Lymphoid Organs and Tissue

Primary Lymphoid Organs

• Ileum and appendix are primary lymphoid organs

Secondary (Effector) Lymphoid Organs/Tissue

• Spleen and lymph nodes are secondary lymphoid organs
• Mucosal associated lymphoid tissue (MALT) is a type of secondary lymphoid tissue
• MALT is composed of:
  • Diffused population of cells mainly consisting of T lymphocytes, macrophages, antigen-presenting cells (APC), reticular cells, and reticular fibers
  • Ground substance produced by reticular cells

Lymphoid Nodules

• Lymphatic nodules are found in MALT
• Tonsils are an example of lymphoid nodules
• Types of tonsils include:
  • Palatine tonsils
  • Pharyngeal tonsil
• Tonsils are covered by stratified ciliated columnar epithelium
• No crypts are present in tonsils

Thymus

• Located in the mediastinum
• Has a CT capsule that penetrates the parenchyma and divides it into incomplete lobules
• Each lobule contains:

Spleen

• The largest lymphoid organ in the body
• The only lymphoid organ interposed in blood circulation
• Covered by a capsule that extends to form trabeculae
• Trabeculae contain trabecular arteries and veins
• Lymphatic nodules with germinal centers constitute the white pulp
• Central artery passes through lymphatic nodules
• Red or splenic pulp contains:
  • Pulp arteries
  • Venous sinuses
  • Splenic cords (Cords of Billroth)