Which is the most common complement pathway activated by immune complexes in the pathogenesis of systemic lupus erythematosus?

Explanation

## Complement Pathway Activation in SLE **Key Point:** The classical pathway is the most common complement pathway activated in SLE and other immune complex-mediated diseases. ### Mechanism of Classical Pathway Activation The classical pathway is initiated when C1q binds directly to the Fc region of IgG or IgM antibodies that are part of circulating immune complexes. In SLE: 1. Antigen-antibody complexes form (DNA-anti-DNA, nucleosome-anti-nucleosome) 2. C1q recognizes and binds to the Fc portion of IgG (especially IgG1 and IgG3) or IgM 3. C1r and C1s are activated sequentially 4. Cascade proceeds through C4 → C2 → C3 → C5-C9 **High-Yield:** Classical pathway activation by immune complexes is the hallmark of SLE pathogenesis and explains why complement levels (especially C3 and C4) are low during active disease. ### Comparison of Complement Pathways | Pathway | Trigger | Primary Antibody | Role in SLE | |---------|---------|------------------|-------------| | Classical | Immune complexes, IgG/IgM-antigen | IgG, IgM | **Most common** — primary mechanism | | Alternative | Microbial surfaces, polysaccharides | None required | Secondary activation; amplification loop | | Lectin | Mannose-binding lectin on pathogens | None required | Rare in SLE; more in infections | | Properdin | Stabilizes C3 convertase | None required | Amplification only; not primary | **Clinical Pearl:** Persistently low C3 and C4 levels in SLE indicate active immune complex disease and ongoing classical pathway activation. Rising complement levels often herald remission. ### Why Classical Pathway Dominates in SLE - SLE is fundamentally an **immune complex disease** - Autoantibodies (anti-dsDNA, anti-nucleosome) form complexes with self-antigens - These complexes deposit in kidneys, skin, joints, and blood vessels - Classical pathway is the **only pathway directly activated by antibody-antigen complexes** - Alternative and lectin pathways serve amplification roles but are not primary initiators **Mnemonic:** **C1q = Complex Recognition** — C1q is the pattern recognition molecule that "sees" immune complexes and starts the classical cascade.