Granulomatous inflammation

Granulomatous Inflammation - The Cellular Crew

• Macrophages: The core component.
  • Transform into epithelioid histiocytes (elongated, pink cytoplasm), which are secretory, not phagocytic.
  • Fuse to form multinucleated giant cells (e.g., Langhans type in TB).
• Lymphocytes: A surrounding collar, primarily CD4+ Th1 cells.
  • Orchestrate the response by secreting cytokines.
• Fibroblasts: Encapsulate the granuloma with a rim of fibrosis over time.

⭐ Macrophage activation and differentiation is driven by IFN-γ, secreted by Th1 cells. This cytokine interaction is a classic high-yield point for exams.

Granuloma Formation - Building the Fortress

A cellular attempt to contain an offending agent that is difficult to eradicate. Key players orchestrate a defense.

• Initiation: Antigen-presenting cells (APCs), mainly macrophages, present the antigen to CD4+ helper T cells.
• Cytokine Cascade:
  • APCs secrete IL-12, inducing CD4+ T cells to differentiate into Th1 cells.
  • Th1 cells then secrete IFN-γ, the critical signal for macrophage activation.
• The Fortress Walls:
  • IFN-γ transforms macrophages into immobile, pink-cytoplasm epithelioid histiocytes and fused multinucleated giant cells (Langhans or foreign-body type).
  • A collar of lymphocytes surrounds this core.

⭐ TNF-α, secreted by macrophages, is crucial for maintaining the granuloma. Anti-TNF-α drugs (e.g., infliximab) can cause reactivation of latent tuberculosis by breaking down existing granulomas.

Granuloma Types - The Case Files

• Immune Granulomas: Type IV hypersensitivity. Driven by activated T-cells and macrophages (epithelioid cells).

  • Caseating: Central necrosis; cheesy, amorphous, eosinophilic core.
    • Tuberculosis: Acid-fast bacilli on Ziehl-Neelsen stain.
    • Fungal Infections: (e.g., Histoplasma). GMS or PAS stain positive.
  • Non-Caseating: No central necrosis; collection of epithelioid cells.
    • Sarcoidosis: May contain Schaumann or Asteroid bodies.
    • Crohn's Disease: Granulomas found in bowel wall.
    • Berylliosis: History of aerospace or electronics industry exposure.

• Foreign Body Granulomas: Response to inert material; not T-cell mediated.

  • Contain foreign body giant cells (nuclei scattered).
  • Examples: Sutures, talc, splinters.

• Suppurative/Stellate Granulomas: Central neutrophilic abscess.

  • Cat-scratch disease, LGV, Yersiniosis.

⭐ In Sarcoidosis, look for bilateral hilar lymphadenopathy on chest X-ray and elevated serum Angiotensin-Converting Enzyme (ACE) levels.

High‑Yield Points - ⚡ Biggest Takeaways

• A pattern of chronic inflammation defined by collections of activated macrophages (epithelioid histiocytes).
• Primarily driven by a Th1-cell response and the secretion of IFN-γ.
• Differentiated into caseating (e.g., Tuberculosis, with central necrosis) and non-caseating (e.g., Sarcoidosis, Crohn's disease).
• Often contains Langhans giant cells, formed by the fusion of multiple macrophages.
• Its main function is to wall off an offending agent that is difficult to eliminate.
• Can cause hypercalcemia due to 1α-hydroxylase activity in activated macrophages.