Children inherit one MHC haplotype from each parent. In haploidentical bone marrow transplantation, T cell–depleted donor cells from one parent are transferred to a recipient child. In this example, the recipient child has leukemia, the mother is the donor, and her NK cells express a set of KIRs (blue) that bind and are inhibited by MHC molecules on the child’s leukemic cells. Other donor uNK cells expressing KIRs (yellow) that do not bind the child’s MHC molecules are not inhibited and will destroy the cancer cells. A third group of donor NK cells expresses activating KIRs (green), which, upon binding to the child’s MHC molecules, are activated to destroy leukemic cells. In the latter two situations, donor NK cells also destroy the patient’s normal hematopoietic cells, including the APCs, and this contributes to preventing graft-versus-host disease caused by residual donor maternal T cells. These three types of interactions are equivalent to the situation between KIRs expressed by maternal uNK cells (donor) and MHC molecules on fetal trophoblast cells (recipient).