Clinical meaning
Myasthenia gravis (MG) is an autoimmune disorder targeting the postsynaptic neuromuscular junction (NMJ). In 85% of cases, IgG1 and IgG3 autoantibodies target nicotinic acetylcholine receptors (AChR) through three mechanisms: (1) complement activation causing membrane attack complex (MAC) destruction of the postsynaptic membrane, simplifying the normal complex folds and reducing AChR density; (2) crosslinking and accelerated endocytosis of AChR (antigenic modulation), reducing surface receptor numbers; (3) direct blockade of ACh binding sites. Anti-MuSK antibodies (5-8%) target muscle-specific kinase, which organizes AChR clustering at the NMJ via the agrin-LRP4-MuSK-rapsyn signaling pathway — disruption causes diffuse receptor dispersal. MuSK-MG is typically more severe with prominent bulbar and respiratory involvement. The thymus plays a central role: thymic hyperplasia (70% of AChR-MG) generates autoimmune responses through ectopic germinal centers; thymoma (10-15%) represents a neoplastic immune dysregulation. Fluctuating weakness with fatigability (worsening with repetitive use, improving with rest) is the clinical hallmark, reflecting progressive failure of NMJ transmission as ACh vesicles are depleted during repetitive stimulation against a background of reduced receptor density.