Harmonized immune recovery monitoring after HCT: evidence and practical guidance from the Westhafen Intercontinental Group

Allogeneic hematopoietic cell transplantation (allo-HCT) is a curative option for patients with high-risk malignancies and nonmalignant disorders. Long-term survival depends on robust immune reconstitution (IR), which governs overall immune homeostasis and risks of infection, graft-versus-host disease, and relapse. However, despite its centrality to posttransplant outcomes, IR is not consistently monitored across transplant centers, limiting ability to generate meaningful, comparable, and translatable data. This review synthesizes current knowledge on numerical and functional IR milestones after allo-HCT, with a primary focus on flow cytometry-based monitoring of key immune cell subsets. Importantly, early CD4⁺ T-cell recovery (achieving >50 cells per μL by day 100 after transplant), is supported by strong clinical evidence and correlates with improved outcomes. Although emerging data suggest that additional subsets (CD8⁺ T cells, natural killer cells, B cells, naïve and recent thymic emigrant T cells, and γδ T cells) may also influence clinical trajectories, further harmonized, multicenter studies are needed to validate prognostic relevance across transplant settings. We propose practical, evidence-based guidelines for IR monitoring, including recommended time points, preferred assays, and flow cytometry panel components. Additionally, we highlight modifiable factors (eg, immunosuppressive drug exposures, graft manipulation) offering interventional opportunities for influencing IR. Harmonized monitoring strategies will support robust correlation between IR and clinical outcomes, guide real-time risk stratification, and facilitate the development of targeted, individualized transplant approaches. Standardization efforts led by consortia and registries are essential for advancing knowledge and optimizing care. We provide a roadmap for implementing uniform IR monitoring to improve outcomes and quality of life for allo-HCT recipients.