KIR ligands play a role in determining frequency and functionality of reconstituting KIR-positive cells. CD107a mobilization in response to the 721.221 BLCL by KIR2DL1 (A) and KIR2DL3 (B) single-positive NK cells from donors (left panels) and patients 75 to 150 days after haploidentical HSCT (right panels), subgrouped according to KIR ligand status. (C) Frequency (highlighted gates in dot plots) and CD107a mobilization in response to the 721.221 BLCL (histograms) of single-KIR2DL3⁺ NK cells in the donor (left) and in the patient 4 months after haploidentical HSCT (right) in the single donor-recipient pair in which both were negative for KIR2DL3 HLA-C1 ligand. (D) Mean percentages, with SE, of total KIR expression over time in CD56^dim NK cells after haploidentical HSCT in patients carrying (filled symbols) or lacking (open symbols) the HLA-ligand for the group of KIRs under analysis. Both donors and patients with a null KIR3DL1 phenotype were excluded from the analysis of KIR3DL1 expression recovery.