Study to Evaluate the Effect of XmAb®5871 on Disease Activity in Patients With IgG4-Related Disease (RD)

Inclusion Criteria: * Active IgG4-RD * Compatible pattern of organ involvement consistent with IgG4-RD that cannot be attributed to other causes * Histopathologically-proven diagnosis of IgG4-RD * Peripheral blood plasmablast count \>900 cells/mL and/or elevated IgG4-RD levels during screening * Able and willing to complete the entire study according to the study schedule * Able and willing to provide written informed consent Exclusion Criteria: * History or evidence of a clinically unstable/uncontrolled disorder, condition or disease other than IgG4-RD that, in the opinion of the Investigator would pose a risk to the patient safety or interfere with the study evaluation, procedures or completion * Malignancy within 5 years (except successfully treated in situ cervical cancer, resected squamous cell or basal cell carcinoma of the skin, or prostate cancer with no recurrence ≥3 years following prostatectomy) * Presence of recurrent or chronic infections, defined as ≥3 infections requiring antimicrobials over the past 6 months prior to screening * Active infection requiring hospitalization or treatment with parenteral antimicrobials within the 60 days prior to randomization or oral antimicrobials within the 21 days prior to enrollment * Patient is taking \>40 mg of prednisone QD * Prior use of rituximab (or other B cell depleting agents) within 6 months of enrollment. Prior use of any B cell depleting agent greater than 6 months from enrollment is allowed if the CD19+ B cell count is within the normal reference range during screening * Use of any investigational agent within 5 half-lives of the agent (or 6 months if the half-life is unknown) prior to enrollment * Immunosuppressive agent use within the three months prior to enrollment * Has received live vaccines within 2 months of enrollment * Patient is pregnant or breast feeding, or planning to become pregnant while enrolled in the study, up to end-of-study visit * Unable or unwilling to partake in the follow-up assessments or required protocol procedures