CD107a expression by KIR3DL1⁺ polyclonal NK cells is inhibited by most Bw4⁺ targets. Polyclonally expanded NK cells from 4 donors (donors 1-4) were incubated alone (NA), with a positive control, the class I–negative BLCL 721.221 (721), and a range of Bw4⁺ and Bw6⁺ targets (Table 2) and CD107a expression were measured on KIR3DL1⁺, CD158b⁻ NK cells. Donors 3 and 4 had NK cells that showed high or low expression of DX9, and for each, the percentage of CD107a⁺ cells among either the NK cells bearing the high level (■) or low level (□) of DX9 expression are shown. All donors lysed the 721.221 cell line and all Bw6-homozygous (Bw4⁻) targets. All donors were inhibited by most of the Bw4⁺ targets except for targets B13, B51, A23, and A25. For donor 3, the low-level expression allele was less cytotoxic against Bw6-expressing targets and not as well inhibited by most Bw4-expressing targets compared with the high-expression allele. By contrast for donor 4, the high- and low-level expression alleles behaved similarly. Error bars represent SEM.