NK cell perforin expression in the 3 groups. (A) Percentage of NK cells expressing perforin (left) and perforin MFI (right). Each point represents 1 patient; bars indicate median values. (B) Evolution of the percentage of NK cells expressing perforin (left) and perforin MFI (right) during (HLH⁺) and after HLH recovery in 15 and 14 patients, respectively. Each line represents 1 patient. *P < .05 by Mann-Whitney U test (2 groups) and Kruskal-Wallis test (3 groups). ns, not significant.