SOCIETAL IMPACT Each year 50,000 stem cell transplants are performed world-wide, providing a crucial, often last-resort treatment for patients with conditions ranging from leukaemia to various congenital diseases. Many of these patients are children, for whom this treatment has a profound impact on their development, both socially and academically. These children endure long absences from school, intensive medical interventions and extended periods of isolation due to immunosuppression. They face regular blood tests and follow-up visits, spending much of their recovery period in hospital settings. The immune system is a complex mechanism that is still not fully comprehended, with intricacies that make it difficult to fully grasp. This is why research on this matter remains so necessary and illuminating. In this dissertation we try to find out what the major influencing factors on the immune recovery are to improve treatment with stem cell transplantation and contribute to reduced child morbidity and mortality. POPULAR SUMMARY The recovery of the immune system after stem cell transplantation depends on many aspects, which have a role in determining the outcome. Haematopoietic stem cell transplantation significantly weakens the immune system, leaving children susceptible to infections and complications that can persist for years. Understanding the elements that impact immune recovery is vital to improve patient outcomes and mitigate risks during this critical period. In this research, we explore various factors affecting immune recovery. We began by reviewing existing studies to gain insights into influences such as donor type, graft type, and viral reactivations, among others. Next, we conducted a comparative analysis between groups where each factor was either present or absent, evaluating the effects on immune recovery, relapse, and other outcomes. We then compared our findings with previous research to identify patterns, discrepancies, and potential areas for further inquiry. Finally, we examined possible limitations in our approach and proposed recommendations for future research. Through this work, we hope to advance understanding of immune recovery after stem cell transplantation, with the ultimate goal of improving treatment strategies and outcomes for young patients in need.