Combined immunodeficiencies (CIDs) are an heterogeneous group of primitive immunodeficiency (PID), which affect T cells development, function or both. These inherited conditions can only be cured by allogeneic hematopoietic stem cell transplantation (HSCT). These procedures have a high risk of morbidity and mortality such a graft versus host disease (GVHD), rejection of the graft and serious infections, especially in this population of children with PID. GVHD is more frequent and severe if the donor is not an identical sibling and/or presents an HLA-mismatch. GVHD requires high immunosuppression as prevention and treatment, and therefore impedes immunity against infections.