Which immunopathological feature best distinguishes rheumatoid arthritis from systemic lupus erythematosus?

Question

Accepted Answer

D. IgG and IgM rheumatoid factor directed against the Fc portion of IgG. ## Distinguishing RA from SLE: Rheumatoid Factor

### Key Pathological Difference

**Key Point:** Rheumatoid factor (RF) — specifically IgG and IgM antibodies against the Fc region of IgG — is the hallmark and most discriminating feature of RA. While RF can occur in other conditions, it is the defining autoantibody in RA pathogenesis.

### Comparative Immunopathology Table

| Feature | Rheumatoid Arthritis | Systemic Lupus Erythematosus |
| --- | --- | --- |
| **Rheumatoid Factor (anti-IgG Fc)** | Present in 70–80% (hallmark) | Absent or rare (<5%) |
| **Anti-Nuclear Antibodies (ANA)** | Negative or low-titre | Present in >95% (diagnostic) |
| **Anti-dsDNA / Anti-Smith** | Absent | Present (SLE-specific) |
| **Immune Complex Deposition** | In synovium (local) | In multiple organs (systemic) |
| **Complement Activation** | Both classical and alternative | Predominantly classical |
| **Target Tissue** | Synovial membrane (joint-specific) | Skin, kidneys, heart, CNS (multisystem) |

### Mechanism of RF in RA

1. **Autoimmune B-cell activation** → production of IgG and IgM against Fc region of IgG
2. **Immune complex formation** → IgG-RF + IgG → circulating and deposited complexes
3. **Complement fixation** → inflammation in synovium and systemic vasculitis
4. **Synovial inflammation** → pannus formation, cartilage/bone erosion

**High-Yield:** RF is present in ~70–80% of RA patients and is part of the 2010 ACR/EULAR classification criteria. Its presence correlates with more aggressive, erosive disease.

### Why ANA and Immune Complexes Are Not the Best Discriminators

- **ANA** is present in >95% of SLE but absent in most RA; however, ANA is also found in other autoimmune conditions (Sjögren's, systemic sclerosis), making it less specific to RA.
- **Immune complexes** occur in both RA (synovial) and SLE (systemic); deposition pattern differs but both activate complement.
- **Complement activation** occurs in both diseases via the classical pathway.

**Clinical Pearl:** A seronegative RA patient (RF and anti-CCP negative) still has RA if clinical and imaging criteria are met; conversely, RF positivity without clinical RA does not define the disease. Thus, RF is necessary but not sufficient — yet it remains the most discriminating single feature between the two conditions.

[cite:Robbins 10e Ch 6]

Answer

A. Deposition of immune complexes in affected tissues

Answer

B. Presence of anti-nuclear antibodies (ANA)

Answer

C. Activation of complement via the classical pathway

Which immunopathological feature best distinguishes rheumatoid arthritis from systemic lupus erythematosus?

Explanation

Distinguishing RA from SLE: Rheumatoid Factor

Key Pathological Difference

Comparative Immunopathology Table

Mechanism of RF in RA

Why ANA and Immune Complexes Are Not the Best Discriminators

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Feature	Rheumatoid Arthritis	Systemic Lupus Erythematosus
Rheumatoid Factor (anti-IgG Fc)	Present in 70–80% (hallmark)	Absent or rare (<5%)
Anti-Nuclear Antibodies (ANA)	Negative or low-titre	Present in >95% (diagnostic)
Anti-dsDNA / Anti-Smith	Absent	Present (SLE-specific)
Immune Complex Deposition	In synovium (local)	In multiple organs (systemic)
Complement Activation	Both classical and alternative	Predominantly classical
Target Tissue	Synovial membrane (joint-specific)	Skin, kidneys, heart, CNS (multisystem)