A 30-year-old man with severe pancreatitis shows a serum IgG4 level of 480 mg/dL (normal <135 mg/dL). Pancreatic biopsy shows storiform fibrosis, lymphoplasmacytic infiltrate rich in IgG4+ plasma cells, and obliterative phlebitis. This is type 1 autoimmune pancreatitis (AIP). Which molecular mechanism underlies IgG4-related disease?
- A IgG4 is a poor activator of complement and has limited Fc receptor binding, yet the tissue damage is mediated by T-regulatory cells and fibroblast activation via TGF-β and IL-13 ✓
- B IgG4 activates classical complement pathway causing C3b deposition in pancreatic acini
- C IgG4 cross-links mast cell FcεRI receptors mimicking type I hypersensitivity
- D IgG4 is a typical opsonizing antibody that directs macrophage phagocytosis of pancreatic cells
Correct answer: A. IgG4 is a poor activator of complement and has limited Fc receptor binding, yet the tissue damage is mediated by T-regulatory cells and fibroblast activation via TGF-β and IL-13
Explanation
In IgG4-related disease (IgG4-RD), IgG4 itself is a poor complement activator (due to its structural features allowing half-antibody exchange — Fab arm exchange) and has weak Fc receptor binding, so it is not directly cytotoxic. The tissue damage is driven by activated CD4+ T cells (particularly Tfh and Th2), M2 macrophages, and regulatory T cells secreting TGF-β and IL-13 that drive storiform fibrosis. CD8+ cytotoxic T cells are also key effectors. The IgG4 is a marker and epiphenomenon of the immune dysregulation rather than the primary effector molecule.
Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.