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

Explanation

## 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]