## Distinguishing Classical from Alternative Pathway Activation ### Key Structural Difference **Key Point:** The classical pathway is antibody-dependent, while the alternative pathway is antibody-independent. This is the fundamental discriminator between the two activation routes. ### Comparison Table | Feature | Classical Pathway | Alternative Pathway | | --- | --- | --- | | **Initiation trigger** | IgG or IgM bound to antigen (C1q recognition) | Direct microbial surfaces (polysaccharides, LPS) | | **Antibody requirement** | Yes — essential | No — spontaneous | | **Initial enzyme complex** | C1qr2s2 (C1 complex) | C3 convertase (C3bBb) | | **Amplification loop** | Occurs downstream | Built-in amplification | | **Evolutionary role** | Adaptive immunity link | Innate immunity first-line | ### Why This Matters Clinically **Clinical Pearl:** Patients with deficiencies in early classical pathway components (C1q, C1r, C1s, C4, C2) present with autoimmune disease (SLE-like) because they cannot clear immune complexes efficiently. Patients with alternative pathway defects (Factor B, Factor D, Properdin) suffer recurrent *Neisseria meningitidis* infections because they lack the primary defense against encapsulated organisms. **High-Yield:** The classical pathway is the only one that requires an antibody-antigen complex. All other features (C3a/C5a generation, MAC formation, activation by microbial patterns) are shared by both pathways once initiated. ### Mnemonic **"C1q Captures Complexes"** — Classical pathway needs Complexes (antibody-antigen); Alternative pathway Activates Automatically on Antigens. [cite:Robbins 10e Ch 6]
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