A Study of Mezagitamab in Adults With Primary Immunoglobulin A Nephropathy Receiving Stable Background Therapy

This study will have two parts. The main aims are to: * check the side effects from mezagitamab. * check for long-term side effects from mezagitamab. Before starting the study, participants will be asked to provide a 24-hour urine sample. A few weeks later, if enrolled they will begin receiving a subcutaneous injection (under the skin) of mezagitamab once a week for 8 weeks then once every 2 weeks for 16 weeks. When treatment has ended, there will be a 24-week follow-up period. Participants who receive benefit from the treatment may continue in the second part of the study where they will be monitored for up to 96 weeks and possibly retreated for another 24 weeks.