For more than two decades, the intravenous administration of high doses of IgG pooled from the plasma of healthy donors (immune globulin therapy, also known as “IVIg”) has benefited patients with a variety of autoimmune disorders. Although its mechanism of action is not known, immune globulin is accepted as an effective and convenient alternative to plasmapheresis for treating diseases that are thought to be mediated by pathogenic autoantibodies or immune complexes.¹ It is used increasingly to treat neurologic diseases such as inflammatory demyelinating neuropathies, multifocal motor neuropathy, inflammatory myopathies, myasthenia gravis, and the Lambert–Eaton syndrome. The efficacy of immune globulin . . .