A 35-year-old woman from Bangalore with a 10-year history of systemic lupus erythematosus (SLE) presents with recurrent Neisseria meningitidis meningitis (second episode in 18 months). Serum complement levels: C3 = 52 mg/dL (normal 90–180), C4 = 10 mg/dL (normal 15–45), CH50 = 18% (normal >50%). Serum anti-C1q antibodies are negative. Which mechanism best explains the low complement levels in this patient?

Explanation

## Clinical Context: SLE and Complement Consumption **Key Point:** SLE is characterized by **immune complex deposition** in tissues and circulation. These immune complexes activate the classical complement pathway, leading to **in vivo consumption** of complement components, particularly C3 and C4. ## Pathophysiology of Complement Consumption in SLE ```mermaid flowchart TD A[SLE: Autoantibodies + Autoantigens]:::outcome --> B[Immune Complex Formation]:::outcome B --> C[Circulating & Tissue-Deposited IC]:::outcome C --> D[Classical Pathway Activation]:::action D --> E[C1q → C4 → C2 → C3 activation]:::action E --> F[Consumption of C3 and C4]:::urgent F --> G[Low serum C3, C4, CH50]:::outcome H[Genetic deficiency?] -.->|Anti-C1q negative| I[Ruled out]:::action J[Neisseria susceptibility] -.->|Recurrent meningitis| K[Suggests low C5 or properdin]:::decision K -->|But C3, C4 also low| L[Consumption, not genetic deficiency]:::outcome ``` ## Why This Is Consumption, Not Genetic Deficiency | Feature | Genetic Deficiency | Immune Complex Consumption (SLE) | |---|---|---| | **Baseline complement** | Always low | Normal initially, ↓ during flares | | **Pattern** | C3 or C4 alone (depends on gene) | **Both C3 AND C4 low** | | **Anti-C1q antibodies** | Absent (genetic) | May be present (autoimmune) | | **Family history** | Often positive | Absent | | **Response to treatment** | No improvement | Improves with immunosuppression | | **Tissue deposition** | Absent | IC in kidneys, skin, joints | **High-Yield:** In SLE, **low C3 + low C4 + low CH50** = **consumption pattern**. This is the most common cause of hypocomplementemia in SLE (>90% of cases). ## Why Neisseria Meningitidis Susceptibility? **Clinical Pearl:** Although this patient has SLE-driven C3/C4 consumption, the **selective susceptibility to Neisseria meningitidis** (recurrent meningitis) suggests an additional defect in the **terminal complement pathway (C5–C9)**. However, the question asks about the mechanism of the **low C3/C4**, which is immune complex consumption. **Mnemonic:** **SLE-IC** = SLE → Immune Complexes → Classical pathway activation → Consumption of C3 and C4. ## Why Anti-C1q Negative Does Not Rule Out Consumption Anti-C1q antibodies are found in ~40% of SLE patients with active disease and lupus nephritis. Their absence does NOT exclude immune complex-mediated consumption; it simply means this patient's IC activation is not driven by anti-C1q antibodies (may be anti-DNA/anti-nucleosome complexes instead).