Publisher Summary Acetylcholine receptor antibodies are the main pathogenic agent in myasthenia gravis, a neurological disorder resulting in weakness and fatigability of voluntary muscles. The antibodies can be measured by a radioimmunoprecipitation assay, which is in use throughout the world and provides a diagnostic test of high sensitivity and specificity. The antibodies cause loss of acetylcholine receptors at the neuromuscular junction, leading to defects in neuromuscular transmission. However, a proportion of patients with myasthenia have, instead, antibodies to muscle specific kinase (MuSK), another membrane protein restricted to the neuromuscular junction in adult muscle. The antibodies in myasthenia gravis (MG) are typically high affinity and heterogeneous and bind to a number of different epitopes on the acetylcholine receptors (AChR), all of which appear to be conformation-dependent. AChR antibodies in patients without MG are rare, but usually associated with other disorders with increased susceptibility to MG. Fetal deformities can occasionally be caused by antibodies specific to fetal AChR in the absence of maternal symptoms. Instead some of the patients without AChR antibodies have antibodies to MuSK. These antibodies define a particular subgroup of patients with often severe disease and muscle wasting.