There are also a variety of non-classical HLA genes, such as HLA-DO and HLA-DM, HLA-E, -F, -G, MICA and MICB, which have received less attention in clinical medical research than the classical HLA genes. The HLA genomic region on chromosome 6p21 encodes more than 200 genes including nine classical HLA genes, HLA-A, -B, -C in the class I region and HLA–DPA1, -DPB1, -DQA1, -DQB1, -DRA, -DRB1 in the class II region, that are the most polymorphic in the human genome contributing to over 7000 alleles and numerous HLA haplotypes implicated in disease resistance or susceptibility. Rejection and autoimmunity is believed to occur because the HLA presents aberrant histocompatibility antigen or public epitopes that play a key role in self-nonself recognition via various mechanisms including molecular mimicry and antibody mediated rejection. Human Leukocyte Antigen (HLA) is the major histocompatibility complex in humans and it is critically involved in the rejection of hematopoietic stem cell or organ transplants and in the pathogenesis of numerous autoimmune diseases. *Address all correspondence to: 10.5772/57556 From the Edited Volume Department of Molecular Life Science, Division of Basic Medical Science and Molecular Medicine, Tokai University School of Medicine, Isehara, Kanagawa, Japan.
Centre for Forensic Science, The University of Western Australia, Nedlands, WA, Australia.