The invention provides compositions and methods for detecting or determining the severity of myasthenia gravis in a subject. The invention also provides compositions and methods for preventing or treating myasthenia gravis in a subject. In some embodiments, the compositions are peptides that bind to monoclonal antibodies that bind to the main immunogenic region of an acetylcholine receptor, and in some instances block pathogenic antibody combining sites. In other embodiments, the peptides further comprise an antibody heavy chain fragment. Kits for detecting or determining the severity of and preventing or treating myasthenia gravis are also provided.