Peritransplantation CXCL9 levels stratified based on day 0 alemtuzumab levels. Peritransplantation CXCL9 median levels for all patients. Stratification using first quartile day 0 alemtuzumab levels (level ≤0.32 μg/mL). Light gray shading represents range (25th-75th percentile) of CXCL9 values in healthy pediatric controls. Dark gray shading represents range (25th-75th percentile) of CXCL9 values in pediatric patients with untreated HLH. D0, day 0 after HCT.