In systemic lupus erythematosus, which complement component deficiency is most strongly associated with increased risk of SLE development and is thought to impair clearance of immune complexes and apoptotic cells?

Question

Accepted Answer

B. C1q deficiency. ## C1q Deficiency and SLE Pathogenesis

**Key Point:** C1q deficiency is the strongest genetic risk factor for SLE development, present in nearly 90% of patients with hereditary C1q deficiency who develop SLE. C1q is the first component of the classical complement pathway and plays a crucial role in immune tolerance.

### Mechanism of C1q in Immune Homeostasis

1. **Clearance of Apoptotic Cells:** C1q binds to apoptotic cells and facilitates their rapid clearance by macrophages, preventing secondary necrosis and release of autoantigens.
2. **Immune Complex Handling:** C1q-mediated complement activation deposits C3b on immune complexes, marking them for clearance by follicular dendritic cells and preventing their deposition in tissues.
3. **B Cell Tolerance:** C1q is involved in suppressing autoreactive B cell responses through complement-dependent mechanisms.

### Comparative Analysis of Complement Deficiencies

| Complement | SLE Risk | Mechanism | Clinical Feature |
|-----------|----------|-----------|------------------|
| **C1q** | Very high (90%) | Impaired apoptotic cell clearance + IC handling | Hereditary C1q deficiency → SLE |
| **C4** | Moderate-high (75%) | Reduced IC solubilization; C4 polymorphisms | Partial C4 deficiency common in SLE |
| **C3** | Moderate | Reduced opsonization; active SLE has low C3 | Reflects disease activity, not predisposition |
| **C5** | Low | Preserved early complement steps | Neisseria meningitidis risk instead |

**High-Yield:** C1q deficiency is the **strongest genetic predisposition** to SLE. Patients with hereditary C1q deficiency have a 90% lifetime risk of developing SLE or lupus-like disease.

**Mnemonic:** **"C1q = Clearance Catastrophe"** — loss of C1q → impaired clearance of apoptotic debris and immune complexes → loss of tolerance → SLE.

**Clinical Pearl:** In SLE patients, low C3 and C4 levels reflect **active disease** (especially nephritis), not genetic predisposition. These are markers of disease activity, not risk factors.

Answer

A. C3 deficiency

Answer

C. C5 deficiency

Answer

D. C4 deficiency

In systemic lupus erythematosus, which complement component deficiency is most strongly associated with increased risk of SLE development and is thought to impair clearance of immune complexes and apoptotic cells?

Explanation

C1q Deficiency and SLE Pathogenesis

Mechanism of C1q in Immune Homeostasis

Comparative Analysis of Complement Deficiencies

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Complement	SLE Risk	Mechanism	Clinical Feature
C1q	Very high (90%)	Impaired apoptotic cell clearance + IC handling	Hereditary C1q deficiency → SLE
C4	Moderate-high (75%)	Reduced IC solubilization; C4 polymorphisms	Partial C4 deficiency common in SLE
C3	Moderate	Reduced opsonization; active SLE has low C3	Reflects disease activity, not predisposition
C5	Low	Preserved early complement steps	Neisseria meningitidis risk instead