A Study to Evaluate the Effect of ARGX-121 on the Change in Proteinuria in Adult Participants With IgAN

The study will assess how ARGX-121 affects immunoglobulin A nephropathy (IgAN) disease outcomes, such as renal function, how it is distributed and eliminated from the body, and how the immune system responds in adults with IgAN. While in the study, participants will continue to receive their usual medical care and medications for IgA nephropathy, unless the study doctor confirms otherwise. The study will include a double blinded period, followed by an active drug period, and conclude with a follow up period. The total study duration for participants will be approximately 72 weeks, with a maximum of 52 weeks on ARGX-121.

Inclusion Criteria: * Males and females (not pregnant) * At least 18 years old * Clinically diagnosed with IgAN with biopsy confirmation Exclusion Criteria: * Diagnosis of another autoimmune disease or known medical condition that would interfere with accurate assessment of clinical systems of IgAN * Has secondary form of IgAN leading to IgA deposits * History of Malignancy * Active infection of concern * Recent Major surgery or organ transplant