Rheumatoid arthritis: synovitis and pannus
- Lesion
In rheumatoid arthritis the immune system inflames the joint lining, which grows into a destructive pannus that erodes cartilage and bone.
What it is
Rheumatoid arthritis is a chronic systemic autoimmune disease that mainly attacks the synovium of small and large joints symmetrically. In genetically susceptible people (HLA-DRB1 'shared epitope', PTPN22), environmental factors such as smoking promote citrullination of proteins, and loss of tolerance produces anti-citrullinated protein antibodies and rheumatoid factor. CD4 Th1 and Th17 cells, B cells, macrophages and synovial fibroblasts sustain inflammation through TNF, IL-6 and IL-17, and the hyperplastic synovium (pannus) destroys cartilage and bone. It also affects skin, lungs, vessels, heart and eyes.
Structure
- Synovium: villous hyperplasia of lining cells, dense perivascular lymphocytes and plasma cells (sometimes lymphoid follicles), new vessels and fibrin.
- Pannus spreads over and under the cartilage from the joint margin, eroding cartilage and juxta-articular bone.
- Neutrophils and fibrin in synovial fluid; periarticular osteopenia on X-ray.
- Rheumatoid nodule: central fibrinoid necrosis surrounded by palisading macrophages and granulation tissue, in skin over pressure points (forearm extensor surface) or in lungs.
- Rheumatoid vasculitis can affect small and medium vessels.
Step by step
- Genetic risk (HLA-DRB1 shared epitope, PTPN22) meets triggers such as smoking and periodontal bacteria that citrullinate proteins.
- Citrullinated peptides bind HLA-DR and activate CD4 T cells, which help B cells make anti-CCP and rheumatoid factor, years before symptoms.
- Th1 and Th17 cells, macrophages and immune complexes inflame the synovium; TNF, IL-1, IL-6 and IL-17 rise.
- Synovial fibroblasts become invasive and, with RANKL from T cells, recruit osteoclasts that erode bone.
- Pannus destroys cartilage; the joint becomes deformed and may fuse.
Osteoarthritis vs rheumatoid arthritis
| Feature | Osteoarthritis | Rheumatoid arthritis |
|---|---|---|
| Primary problem | Cartilage degeneration | Autoimmune synovitis |
| Joints | Weight-bearing joints, DIP and PIP, first CMC; asymmetric | MCP, PIP, wrists, feet; symmetric; DIP spared |
| Morning stiffness | Short (under 30 minutes) | Long (over an hour) |
| Synovium | Mild inflammation | Pannus with lymphocytes and plasma cells |
| Bone changes | Osteophytes, sclerosis, subchondral cysts | Marginal erosions, periarticular osteopenia, ankylosis |
| Labs and systemic disease | Normal labs; none | Anti-CCP, RF, high ESR and CRP; nodules, lung, vessels, anaemia |
Clinical notes
- Treatment aims at early disease control with disease-modifying drugs (methotrexate) and biologics against TNF, IL-6, B cells or T-cell co-stimulation.
- Cardiovascular disease is the main cause of excess death.
- In Latin America, rheumatoid arthritis is common and access to biologics varies; early referral prevents deformity.
In the lab
- Anti-CCP antibodies: high specificity; rheumatoid factor: sensitive but less specific.
- Synovial fluid: inflammatory, with many neutrophils and low viscosity.
Test yourself
Think of the answer, then tap the question to see it.
What is pannus?
Proliferating inflamed synovium with granulation tissue that grows over cartilage and erodes cartilage and bone.
Which antibody test is most specific for rheumatoid arthritis?
Anti-citrullinated protein (anti-CCP) antibodies.
Which finger joints are typically spared in rheumatoid arthritis?
The distal interphalangeal joints.
What is a rheumatoid nodule under the microscope?
Central fibrinoid necrosis surrounded by palisading macrophages and granulation tissue.
Elsewhere in DurgiPedia
- Autoantibodies: the table to know
- HLA associations with disease
- TNF-α
- Interleukin-17
- Type III hypersensitivity (immune complexes)
- Synovial joints: capsule, synovial membrane and articular cartilage
Connections
Educational content on the mechanisms of disease; not a diagnostic or treatment guide and no substitute for a health professional's advice. No doses are given.
Sources: Rheumatoid Arthritis (StatPearls (NCBI Bookshelf), 2025); Burden of illness of rheumatoid arthritis in Latin America: a regional perspective (Clinical rheumatology, 2025).
Spotted a mistake?We are always improving for the community. Tell us what is wrong and our team will review it.Report a mistake
Representative model, for educational purposes only.