A Reduced-Intensity Conditioning Regimen (Cyclophosphamide, Pentostatin, Anti-thymocyte Globulin) Followed by Haploidentical Hematopoietic Stem Cell Transplant for the Treatment of Patients With Refractory or Recurrent Severe Aplastic Anemia

Inclusion Criteria: * RECIPIENT: Documented informed consent of the participant. * RECIPIENT: Age: \>= 40 years but =\< 75 years of age at time of enrollment. * RECIPIENT: Failed at least one trial of immunosuppressive therapy (IST) by being refractory (persistence of severe cytopenia and fulfillment of SAA disease criteria at least 3 months after initial IST) or having relapsed (initial improvement of cytopenia after first-line IST but then a later return to fulfillment of SAA disease criteria when IST is decreased or ceased). IST could have included anti-thymocyte globulin (ATG) based regimens, calcineuPririn inhibitors and/or other higher dose therapy directed at the treatment of primary SAA. * RECIPIENT: Karnofsky performance score \>= 60%. * RECIPIENT: Confirmed diagnosis of severe aplastic anemia as: * Bone marrow cellularity \< 25% or marrow cellularity \< 50% but with \< 30% residual hematopoietic cells; * Two out of three of the following (in peripheral blood): * Neutrophils \< 0.5 x 10\^9/L; * Platelets \< 20 x 10\^9/L; * Reticulocyte count \< 20 x 10\^9/L (\< 60 x 10\^9/L using an automated analysis). * RECIPIENT: No suitable fully matched related sibling donor (6/6 match for human leukocyte antigen \[HLA\]-A and B at intermediate or high resolution and DRbetaA1 at high resolution using deoxyribonucleic acid \[DNA\]-based typing) available. * RECIPIENT: Available relative of the patient who is a haploidentical match, including biological parents,…