A 35-year-old woman from Bangalore with a 10-year history of systemic lupus erythematosus (SLE) presents with recurrent bacterial infections (Streptococcus pneumoniae, Haemophilus influenzae) despite adequate antibiotic therapy. Laboratory investigations reveal low serum C3 and C4 levels, positive anti-C1q antibodies, and evidence of immune complex deposition in the glomeruli on kidney biopsy. Which mechanism best explains her increased susceptibility to bacterial infections?

Explanation

## Pathophysiology of SLE-Associated Complement Deficiency **Key Point:** SLE patients with low C3/C4 levels and anti-C1q antibodies have **acquired complement deficiency due to classical pathway activation and consumption**, not genetic deficiency. The loss of C3 impairs opsonization, the most critical function for bacterial clearance. ## Why Impaired Opsonization? ### Mechanism of C3 Consumption in SLE ```mermaid flowchart TD A[Immune complexes in SLE]:::outcome --> B[Classical pathway activation]:::action B --> C[C1q activation]:::action C --> D[Cascade amplification]:::action D --> E[Massive C3 consumption]:::urgent E --> F[Low serum C3 levels]:::outcome F --> G[Impaired C3b opsonization]:::urgent G --> H[Defective bacterial phagocytosis]:::urgent H --> I[Recurrent pyogenic infections]:::outcome ``` ### C3b: The Opsonin | Function | Role in Infection Control | Impact of C3 Deficiency | |----------|--------------------------|------------------------| | **Opsonization** | Tags bacteria for phagocytosis | ↓↓↓ Severe impairment | | **Chemotaxis** | Recruits phagocytes via C3a | ↓ Mild impairment | | **MAC formation** | Bactericidal for gram-negatives | ↓ Moderate impairment | | **Alternative pathway amplification** | Amplifies complement response | ↓↓ Significant loss | **High-Yield:** In SLE, **C3 is the most consumed complement component** because immune complexes continuously activate the classical pathway. Low C3 is a marker of active disease and increased infection risk. ### Why This Patient Is Susceptible 1. **Opsonization is the primary defense** against encapsulated organisms (S. pneumoniae, H. influenzae). 2. C3b deposition on bacterial surface is essential for CR1-mediated phagocytosis. 3. Without adequate C3b, even functional phagocytes cannot efficiently engulf bacteria. 4. Anti-C1q antibodies perpetuate classical pathway activation, worsening consumption. **Clinical Pearl:** SLE patients with **persistently low C3 levels** have a 3–5-fold increased risk of serious infections. Complement levels should be monitored as part of disease activity assessment. **Mnemonic:** **SLE-C3-SICK** = SLE + Low C3 + Immune Complex consumption + Compromised opsonization = Increased infection risk.