## Diagnosis: C3 Deficiency Secondary to Classical Pathway Over-Activation ### Clinical Context This patient has SLE with recurrent infections (sinusitis, meningitis) and evidence of complement activation. The pattern of low C3, low C4, and low CH50 indicates **classical pathway activation with secondary C3 consumption**. ### Complement Activation Pattern in SLE | Parameter | Finding | Interpretation | |-----------|---------|----------------| | **C3** | 45 mg/dL (↓) | Consumed | | **C4** | 12 mg/dL (↓↓) | Severely consumed (earliest to drop) | | **CH50** | 18 U/mL (↓) | Overall complement activity reduced | | **Anti-C1q Ab** | Positive | Immune complex activation of classical pathway | | **ANA + anti-dsDNA** | Positive | Active SLE with immune complex formation | ### Pathophysiology **Key Point:** In SLE, circulating immune complexes (antigen–antibody) activate the classical complement pathway: 1. **Immune complex formation** → IgG anti-dsDNA + dsDNA 2. **Classical pathway activation** → C1q binds to IgG Fc region 3. **C1s activation** → C4 cleavage (C4 drops first and most severely) 4. **C2 and C3 activation** → C3 consumption 5. **Net result** → Low C3, low C4, low CH50 ### Why C3 Deficiency Causes Recurrent Infections **High-Yield:** C3 is the **central hub** of all three complement pathways (classical, alternative, lectin). Its deficiency impairs: - **Opsonization:** C3b coating of pathogens for enhanced phagocytosis - **Chemotaxis:** C3a generation (anaphylatoxin) recruits neutrophils and macrophages - **Membrane attack complex (MAC) formation:** C3 is required for C5 activation and MAC assembly - **Immune tolerance:** C3-mediated clearance of apoptotic cells and immune complexes ### Why Sinusitis and Meningitis? **Clinical Pearl:** Patients with C3 deficiency are susceptible to **encapsulated bacteria** (especially *Neisseria meningitidis*, *Streptococcus pneumoniae*, *Haemophilus influenzae*) because: - Polysaccharide capsules are poorly opsonized without C3b - C3a-mediated neutrophil recruitment is impaired - Complement-mediated bacteriolysis is absent Meningitis caused by *Neisseria meningitidis* is a classic presentation of complement deficiency, particularly C3 or terminal complement defects. ### Mechanism of C3 Consumption in This Patient ```mermaid flowchart TD A[SLE: Immune complex formation]:::outcome --> B[Anti-dsDNA + dsDNA complexes]:::outcome B --> C[Classical pathway activation]:::action C --> D[C1q binds to IgG Fc]:::action D --> E[C1s → C4 cleavage]:::action E --> F[C2 → C3 activation]:::action F --> G[C3 consumption]:::outcome G --> H{Severity of C3 deficiency?}:::decision H -->|Severe| I[Impaired opsonization & chemotaxis]:::urgent I --> J[Recurrent infections: Neisseria, Strep]:::urgent H -->|Mild| K[Subclinical complement deficiency]:::outcome ``` ### Differentiation from Other Causes **Mnemonic: "CLASSIC" complement deficiency in SLE** - **C**lassical pathway activation (immune complexes) - **L**ow C4 (earliest marker) - **A**ctivation of C1q by IgG - **S**evere C3 consumption - **S**usceptibility to encapsulated bacteria - **I**mmune complex-mediated disease - **C**linical SLE activity correlates with complement levels ### Why Not the Other Options? **C1q deficiency (Option A):** - Anti-C1q antibodies are present, but this indicates immune complex-mediated activation of C1q, not primary C1q deficiency - Primary C1q deficiency would show isolated low C1q with normal C3 and C4 initially - C1q deficiency is associated with SLE-like disease but is rare **C5 deficiency (Option C):** - C5 is downstream of C3 in the complement cascade - C5 deficiency would NOT cause C3 or C4 consumption - C5 deficiency presents with recurrent *Neisseria* meningitidis infections but does NOT explain the low C3 and C4 pattern **Factor H deficiency (Option D):** - Factor H is a regulator of the **alternative pathway**, not classical pathway - Factor H deficiency causes uncontrolled alternative pathway activation, leading to low C3 but typically normal C4 - This patient has low C4, indicating classical pathway activation, not isolated alternative pathway disease ### Management Implications 1. **Treat underlying SLE** → Reduce immune complex formation → Reduce complement consumption 2. **Vaccinate** against *Neisseria meningitidis*, *Streptococcus pneumoniae*, *Haemophilus influenzae* 3. **Prophylactic antibiotics** if recurrent meningitis 4. **Monitor C3 and C4** as markers of SLE activity 5. **Avoid complement-depleting agents** (e.g., eculizumab) unless indicated for specific complications [cite:Harrison 21e Ch 310; Robbins 10e Ch 6]
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