- Donors: If a patient is homozygous at a particular loci, mismatching at that loci is not allowed due to an isolated graft rejection vector, i.e., patient A*0101 and the donor is A*0101, A*0201; such a mismatch may increase the risk of graft rejection; if patient and donor pairs are both homozygous at a mismatched loci, they are considered a two-HLA antigen mismatch, i.e., the patient is A*0101 and the donor is A*0201, and this type of mismatch is not allowed