Which of the following mechanisms best explains the pathogenesis of lupus nephritis in systemic lupus erythematosus?

## Pathogenesis of Lupus Nephritis: Immune Complex-Mediated Disease **Key Point:** Lupus nephritis is a **Type III hypersensitivity reaction** (immune complex-mediated disease). Circulating immune complexes (CICs) composed of autoantibodies (anti-dsDNA, anti-nucleosome, anti-histone) bound to nuclear antigens deposit in the glomeruli, activating complement and causing inflammation. ### Mechanism of Lupus Nephritis ```mermaid flowchart TD A[SLE: Defective apoptosis + impaired clearance]:::outcome --> B[Accumulation of nuclear antigens]:::outcome B --> C[Production of anti-dsDNA and anti-nucleosome antibodies]:::outcome C --> D[Formation of circulating immune complexes]:::outcome D --> E[Glomerular deposition via charge/size]:::action E --> F[Complement activation C1q, C3, C4]:::action F --> G[Glomerular inflammation and injury]:::urgent G --> H[Proteinuria, hematuria, renal dysfunction]:::outcome ``` ### Hypersensitivity Reaction Classification in Kidney Disease | Type | Mechanism | Kidney Disease Example | Pathology | |---|---|---|---| | Type I (Immediate) | IgE-mediated mast cell degranulation | Anaphylaxis (rare renal involvement) | Acute vasodilation, edema | | **Type III (Immune Complex)** | **Circulating IC deposition + complement** | **Lupus nephritis, post-streptococcal GN** | **Subendothelial IC deposits, C3 dominant** | | Type II (Cytotoxic) | IgG/IgM against cell surface antigens | Anti-GBM disease (Goodpasture) | Linear IgG along GBM | | Type IV (Delayed) | CD8+ T cell infiltration | Interstitial nephritis (drug-induced) | T cell infiltrates, no antibodies | **High-Yield:** Lupus nephritis shows **granular (not linear) immunofluorescence** with deposits of IgG, IgA, IgM, C3, and C1q — the hallmark of immune complex disease. This distinguishes it from anti-GBM disease (linear IgG). **Clinical Pearl:** The **WHO classification** of lupus nephritis (Class I–VI) is based on light microscopy patterns (proliferative vs. membranous) and electron microscopy location of immune complexes (subendothelial, subepithelial, mesangial). **Mnemonic:** **"CIC in SLE"** — Circulating Immune Complexes in SLE = Type III hypersensitivity. Complement activation (C3, C4 ↓) and glomerular IC deposition drive the pathology.