Lymphoid tissue histology

Lymphoid Architecture - The Body's Network

• Primary Organs: Sites of lymphocyte maturation.
  • Bone Marrow (B cells), Thymus (T cells).
• Secondary Organs: Sites of lymphocyte activation.
  • Arranged to encounter antigens; includes spleen, lymph nodes, and MALT (Mucosa-Associated Lymphoid Tissue).

• Lymph Node Structure:
  • Cortex: B-cell follicles (primary/secondary).
  • Paracortex: T-cell region with high endothelial venules (HEVs) for lymphocyte entry from blood.
  • Medulla: Cords (plasma cells) and sinuses (macrophages).

⭐ DiGeorge syndrome results in a poorly developed paracortex due to T-cell deficiency.

Primary Organs - T&B Cell University

• Thymus: Encapsulated organ in the anterior mediastinum where T-cells mature. Fully formed and active at birth, it involutes with age.

  • Cortex: Darkly stained, densely packed with immature thymocytes undergoing positive selection (recognizing self-MHC).
  • Medulla: Paler stained, contains mature T-cells undergoing negative selection (eliminating self-reactive cells via AIRE). Features Hassall's corpuscles (keratinized epithelial whorls).

• Bone Marrow: Central factory for all hematopoietic cells. Serves as the site for B-cell origin, maturation, and selection.

⭐ The thymus is derived from the 3rd pharyngeal pouch. Its absence in DiGeorge syndrome (22q11 deletion) causes severe T-cell deficiency.

Secondary Organs - Sites of Battle

• Lymph Nodes: Encapsulated filters of lymph.

  • Cortex: B-cell zone. Contains follicles.
    • Primary follicles: Inactive, dense B-cells.
    • Secondary follicles: Active, with pale germinal centers (site of B-cell proliferation).
  • Paracortex: T-cell zone. Contains high-endothelial venules (HEVs) for lymphocyte entry.
  • Medulla: Medullary cords (plasma cells) & sinuses (macrophages).

• Spleen: Encapsulated filter of blood.

  • White Pulp: Lymphoid tissue.
    • Periarteriolar Lymphoid Sheath (PALS): T-cells surrounding central artery.
    • Follicles: B-cells.
  • Red Pulp: Sinusoids & Cords of Billroth for RBC filtration.

• Mucosa-Associated Lymphoid Tissue (MALT): Unencapsulated.

  • e.g., Peyer's Patches (ileum), tonsils, appendix.
  • Specialized M cells transport antigens.

⭐ In DiGeorge syndrome (22q11 deletion), the T-cell-rich paracortex of the lymph node is poorly developed due to thymic aplasia.

High‑Yield Points - ⚡ Biggest Takeaways

• Lymph node paracortex houses T-cells and expands during viral infections. The cortex contains B-cell follicles.
• Spleen's white pulp consists of T-cell rich periarteriolar lymphoid sheaths (PALS) and B-cell follicles.
• The thymus matures T-cells: positive selection in the cortex, negative selection in the medulla with Hassall's corpuscles.
• Peyer's patches in the ileum utilize M cells for antigen sampling from the gut lumen.
• Secondary follicles with pale germinal centers are hallmarks of B-cell activation and proliferation.