A Study to Find Out How Safe and Effective Gammaplex® is in Young People With Primary Immunodeficiency

Inclusion Criteria: * The subject is between the ages of or is equal to 2 and 16 years of age, of either sex, belonging to any ethnic group, and above a minimum weight of 10 kg. This weight is based on the amount of blood required for testing. * The subject has a primary immunodeficiency disease, which has as a significant component of hypogammaglobulinemia and/or antibody deficiency (e.g. common variable immunodeficiency, X-linked and autosomal forms of agammaglobulinemia, hyper-IgM syndrome, Wiskott-Aldrich Syndrome). NB Isolated deficiency of a single IgG subclass, or of specific antibodies without hypogammaglobulinemia per se, does not qualify for inclusion. * Subjects already receiving IGIV replacement therapy require the following before their first infusion of Gammaplex: * Documented IGIV dose(s) and treatment intervals for the last 2 consecutive routine IGIV treatments (one of which can be the screening visit result). The previous doses should also meet the following conditions before study entry: Have not changed by ± 50% of the mean dose for at least 3 months; be between 300 and 800 mg/kg/infusion; be given every 21-28 days, inclusive; be a licensed or investigational product (Phase III or IIIb). * Documented previous IgG trough levels for the last 2 consecutive routine IGIV treatments for the last 2 consecutive routine IGIV treatments: Maintained at least 300 mg/dL above baseline serum IgG levels (defined as before initiation of any gamma globulin treatment for th…