The process of identifying the major histocompatibility (MHC) antigens in humans encoded by the HLA system in both the recipient and the potential donor before organ transplantation. If the donor and recipient tissue types do not match exactly, the transplanted organ will be rejected. HLA typing is imprecise, and individuals who are shown on testing to be HLA identical very rarely have identical MHC genotypes. This follows from the complexity and polymorphism of the alleles determining histocompatibility. Even a graft from an HLA-identical sibling will provoke rejection (albeit more slowly), because of differences between non-MHC proteins that act as minor histocompatibility antigens. Only grafts between identical twins are likely to be free of rejection problems.