IFN-γ to Treat Acute Myeloid Leukemia (AML) and Myelodysplastic Syndrome (MDS) That Has Relapsed After Allogeneic Hematopoietic Stem Cell Transplantation

Inclusion Criteria: * Recipients of allogeneic stem cell transplantation for acute myeloid leukemia or myelodysplastic syndrome from a human leukocyte antigen (HLA) matched donor * Relapsed of primary disease with 5% to 20% of blasts in the bone marrow by flow cytometry in the bone marrow with an clear leukemia-associated immunophenotype (If the patient received therapy to treat the relapse, he or she must have 5-20% residual blasts prior to enrollment on this study) * Performance status KPS score \>60% (ECOG 0-2) * No increases in systemic immunosuppression in the prior four weeks other than to maintain therapeutic levels * No systemic corticosteroid with a dose higher than 0.5mg/kg/day prednisone or equivalent * No history of grade IV acute GVHD * No new systemic immunosuppressive medications in the prior two weeks initiated due to GVHD * Willingness to have bone marrow and peripheral blood collected as per the study protocol * Must be able to give informed consent * Age 18 or older Exclusion Criteria: * Contraindication to receive IFN-γ including known hypersensitivity to interferon-gamma, E. coli derived products or any component of the product * Subjects with a positive pregnancy test or who are breastfeeding * For men or women of childing bearing potential (age \< 50 without hysterectomy or oophorectomy or documented menopause), unwilling to use effective contraception for the duration of the study. * Primary engraftment failure * Active cardiac arrhythmias not controlled by medical management or current NYHA class II or higher congestive heart failure * Active ischemic heart disease not well controlled with medications * A seizure disorder not well controlled by medications * Estimated GFR \<30 mL/min * AST/SGOT or ALT/SPOT \> 5 x ULN * Total bilirubin \> 3 x ULN * Chemotherapy (other than hypomethylating and/or venetoclax therapy) within the prior 4 weeks * Body surface area at or less than 1.5 m2, or greater than 2.5 m2 so as to minimize the variation in IFN-γ exposure based on differences in BSA. * Patients less than 18 years old. * Pregnant or breastfeeding patients.