A 5-year-old boy from Mumbai is brought to the pediatric clinic with a history of recurrent pyogenic infections (Staphylococcus aureus and Streptococcus pneumoniae) since infancy. He has had meningitis twice and recurrent otitis media. Serum immunoglobulin levels are normal. Complement assays reveal absent C3 in serum, with normal C1q and C4 levels. The alternative pathway hemolytic assay is markedly abnormal. Which genetic defect is most likely responsible for this presentation?

Explanation

## Clinical Presentation The patient presents with: - **Recurrent pyogenic infections** (S. aureus, S. pneumoniae) — hallmark of complement deficiency - **Meningitis** — especially concerning for complement deficiency - **Recurrent otitis media** - **Normal immunoglobulin levels** — rules out primary antibody deficiency - **Absent serum C3 with normal C1q and C4** — indicates alternative pathway defect - **Abnormal alternative pathway hemolytic assay** — confirms alternative pathway dysfunction ## Complement Pathway Overview ```mermaid flowchart TD A[Complement Activation Pathways]:::outcome --> B[Classical Pathway]:::action A --> C[Alternative Pathway]:::action A --> D[Lectin Pathway]:::action B --> E[C1q → C4 → C2]:::action C --> F[Factor B + Factor D]:::action D --> G[Mannose-binding Lectin]:::action F --> H[C3 Convertase: C3bBb]:::action E --> I[C3 Convertase: C4b2a]:::action H --> J[C3 Cleavage]:::outcome I --> J J --> K[C3a + C3b]:::outcome K --> L[Amplification Loop]:::action L --> M[MAC Formation]:::action ``` ## Why C3 is Depleted: The Role of Factor H **Key Point:** Factor H is a **soluble regulatory protein** that acts as a cofactor for Factor I, which cleaves C3b. Without Factor H, C3b cannot be inactivated, leading to: 1. **Uncontrolled alternative pathway activation** 2. **Continuous C3 consumption** (C3 → C3a + C3b) 3. **Severe C3 depletion** in serum 4. **Loss of opsonization** (C3b is the major opsonin) 5. **Impaired chemotaxis** (C3a is a potent chemoattractant) **High-Yield:** In Factor H deficiency, the **alternative pathway is constitutively active** because C3b cannot be regulated. This leads to: - Rapid consumption of C3 - Inability to opsonize encapsulated bacteria (S. pneumoniae, H. influenzae) - Increased susceptibility to meningitis ## Differential Diagnosis of C3 Depletion | Deficiency | C3 Level | C1q/C4 | AP Assay | Clinical Features | |------------|----------|--------|---------|-------------------| | **Factor H** | Absent/Very Low | Normal | Abnormal | Recurrent pyogenic infections, meningitis, early-onset | | **Factor B** | Markedly Low | Normal | Abnormal | Similar to Factor H but rarer | | **Factor D** | Markedly Low | Normal | Abnormal | Very rare; severe infections | | **C3 Deficiency (primary)** | Absent | Normal | Abnormal | Severe recurrent infections, SLE-like syndrome | **Clinical Pearl:** Factor H deficiency is the **most common genetic cause of C3 depletion** and accounts for ~40% of post-infectious glomerulonephritis cases. It is also associated with **membranoproliferative glomerulonephritis (MPGN)** and **atypical hemolytic uremic syndrome (aHUS)**. ## Why This Patient Has Factor H Deficiency 1. **C3 is absent** — indicates alternative pathway hyperactivation 2. **C1q and C4 are normal** — classical pathway is intact 3. **Alternative pathway assay is abnormal** — confirms alternative pathway defect 4. **Recurrent pyogenic infections + meningitis** — typical presentation 5. **Early age of onset (infancy)** — consistent with genetic defect **Mnemonic:** **CHAMP** for complement deficiency infections — **C**apsulated organisms (S. pneumoniae, H. influenzae), **H**aemophilus, **A**ssociated with meningitis, **M**eningococcus, **P**yogenic infections.