Efficacy, Safety, Pharmacokinetics, and Pharmacodynamics of KP104 to Treat Glomerulonephritis

Inclusion Criteria: * Weight of \>35 kilograms (kg) at Screening * Body mass index (BMI) of \<35 kilograms per square meter (kg/m\^2) * UPCR \>1.0 grams per gram (g/g) by 24-hour urine collection at Screening * Documented diagnosis and clinical status of IgAN or C3G as follows: IgAN: * Diagnosis of IgAN verified by biopsy taken within the past 3 years prior to enrolment. * On stable regimen of angiotensin converting enzyme or angiotensin blocking agents for 12 weeks and/or sodium-glucose cotransporter-2 (SGLT2) inhibitors for 6 weeks at Screening C3G: * Diagnosis of C3G verified by biopsy taken within the past 3 years prior to enrolment. * On stable regimen of angiotensin converting enzyme or angiotensin blocking agents for 12 weeks and/or SGLT2 inhibitors for 6 weeks at Screening * Females of childbearing potential and males must practice effective contraception from Screening until 28 days after the end of study (EOS) visit. * Females of childbearing potential must have a negative pregnancy test at Screening and within 1 day prior to dosing of study drug Exclusion Criteria: * Any clinically significant, poorly controlled underlying illness other than IgAN or C3G, as determined by the investigator * Treatment of any infection with IV (within 30 days of Screening) or oral (within 14 days of Screening) antibiotics, antivirals, or antifungals * History of infections with encapsulated organisms * History of untreated tuberculosis * Positive serology for hepatitis C virus (HCV) ribonucleic acid (RNA) or human immunodeficiency virus (HIV) at Screening * History of bone marrow or stem cell transplantation * Absolute neutrophil count (ANC) \<500 cells per microliter (cells/μL) * eGFR \<30 milliliters per minute per 1.73 square meter (mL/min/1.73 m\^2) using the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) formula * Presence of crescent formation in \>50 percent (%) of glomeruli assessed on renal biopsy * Nephrotic syndrome defined as presence of substantial proteinuria (\> 3.5 g/24 hours), hypoalbuminemia (\< 30 grams per liter \[g/L\]), and edema/hyperlipidemia. Nephrotic range proteinuria alone is acceptable. * Rapidly progressive glomerulonephritis, defined as a fall in eGFR of \> 30 mL/min/1.73 m\^2 within 24 weeks prior to the Screening Visit * Receiving renal replacement therapy or anticipated to require renal replacement therapy during the duration of the study NOTE: Other protocol defined Inclusion/Exclusion criteria may apply.