There is actually a higher risk of developing Graft vs Host disease (deadly if not mitigated appropriately) from a related donor than an unrelated donor. There are hundreds of other antigens on RBCs than just ABORh. 50% of Baby Will's antigens come from each parent, so there is a high likelihood he would have mismatches from either parent. The risk of GVH can be mitigated by treating the blood with radiation to render the remaining donor lymphocytes (in the transfused blood) unable to replicate inside the new "foreign" home ie: baby Will. And ALL blood donations are tested for the same transfusion transmitted diseases, whether the blood is from a directed (known) donor or from the voluntarily donated blood from the community. Just a little blood bank FYI from a BB nerd, but I appreciate your thoughtful questions! :)