Additional Study Information: This study is designed to test how well an intravenous human immunoglobulin concentrate, IGIV-C (compared with placebo) improves the ability to taper participant's dose of corticosteroids without causing significant increase in your muscle weakness and worsening MG symptoms. A placebo is a medication that looks like the study medication but has no active ingredient. Immunoglobulin is an antibody produced by white blood cells that is used by the immune system (body protection system) to identify and neutralize foreign objects (such as viruses, bacteria, antibodies, etc). In MG treatment, human immunoglobulins are used to potentially affect the function or production of the abnormal antibodies occurring in MG patients. This study will be used to analyze the effectiveness of IGIV-C (versus placebo) in allowing corticosteroid doses to be tapered to the lowest dose possible without significant worsening of MG symptoms or weakness.