Granulomatous Inflammation

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  • Granulomatous inflammation is usually caused by organisms of low virulence but great persistence, or by implanted foreign bodies.
  • Classically appears as a granuloma.
    • NOT a tumour, despite the suffix "-oma".
    • A circumscribed sphere of chronic inflammatory cells enveloped by poorly organised attempts at encapsulation by local connective tissue.
  • The differences between a granuloma and an abscess must be appreciated.
    • The fibrous envelope is never as well developed as that of an abscess.
    • The inner contents are never as completely fluid as in an abscess.

Structure of a Granuloma

Central core

  • The central core which contains the agent.
  • The agent may be visible with H&E staining in section, e.g.
    • Actinobacillus lignieresii
      • The cause of "Wooden tongue" in cattle.
      • Appears as a granule, with a central core of the bacterial colony surrounded by radiating eosinophilic "clubs".
        • Clubs are considered to be formed from degenerating collagen and antigen-antibody complexes.
    • Actinomyces bovis
      • The cause of "Lumpy Jaw" in cattle.
      • Forms granules containing bacteria and "clubs".
    • Fungal hyphae
    • Parasitic larvae
    • Foreign bodies
  • The agent might not be visible without being selectively stained.

Chronic Inflammatory Cells

  • Outside the core is a substantial number of chronic inflammatory cells.
    • Mainly macrophages.
      • Often appear as epithelioid cells.
    • Lymphocytes
    • Plasma cells.
  • Neutrophils and necrotic remnants of cells can be quite prominent in the granulomas of Actinobacillus and Actinomyces species.
  • Eosinophils are prominent in parasitic granulomas.
  • A scattered and variable number of Giant cells are often seen, but not always in every granuloma.

Outer Envelope

  • The final layer is an outer envelope of incomplete fibrous tissue.
  • Giant cells can also be seen in this area.

Gross Appearance of Granulomas

  • The cut surface of granulomas varies considerably;
    • Tuberculous granulomas tend to have solid whitish cores which are often calcified.
      • Grate on the knife when cut through.
    • Parasitic granulomas are often greenish in colour due to the substantial numbers of eosinophils.
      • Older ones are also often calcified.
    • Actinobacillus and Actinomyces species often have liquefied cores due to the necrosis and neutrophils.
      • I.e. they are purulent.
      • May discharge to the surface along sinus tracts.
      • The central core of bacteria and ‘clubs’ may appear as yellowish granules in this pus.
        • Often called "sulphur granules".