Approximately 60 subjects will be enrolled in order to have approximately 20 adult subjects and 20 pediatric subjects treated with subcutaneously administered IGSC 20% who complete the entire study. This study will include 3 study stages: Screening/Previous Regimen Phase, IGSC 20% Treatment Stage 1 (13 IGSC 20% weekly doses), and IGSC 20% Treatment Stage 2 (39 IGSC 20% weekly doses). A total of 52 doses of IGSC 20% will be administered with a final follow-up visit 1 week after the last dose at Week 53. Subjects/caregivers will be trained on self-administration of IGSC 20% by the clinical site personnel.