Rituximab prophylaxis prevents H-Y allogeneic Ab development. Blood IgGs against 5 H-Y antigens were determined by ELISA in 25 F → M HCT patients who never received rituximab after TLI-ATG (left panel) and 10 study patients treated with rituximab on days 56, 63, 70, and 77. These heat maps show that no alloreactive H-Y Abs developed in study patients receiving rituximab 2 months after TLI-ATG alloHCT, whereas 1 or more H-Y Abs developed in 56% (14 of 25) of patients receiving TLI-ATG without posttransplantation rituximab. Considering all patients who survived 9 months, rituximab prophylaxis prevents H-Y Ab development (P = .01).