When selecting an HLA mismatched stem cell donor consider donor immune status.

Until now, hematopoietic stem cell (HSC) donor selection has been almost exclusively based on selecting a (near) HLA identical donor but many patients fail to find a suitable donor. However all donors have been exposed during fetal life and by breastfeeding to microchimeric maternal cells and molecules, which may induce long lasting T regulator cells. Likewise fetal cells invade the maternal circulation and one-third of the mothers are immunized to the paternal antigens of the child, while part of the mothers become tolerant to them. By taking the alloimmune status of the HSC donor into account, HLA mismatched grafts are an attractive alternative for those patients who do not have an HLA identical sibling donor.