Agents, compositions, and medicaments that reduce interactions between muscle specific kinase receptor (MuSK) and pathogenic immunoglobulin G4 (IgG4) antibodies specific for the first Ig-like domain of MuSK and methods and uses thereof to reduce such interactions are encompassed herein. Also encompassed are screening assays to identify inhibitors of these pathogenic antibodies, particularly those that reduce binding to MuSK. Agents identified using the screening assays described herein are envisioned for use as therapeutics, alone or in compositions or in medicaments, to improve motor function in subjects afflicted MuSK-MG.