## Pathophysiology of Seropositive Myasthenia Gravis **Key Point:** Seropositive myasthenia gravis (MG) is characterized by IgG autoantibodies directed against nicotinic acetylcholine receptors (AChR) at the neuromuscular junction. ### Mechanism of Antibody-Mediated Damage 1. **Antibody binding** → Cross-linking of AChR molecules 2. **Complement activation** → MAC (membrane attack complex) formation 3. **Structural damage** → Loss of junctional folds and receptor internalization 4. **Functional consequence** → Reduced number of functional AChR → Impaired neuromuscular transmission ### Key Features | Feature | Details | |---------|----------| | **Antibody prevalence** | 80–90% of generalized MG; 50% of ocular MG | | **Antibody type** | IgG (crosses placenta → neonatal MG) | | **Associated condition** | Thymoma in 10–15% of seropositive patients | | **Diagnosis** | Anti-AChR antibody serology (most specific test) | **High-Yield:** The presence of anti-AChR antibodies in ~85% of generalized MG makes serology a cornerstone diagnostic test. Seropositive patients typically have more severe disease than seronegative patients. **Clinical Pearl:** Neonatal myasthenia gravis occurs in ~12% of infants born to seropositive mothers because maternal IgG antibodies cross the placenta; symptoms resolve within 2–3 weeks as maternal antibodies are cleared. ### Why This Mechanism Matters Understanding antibody-mediated destruction explains: - Why immunosuppression and plasma exchange are effective - Why thymectomy helps (removes source of autoreactive B cells) - Why disease severity correlates with antibody titre - Why neonatal MG is transient (maternal antibodies are temporary)
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