Some donor "self" antigens are more antigenic than others, which means they are better at triggering an unwanted immune reaction in the transplant patient. By studying successful cases of blood stem cell transplantations, doctors have identified the three most important antigens to match when choosing "non-self" donors for a transplant. Matching these helps to minimize the chances of graft vs. host or host vs. graft attacks. The antigens are HLA-A, HLA-B, and HLA-DR.