Primary IgA Nephropathy is a kidney disease where the immune system builds up proteins in the kidneys, causing swelling and damage. This study tests how well a drug named mezagitamab works compared to a placebo (a fake treatment with no active ingredients) for adults with this condition. The study will also see if mezagitamab is safe and how it affects kidney function over time.

• The study lasts about two years, with regular check-ups every month.
• Participants are randomly assigned to either mezagitamab or placebo groups.
• Participants will have several clinic visits during the study.

Eligibility includes being at least 18 years old and meeting specific kidney function criteria. Participants must not have significant other kidney diseases, recent infections, or have taken certain medications recently. This trial involves signing a consent form and agreeing to use effective birth control if needed.