Although blood transfusion is an extremely important therapeutic procedure that usually proceeds without complications, there are some risks associated with donated blood. Investigations into the causes of transfusion reactions and their prevention are important issues for transfusion therapy. In addition to nucleic acid amplification testing (NAT) for infectious diseases and the irradiation of blood to prevent post-transfusion GVHD, prestorage leukocyte reduction and diversion of the first part of the donation of blood were recently introduced into transfusion therapy. This symposium, entitled "Immunoreaction and blood transfusion", reviewed the immune responses associated with blood transfusion, which is probably the most frequent medical procedure performed in allogeneic organ transplantation, with four themes provided by the four featured invited speakers: transfusion-related acute lung injury (TRALI) and transfusion-associated circulatory overload (TACO), high-dose intravenous immunoglobulin therapy for chronic inflammatory demyelinating polyradiculoneuropathy, transfusion-transmitted infectious disease surveillance, and transfusion-related immunomodulation.