Pretransplantation and posttransplantation immunofluorescence analyses in 2 representative transplant pairs of NK cells of donor origin expressing the KIR for which there is no class I ligand in the recipient as their only inhibitory receptor for self. Panels A (donor) and B (recipient 1–3 months after transplantation) illustrate a transplant pair in whom the donor KIR ligand HLA-C 1 was missing in the recipient. KIR genotyping showed the donor possessed only group A haplotype genes, specifically KIR2DL1, KIR2DL3, and KIR3DL1 inhibitory KIR genes. Consequently the potentially alloreactive population is represented by cells expressing only KIR2DL3 (upper left quadrants in panels A and B where the percentage is indicated). Cells expressing or coexpressing all other KIRs and/or NKG2A are shown in the right quadrants. Panels C (donor) and D (recipient 4-7 months after transplantation) illustrate a transplant pair in whom the donor KIR ligand HLA-C 2 was missing in the recipient. KIR genotyping showed the donor possessed group B haplotype genes, specifically KIR2DL1, KIR2DL2, KIR2DL3, and KIR3DL1 inhibitory genes and KIR2DS1, KIR2DS2, KIR2DS3, and KIR3DS1 activating genes. Consequently, the potentially alloreactive population is represented by cells expressing only the KIR2DL1 inhibitory receptor (lower right quadrants in panels C and D where the percentage of cells expressing KIR2DL1 with or without KIR2DS1 is indicated). Cells expressing or coexpressing all other KIRs or NKG2A or both are shown in the upper quadrants.