Tolerance: Pregnancy matters

In this issue of Blood, van Halteren and colleagues demonstrate that not only antigen-specific CD8 ⁺ CTLs, but also antigen-specific CD8 ⁺ T _regs can emerge during pregnancy and persist over time when mother and offspring differ for minor histocompatibility antigens.1 The relative ratio between these 2 populations, either promoting aggression against allogeneic tissues or tumor cells or tolerance toward alloantigens, is of potential great relevance in the context of HSCT.