This study aims to learn more about the safety and effectiveness of an experimental drug called felzartamab in people who have received a kidney transplant and may have late antibody-mediated rejection (AMR). AMR is a type of transplant rejection in which the patient’s immune system produces antibodies that attack the transplanted organ. Study procedures include electrocardiograms, kidney biopsies, felzartamab and/or placebo (inactive substance) infusions, and laboratory assessments. Felzartamab is a monoclonal antibody, which is a type of laboratory-made substance designed to target cells that produce harmful donor-specific antibodies. Donor-specific antibodies are immune proteins that the body produces in response to a transplanted kidney, potentially leading to organ rejection. By targeting and eliminating these plasma cells, felzartamab helps reduce the risk of rejection and supports the success of the transplant. Researchers want to learn how well felzartamab works, as well as assess the drug’s pharmacokinetics (how the drug is processed through the body) and immunogenicity (how the drug works on the immune system).