U01.04.018 Granulomatous inflammation

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Learning Objectives

Master the structural and cellular components of Granulomatous Inflammation. Distinguish between Caseating and Noncaseating granulomas and identify the cytokine signaling pathway (IL-12, IFN-\gamma, TNF) required for their formation. Understand the clinical risk of Anti-TNF therapy and the metabolic link to Hypercalcemia.

1. Histology and Structure

A granuloma is a microscopic aggregation of macrophages that have been transformed into epithelioid cells (activated macrophages with abundant pink cytoplasm), surrounded by a collar of mononuclear leukocytes (lymphocytes and plasma cells).

Type Characteristics Common Etiologies
Caseating Central granular debris (necrosis); “cheese-like” appearance. Infectious: TB, Fungal (Histoplasmosis).
Noncaseating No central necrosis; preserved cellular architecture. Sarcoidosis, Crohn disease, Berylliosis.
Giant Cells Fusion of activated macrophages (e.g., Langhans type). Common to both types.

2. Mechanism of Formation

Granuloma formation depends on a complex interplay between Antigen-Presenting Cells (APCs) and T-cells, requiring a specific sequence of cytokine signals.

Step Mediator / Action Result
1. Initiation Macrophages (APCs) secrete IL-12. T-cells differentiate into Th1 cells.
2. Activation Th1 cells secrete IFN-\gamma. Macrophages become “Activated” (Epithelioid).
3. Maintenance Macrophages secrete TNF-\alpha. Maintains granuloma structural integrity.

3. Etiologies and Complications

Granulomatous diseases are classified into infectious and non-infectious categories. These conditions can cause systemic metabolic disturbances.

Category Examples Unique Pathology Note
Bacterial TB, Leprosy, Cat Scratch Disease. Cat scratch = Stellate necrotizing granulomas.
Immune / Vascular Sarcoidosis, Wegener’s (GPA), Temporal Arteritis. GPA involves necrotizing granulomas in the airway.
Hypercalcemia Seen in Sarcoidosis. Macrophages express 1\alpha-hydroxylase → ↑ Vitamin D.

Activity:

Activity

High-Yield Mnemonics & Tips:

  • Anti-TNF Danger: Drugs like Infliximab or Etanercept can break down a granuloma. If the patient has latent TB, the bacteria will escape, leading to disseminated disease. Always PPD/IGRA test first!
  • Hypercalcemia Mechanism: Granulomatous diseases (especially Sarcoidosis) cause hypercalcemia because the macrophages themselves activate Vitamin D independently of the kidney/PTH.
  • The “Th1” Pathway: Remember IL-12 makes Th1, and IFN-\gamma makes the Macrophage “Angry” (epithelioid).