**Study of Mezagitamab for IgA Nephropathy** IgA Nephropathy (IgAN) is a kidney disease caused by protein buildup that leads to swelling and damage over time. The study aims to see if the medicine, mezagitamab, can reduce protein in urine better than a non-active substance called a placebo. It also checks if mezagitamab is safe and helps maintain kidney function. Participants will be put into two groups: one main and one open-label. In the main group, participants have a 2 in 3 chance of getting mezagitamab. The study lasts for up to 2 years, with 22 weeks of treatment followed by regular check-ups for the rest. The open-label group involves participants who meet certain conditions and have previously used mezagitamab. **Key Points:** - **Study Duration:** Up to 2 years with multiple clinic visits. - **Treatment:** Possible mezagitamab or placebo for half a year. - **Eligibility:** Must meet specific health criteria, like certain protein levels in urine and kidney function. Consider participating if you meet the criteria and seek potential benefits from mezagitamab, but be ready for regular clinic visits.