What makes it more likely to happen
As mentioned above, GVHD is an unfortunate result of even modern stem cell transplantation and its occurrence is related to several factors, the most important of which are:
1) Mismatching
The more unlike the stem cells are to the body in which they are transplanted, the more likely for GVHD to occur. This includes different sex between the donor and the recipient and different HLA typing. That is why we consider twins to be the perfect donor-recipient, siblings follow and are followed by first degree relatives. For unrelated donors, we run HLA typing to ensure the greatest degree of similarity between the donor and the recipient.