Thrombotic thrombocytopenic purpura (TTP) is an ultra-rare and fatal thrombotic disease characterized by systemic ischemic organ damage due to peripheral capillary occlusion by microthrombi. An acquired form of TTP, immune-mediated TTP (iTTP), is caused by the production of auto-antibodies against von Willebrand cleaving protease, also known as ADAMTS13. At the beginning of the 2010s, three independent groups in Europe reported that DRB1*11 was one of the strongest susceptible alleles to develop iTTP among European population. Several in silico predictions for the allele-restricted ADAMTS13 epitopes against T-cell are performed, followed by in vitro experiments to validate their functional implications, including mass spectrometry analysis of eluted peptides and T-cell assay. However, these analyses had not been performed in Japanese population so far. Here, we performed HLA typing for 52 patients with iTTP from 19 institutes across Japan, using the next-generation sequencing method. Our analysis revealed that DRB1*08:03 was associated with iTTP among the Japanese population while there were no statistical differences of the allele frequency of DRB1*11 between iTTP and healthy control. Subsequently, we predicted the strong binders for DRB1*08:03 molecules using NetMHCIIpan and found ADAMTS13 peptides in several domains had the possibility to bind to the molecule. To confirm this prediction, we performed in vitro MHC-density assay that evaluated the binding strength of the candidate peptides to DR molecules. This article reviews the current status of genetic and immunological studies on iTTP and our findings for iTTP in Japanese population.