Immunopathogenesis: Cellular and Humoral Components
The disease is triggered by an unknown antigen or nonspecific stimulation. The activation cascade begins when activated macrophages stimulate T lymphocytes.
The cellular component plays a dominant role. The primary mechanism is a T-cell-mediated response dominated by CD4+ type 1 T-helper cells (Th1). Cytokines are heavily involved:
- Synovial endothelium and fibroblasts secrete IL-33, which enhances early Th2 responses and increases Th17 activity.
- A massive proinflammatory cascade is triggered, releasing interleukins (IL-1, IL-6, IL-8, IL-15, IL-16, IL-17), tumor necrosis factor alpha (TNF-α), and interferon alpha (IFN-α).
The humoral component is characterized by polyclonal activation of B lymphocytes, leading to uncontrolled hyperproduction of immunoglobulins. The main product of this activation is rheumatoid factor (RF) — autoantibodies (IgM, IgG, IgA classes) that specifically target the Fc fragment of the IgG molecule. When bound together, they form immune complexes that deposit in joint tissues and cause damage.
Mechanisms of Joint Tissue Destruction
Joint destruction in rheumatoid arthritis is a multi-component process involving various cells and biochemical cascades.
Key damage factors include:
- Immune complexes. Upon tissue deposition, they activate the complement system.
- Enzymatic aggression. Neutrophils are recruited to the site of inflammation, where they undergo lysis and release aggressive lysosomal enzymes.
- Inflammatory mediators. Arachidonic acid metabolites play an active role.
Macrophage secretion of key proinflammatory cytokines (TNF-α, IL-1, IL-6) drives major structural damage by acting on other cells:
- Activating osteoclasts, leading to pathological bone resorption.
- Activating chondrocytes, whose altered activity causes direct destruction of articular cartilage.
In addition, marked vascular changes occur. Local vasculitis develops, sharply increasing vascular permeability and causing massive edema of the synovial membrane. Neutrophils, T lymphocytes, plasma cells, and dendritic cells actively infiltrate the synovial tissue.
Pannus Formation
One of the most specific morphological hallmarks of rheumatoid arthritis is the formation of a pannus.
The process begins with synovial hypertrophy, thickening exclusively through the active proliferation of synoviocytes. This creates a pannus — an aggressive granulation tissue that gradually grows over the surface of the articular cartilage, destroying its structure.
This granulation tissue comprises not only proliferating synovial cells but also a dense inflammatory infiltrate containing macrophages, T-helper cells, and plasma cells.
Laboratory Diagnostics
Diagnosis is based on identifying markers of systemic inflammation and specific autoantibodies.
- Rheumatoid factor (RF). An important marker, though nonspecific for rheumatoid arthritis alone. RF can also be detected in other autoimmune disorders (e.g., systemic lupus erythematosus [SLE]).
- Specific autoantibodies. Their presence directly indicates polyclonal B-cell activation. These include anti-perinuclear factor, antibodies to modified citrullinated vimentin (anti-MCV), as well as anti-keratin and anti-collagen antibodies.
- Inflammatory markers. During acute flares, blood tests show a significant elevation in C-reactive protein (CRP) levels.