Freshly isolated KIR2DS1sp NK cells from HLA-C2 homozygous donors are hyporesponsive to stimulation with HLA-C2⁺ target cells. Degranulation in response to stimulation with HLA-C2 homozygous DG75 cells in (A) KIR⁻ NK cells, (B) KIR2DS1sp NK cells, (C) KIR2DL3sp NK cells coexpressing KIR2DS1, (D) KIR2DL3sp NK cells, (E) KIR2DL1sp NK cells coexpressing KIR2DS1, and (F) KIR2DL1sp NK cells. In each graph, the responses in HLA-C1 homozygous and HLA-C2 homozygous donors are compared. All subsets were gated on NKG2A⁻ NK cells. Horizontal bars represent the mean expression of CD107a on the cell surface. P values indicate a statistically significant difference between groups, tested by a 2-tailed Mann-Whitney test.