The interaction of immunoglobulin G fractions from the sera and thymus glands of patients with myasthenia gravis with acetylcholine receptors from denervated rat muscle was studied using a combination of immunoprecipitation and an assay for binding to concanavalin A-Sepharose. Results showed that myasthenic IgG distinguishes two different α-bungarotoxin-binding components, each associated with approximately half the total receptor toxin binding capacity. The receptor components can be separated as immune complexes by different properties of binding to concanavalin A and are characterized by different kinetics of toxin binding. The majority of myasthenia gravis sera contain at least two antibodies, directed against determinants on one or both components of the receptor, which do not affect the total binding capacity for α-bungarotoxin. A few sera contain a third antibody type, which prevents the binding of α-bungarotoxin to receptors.
ACKNOWLEDGMENTS We are grateful to G. Genkins, P. Kornfeld, and A. Papatestas for making sera and thymus available for these studies.
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