EBV reactivation induced in vivo expansion of haploidentical B cells. Using the informative HLA-B7 allele, blood and bone marrow mononuclear cells (BMs) were evaluated in a patient who developed fevers, hemophagocytosis, and EBV reactivation 126 days after haploidentical NK-cell infusions and about 226 days following umbilical cord blood transplantation. PB containing 5% CD56⁺/CD3^– NK cells and 68% CD19⁺ B cells were sorted to purity (documented by postsort analysis) and their origin was evaluated. Haploidentical NK cells were minimally present in the 120-day sample, unlike that observed at earlier time points (14 days) after infusion. However, most of the EBV-driven B cells were from the HLA-B7 haploidentical NK-cell donor.