This study aims to assess the safety, tolerability, efficacy and pharmacokinetics of Ig NextGen 16% in people with antibody deficiency currently being treated with IntragamP. Ig NextGen 16% is a liquid immunoglobulin (antibody) preparation manufactured using predominately chromatographic techniques. Eligible patients will switch from monthly intravenous IntragamP therapy to weekly subcutaneous Ig NextGen 16% treatment. Initial hospital training will be required for subcutaneous administration and then the patient will perform the infusion in their own home, returning once a month for a supervised infusion. Patients will be monitored on the study for up to 10 months to assess blood IgG levels and rate of serious bacterial infections.