A 28-year-old man from Delhi presents with recurrent pyogenic infections (Staphylococcus aureus, Streptococcus pneumoniae) and a history of two episodes of meningitis caused by Neisseria meningitidis. Physical examination reveals hepatosplenomegaly. Serum complement levels are normal, but serum bactericidal activity is absent. Which complement component deficiency is most likely responsible for his clinical presentation?

## Clinical Presentation Analysis **Key Point:** The combination of recurrent pyogenic infections (S. aureus, S. pneumoniae), recurrent Neisseria meningitidis meningitis, hepatosplenomegaly, **normal serum complement levels by immunoassay**, and **absent serum bactericidal activity** is most consistent with **C3 deficiency**. ## Why C3 Deficiency? ### Central Role of C3 in Complement C3 is the pivotal component of all three complement pathways (classical, lectin, and alternative). Its cleavage product C3b is essential for: 1. **Opsonization** — coating bacteria for phagocytosis (explains recurrent pyogenic infections with encapsulated organisms like S. aureus and S. pneumoniae) 2. **MAC formation** — C3b feeds into C5 convertase; without C3 activation, C5 is not cleaved, so the terminal pathway (C5b-9/MAC) cannot form (explains absent bactericidal activity) 3. **Immune complex clearance** — deficiency leads to hepatosplenomegaly from immune complex deposition ### Why "Normal" Complement Levels? Standard complement screening tests (CH50, AH50) measure **functional** activity. However, in some C3 deficiency states, immunoassay-based quantification of individual proteins (e.g., nephelometry) may report near-normal antigen levels if a dysfunctional C3 protein is present. The critical finding is **absent bactericidal activity**, reflecting failure of the entire downstream cascade. ### Clinical Correlates | Feature | C3 Deficiency | C5 Deficiency | Factor H Deficiency | C1q Deficiency | |---------|---------------|---------------|---------------------|----------------| | **Recurrent Neisseria** | ✓ | ✓ (hallmark) | ✓ | ✗ | | **Recurrent pyogenic (encapsulated)** | ✓✓ (severe) | Moderate | ✓ | Moderate | | **Hepatosplenomegaly** | ✓ | ✗ | ✓ | ✗ | | **Bactericidal activity** | Absent | Absent | Reduced | Reduced | | **Opsonization** | Absent | Intact | Reduced | Intact | **High-Yield:** C3 deficiency causes the **broadest susceptibility** — both encapsulated pyogenic bacteria (due to absent opsonization) AND Neisseria species (due to absent MAC formation). This dual susceptibility pattern, combined with hepatosplenomegaly (immune complex accumulation), is the hallmark of C3 deficiency. C5 deficiency, by contrast, spares opsonization and primarily predisposes to Neisseria alone. ### Why Not C5 Deficiency? C5 deficiency selectively impairs MAC formation but leaves C3b-mediated opsonization intact. Therefore, C5-deficient patients are predominantly susceptible to Neisseria species, not to encapsulated pyogenic organisms like S. aureus and S. pneumoniae. The co-occurrence of severe pyogenic infections points upstream to C3. ### Why Not Factor H Deficiency? Factor H is a regulatory protein of the alternative pathway. Its deficiency leads to uncontrolled C3 consumption, resulting in **low** serum C3 levels — inconsistent with "normal" complement levels in this patient. ### Why Not C1q Deficiency? C1q deficiency impairs only the classical pathway and is classically associated with SLE-like autoimmune disease, not recurrent bacterial infections of this pattern. **Clinical Pearl (Harrison's Principles, 21st ed.):** C3 deficiency is associated with recurrent infections by encapsulated bacteria AND Neisseria species, immune complex disease, and hepatosplenomegaly — making it the most clinically severe complement deficiency. Screening with CH50 (which requires all components C1–C9) will be abnormal, but individual protein immunoassays may vary. **Mnemonic:** **C3 = Central 3-way threat** → Pyogenic + Neisseria + Immune complex disease.