abdomen, posterior to stomach
• Function:
  • Filters blood (not lymph)
  • Removes old or damaged red blood cells
  • Initiates immune responses to blood-borne antigens
  • Stores platelets and monocytes
• Structure: White pulp (lymphoid tissue), red pulp (filters blood and recycles erythrocytes)

5. Tonsils

• Location: Surround the entrance of the pharynx (palatine, pharyngeal/adenoids, lingual)
• Function:
  • Protect against inhaled or ingested pathogens
  • Initiate immune responses via crypt epithelium exposure

Other Lymphoid Tissues

MALT (Mucosa-Associated Lymphoid Tissue)

• Includes: Peyer’s patches (ileum), appendix, bronchial-associated lymphoid tissue (BALT)
• Function:
  • Provides immune surveillance at mucosal surfaces (GI, respiratory, urogenital tracts)

Summary Table of Lymphatic Organs

Clinical Significance

• Lymphadenopathy: Enlargement of lymph nodes due to infection, inflammation, or malignancy
• Splenectomy: Removal of spleen impairs immune response to encapsulated bacteria
• Thymic disorders: Can affect T cell development and result in immunodeficiency (e.g., DiGeorge syndrome)