[Pathogenic antibodies in myasthenia gravis].

Myasthenia gravis (MG) is the most common autoimmune disorder of the neuromuscular junction and is clinically characterized by weakness and muscle fatigue. We have classified MG into 3 types on the basis of the antibody pattern. The first type associated with acetylcholine receptor (AChR) autoantibodies, which predominantly belong to IgG1 subclass and are measured by a conventional radioimmunoprecipitation assay with 125I-alpha-bungarotoxin. This subtype occurs in approximately 80% of patients with MG and leads to the loss of AChR number and function, mainly by complement-mediated destruction of the neuromuscular junction. Approximately 40% of patients with MG who have AChR antibodes have a thymoma as a paraneoplastic neurological syndrome. However, the role of antigen expression by thymomas is unclear. The second type of MG occurs in a proportion of "seronegative" patients who did not have AChR autoantibodies. These patients process IgG autoantibodies to muscle-specific tyrosine kinase (MuSK); these antibodies are predominantly of the IgG4 subclass but are not associated with complement-mediated damage to the neuromuscular junction or with the presence of thymomas. In most patients with MuSK antibodies, the symptoms of MG improve after plasma exchange; these patients show a good response to steroid and immunosuppressive drugs but a poor response to thymectomy. MG not associated with the presence of the 2 abovementioned pathogenic autoantibodies is classified as heterogeneous "double seronegative" MG. Our classification is superior to the present classifications with regard to the mechanism, treatment, and prognosis of the disease.