Thus, an A.B mouse represents a congenic strain carrying background genes of the A strain a homozygous pair of histocompatibility genes derived from the B strain. With the development of inbred mice, congenic strains, and mutated strains of mice, genetic differences could be detected by immunological analysis of allo-antigens. With the development of inbred, congenic, and mutated strains of mice, genetic differences could be detected by immunological analysis of the allo-antigen or gene products. These were seen as major or minor histocompatibility antigens. Major hsitocompatibility antigens stimulate acute, rapid, intense forms of graft rejection.
Minor histocompatibility antigens stimulate chronic, slow, less intense reactions.
The genes encoding these proteins were found to occur in clusters. The clusters of genes became known as major and minor histocompatibility complexes, responsible for transplantation antigens. Major histocompatibility genes are located on a single chromosome, whereas minor histocompatibility genes are scattered throughout the genome. In mice 15 of the 20 chromosome pairs contain minor histocompatibility genes. Major histocompatibility antigens are located on chromosome 17 in mice and 6 for humans.