Most of the histocompatibility antigens, also known as HLA or Human Leucocyte Antigens, are located on a portion of our chromosome number 6. This is called MHC (Major Histocompatibility Complex) or HLA complex (Human Leucocyte Antigen complex). The recipient's immune system can recognize the histocompatibility antigens on the donor organ, and accordingly accepts or rejects it. Hence, HLA matching and blood group matching are essential before undergoing any transplant.
The array of HLA alleles on a homologue of our chromosome number 6 is known as a haplotype. An individual inherits one haplotype from each parent. As identical twins (monozygotic twins) are the outcome of a single egg being fertilized by a single sperm where the fertilized egg is separated into two, their genomes are almost identical, and hence they can have identical haplotype. The best HLA matching can be observed within the same family.
Thus, the preference order of the best HLA matching is as follows:
Identical twins > Siblings > Parents > Unrelated donor