Book excerpt: CONTEXT: Allogeneic hematopoietic stem cell transplantation (HSCT) has become a well-defined therapeutic strategy for a wide variety of malignant and non-malignant diseases with curative potential. Immune reconstitution post-HSCT is a complex and dynamic process, achieved in stages, depending on the type of transplant, the characteristics of the donor and the recipient, and the occurrence and stage of the graft versus host disease (GvHD).OBJECTIVE: The objective of our study is to identify the correlations between regenerating cell lineage and the time required for complete reconstitution of the immune status depending on the type of allogeneic transplantation.DESIGN: We retrospectively evaluated 16 patients, who have been undergoing allogeneic transplantation between January 2015 and March 2018, and who survived at least 100 days after transplantation in the Pediatric Department, Fundeni Clinical Institute, Bucharest. We analyzed the immune status (B, T and NK cells) by immunophenotyping, using the following markers: Lambda, Kappa, CD56, CD5, CD19, TCRgd/CD16, CD3, CD38, CD20, CD8, CD4, CD45 at well-defined checkpoint intervals (3, 6, 9, 12, and 24 months after the transplant). Absolute values of each subgroup were calculated using the percentage of each subset and absolute count from the leukocytes. RESULTS: In this study, 16 patients have been undergoing allogeneic HSCT transplantation for malignant and non-malignant diseases. There were achieved 5 haploidentical HSCT, 8 HSTC from a matched unrelated donor (MUD), and 4 matched sibling donor (MSD). 5 patients experienced immune recovery during the first 6 months, the NK cells subset has reconstituted in the first 3 months, and the last one to recover was the T cells subset (CD4+). In our study group, the immune reconstitution rate was 20% for haploidentical transplantation, 37.5% for MUD and 25% for MSD. Moreover, delayed immune reconstitution has been associated with the occurrence/reactivation of viral infections (CMV, EBV, BKV, and JCV) with increased frequency in the post-grafting period.CONCLUSIONS: In our center, patients undergoing transplantation from MSD have the fastest immune reconstitution. The immune reconstitution of patients after allogeneic HSCT influences long-term survival by contributing to the management of infectious complications and of GvHD.