Study of Ravulizumab in Immunoglobulin A Nephropathy (IgAN)

Inclusion Criteria: * Documentation of IgAN diagnosis established on kidney biopsy obtained any time prior to or during the Screening Period for participants with eGFR ≥ 30 mL/min/1.73 m\^2. * For participants in the AdKD cohorts, eGFR 20 to 29 mL/min/1.73 m2 a kidney biopsy is required within 6 months prior to Screening or during the Screening Period. * UPCR ≥ 0.75 g/g or UP ≥1 g/day calculated from the mean of two 24-hour urine during the Screening Period. * Estimated GFR ≥ 30 mL/min/1.73 m2 at Screening. * Stable and maximum allowed or tolerated RASI (ACEI and/or ARB) dose for ≥ 3 months prior to Screening with no planned change during Screening through Week 106. * Participants who are receiving SGLT2I, DEARA, MRA or ERA must be on a stable and maximum allowed or tolerated dose for ≥ 3 months prior to Screening with no planned change in dose through Week 106. Exclusion Criteria: * Diagnosis of rapid progressive glomerulonephritis as measured by eGFR loss ≥ 50% over a period of 3 months prior to Screening. * Secondary IgAN (eg, due to systemic lupus erythematosus (SLE), cirrhosis, or celiac disease; IgAV-N may be eligible). * Concomitant clinically significant renal disease other than IgAN. * Prior use of immunosuppressive treatment within 3 months of screening. * Uncontrolled diabetes mellitus with glycosylated hemoglobin (HbA1c) \> 8.5%. * Henoch-Schonlein purpura (IgAV) requiring systemic immunosuppressive therapy within 12 months of Screening. * History of kidney transplant or planned kidney transplant during the Treatment Period. * Splenectomy or functional asplenia. * History of Neisseria meningitidis infection. * Active systemic bacterial, viral, or fungal infection within 14 days prior to randomization.