A procedure involving a test or a series of tests to determine the compatibility of tissues from a prospective donor and a recipient prior to transplantation. The immunological response of a recipient to a transplant from a donor is directed against many cell-surface histocompatibility antigens controlled by genes at many different loci. However, one of these loci, the major histocompatibility complex (MHC), has the greatest genetic complexity and controls antigens that evoke the strongest immunological response. The MHC is a cluster of closely linked gene loci and is conserved in all vertebrate species, including humans. The human MHC is known as the HLA system, which stands for the first (A) Human Leukocyte blood group system discovered. The HLA complex was first described as a gene locus on chromosome 6 that controls the allograft rejection response, which is the rejection of a graft from a donor by a genetically dissimilar recipient of the same species. It has since been found, however, that these genes are also physiologically important in the regulation of the immune response to highly foreign antigens such as bacterial or viral antigens, as well as to self-antigens.