Stem Cell Transplant for Immunologic or Histiocytic Disorders

Inclusion Criteria: Patients with immunodeficiencies or histiocytic disorders 0-35 years of age with an acceptable stem cell donor and disease characteristic defined by the following: * Patients with histocytic disorders (hemophagocytic lymphohistiocytosis of any etiology and refractory Langerhans cell histiocytosis) who do not meet eligibility criteria for a myeloablative transplant procedure * Patients with immunodeficiency disorders in whom residual immune function may not require a fully myeloablative preparative regimen or patient is ineligible for standard myeloablative preparative regimen (any form of severe combined immunodeficiency \[SCID\], or other immunodeficiency with T cell defect) * Patients with immunodeficiency disorders that have had poor outcome with myeloablative stem cell transplants (including, but not limited to, common variable immunodeficiency \[CVID\], Wiskott Aldrich Syndrome \[WAS\] if \> 5 years of age, ataxia telangiectasia) * Patients with immunodeficiencies or histocytic disorders that require a second stem cell transplant (SCT) for any reason Exclusion Criteria: * Karnofsky or Lansky performance score \<70 * Glomerular filtration rate (GFR)\<30% predicted * Cardiac function \<50% normal by echocardiogram * Serum creatinine \> 2x normal for age/weight * Pregnant or lactating females * Active serious infection that has not had an adequate course of therapy pre-SCT. Any patient with acquired immune deficiency syndrome (AIDS) or AIDS-related c…