A Study to Learn About the Effects of Felzartamab on the Kidneys of Adults With Immunoglobulin A Nephropathy (IgAN)

In this study, researchers will learn more about the use of felzartamab in participants with immunoglobulin A nephropathy (IgAN). IgAN is a kidney disease caused by the buildup of an antibody called IgA in the kidneys over time. In people with IgAN, abnormal IgA and other antibodies form clusters that build up in the small filters of the kidneys, which leads to inflammation and damage. Felzartamab is designed to target certain immune cells that produce these abnormal antibodies. The main goal of the study is to learn about the effect felzartamab has on the buildup of IgA clusters in the kidneys. The main question that researchers want to answer is: • How much does the buildup of IgA in the kidney change from the start of the study to Week 36? Researchers will also learn about the effect felzartamab has on kidney health by measuring protein levels in the urine and the kidneys\' ability to filter blood. They will also learn more about the safety of felzartamab and how the body processes the drug. The study will be done as follows: * Participants will be screened to check if they can join the study. * This is an open-label study, which means both the researchers and the participants will know that all participants are receiving felzartamab. * Participants will receive felzartamab as intravenous (IV) infusions. * Participants will have kidney biopsies done over the course of the study to help measure changes in the health of the kidneys. * Participants will have multiple scheduled study visits over about 104 weeks. An extra study visit may be needed if a participant decides to leave the study early. * In total, participants will stay in the study for about 2 years (up to 112 weeks).