Minor histocompatibility antigens consist of HLA-bound peptides derived from cellular proteins encoded by polymorphic genes that differ between transplant donor and recipient. More than 20 of human minor histocompatibility antigens have been molecularly characterized. In HLA-matched hematopoietic stem cell transplantation, donor T cell recognition of recipient minor histocompatibility antigens is responsible for graft-versus-host disease and the graft-versus-tumor effect.

Idiopathic thrombocytopnenic purpura (ITP) is a disease caused by circulating autoantibodies that react with platelet membrane． Many authors have recently found a positive correlation between Helicobacter pylori infection and ITP. In order to clarify the pathogenic mechanism of Helicobacter pylori -associated ITP, we have investigated 72 ITP patients for Helicobacter pylori infection and HLA class II alleles. Fourty-six ITP patients (63.9%) were infected by Helicobacter pylori and bacterium eradication was accompanied by a long-term platelet response in 26 (56.5%) of them. The ITP patients with Helicobacter pylori infection showed DRB1*0405, DRB1*0410, DQB1*0401, DQB1*0402 frequencies significantly higher and DRB1*0901, DQB1*0303 frequencies significantly lower than in Helicobacter pylori-negative patients. Moreover, an DQB1*0301 was associated with a higher probability of platelet response to eradication treatment. These results suggest that the response to eradication in some ITP patients are likely to associate with HLA class II allele.