American Board of Internal Medicine (ABIM) Certification Practice Exam

What histological characteristic helps diagnose IgG4-related disease?

1. Presence of granulomas

2. Fibrin deposition

3. IgG4-staining plasma cells

4. Necrotizing inflammation

The correct answer is: IgG4-staining plasma cells

IgG4-related disease is characterized by the presence of abundant IgG4-positive plasma cells in affected tissues. This histological hallmark is essential for diagnosing the condition, as IgG4-related disease involves a spectrum of disorders that can affect various organs, leading to a specific type of tissue response. The diagnosis is further supported by the observation of a significant increase in the number of IgG4-positive plasma cells relative to total IgG-positive plasma cells, typically with a ratio greater than 40%. This distinction is critical because it helps differentiate IgG4-related disease from other inflammatory or autoimmune conditions that may present similarly but do not exhibit this specific plasma cell profile. Other histological features may be present in the context of IgG4-related disease, such as storiform fibrosis or a lymphoplasmacytic infiltrate, but the identification of IgG4-staining plasma cells is the definitive characteristic that underscores the diagnosis of this disorder.