A 32-year-old woman with a 5-year history of SLE presents with new-onset photosensitive rash, arthralgia, and positive ANA. All of the following are mechanisms of tissue injury in SLE EXCEPT:

Explanation

## Pathogenic Mechanisms in SLE **Key Point:** SLE is primarily a **Type III (immune complex)** and **Type II (antibody-mediated)** hypersensitivity disease. Type IV hypersensitivity (cell-mediated, CD8+ T-cell driven) is NOT a primary mechanism in SLE; it is characteristic of delayed-type hypersensitivity diseases like tuberculosis, contact dermatitis, and type 1 diabetes. ### Classification of SLE Tissue Injury Mechanisms | Mechanism | Hypersensitivity Type | Role in SLE | Evidence | |---|---|---|---| | Immune complex deposition | Type III | Primary | IC found in kidneys, skin, joints; complement activation (↓C3, C4) | | Antibody-mediated cytotoxicity | Type II | Major | Anti-RBC, anti-platelet, anti-neutrophil antibodies cause hemolysis, thrombocytopenia | | T-cell infiltration (CD8+) | Type IV | **Minimal** | Not the primary driver; T cells are secondary to IC and antibody pathology | | Neutrophil extracellular traps (NETs) | Innate inflammation | Emerging | NET-derived autoantigens (dsDNA, histones) perpetuate autoimmunity | **High-Yield:** SLE is **NOT** a Type IV hypersensitivity disease. Type IV hypersensitivity is seen in: - Tuberculosis - Contact dermatitis - Type 1 diabetes mellitus - Hashimoto thyroiditis (T-cell infiltration) - Delayed-type drug reactions ### Correct Mechanisms in SLE #### 1. Immune Complex Deposition (Type III Hypersensitivity) ```mermaid flowchart TD A[Autoantigen + Autoantibody]:::outcome --> B[Immune Complex Formation]:::outcome B --> C[Deposition in Kidney, Skin, Joints]:::outcome C --> D[Complement Activation via Classical Pathway]:::action D --> E[C3a, C5a Generation]:::outcome E --> F[Neutrophil Infiltration & Inflammation]:::action F --> G[Tissue Damage: Glomerulonephritis, Vasculitis]:::urgent ``` **Clinical Pearl:** Lupus nephritis (Class III–IV) is the hallmark of IC-mediated injury; low serum C3 and C4 reflect complement consumption and correlate with disease activity. #### 2. Antibody-Mediated Cytotoxicity (Type II Hypersensitivity) - **Anti-RBC antibodies** → hemolytic anemia - **Anti-platelet antibodies** → thrombocytopenia - **Anti-neutrophil antibodies** → neutropenia - **Anti-phospholipid antibodies** → thrombosis, recurrent miscarriage #### 3. Neutrophil Extracellular Traps (NETs) — Emerging Mechanism **Key Point:** Defective clearance of apoptotic cells and NETs allows release of nuclear autoantigens (dsDNA, histones, nucleosomes), which drive further autoimmunity and perpetuate the disease cycle. **Clinical Pearl:** Impaired clearance of apoptotic material is a hallmark of SLE pathogenesis; defects in C1q, C3, and C4 (which normally opsonize apoptotic cells) predispose to SLE. **Warning:** Do NOT confuse SLE with Type IV hypersensitivity diseases — while T cells are present in SLE lesions, they are **secondary** to IC and antibody-mediated injury, not the primary driver.