The general objective is the study, through scientific approaches, of the main components of immunological risks linked to red blood cell transfusions, as well as their consequences, in order to define precise rules for prevention, taking into account that 2,700,000 units were transfused in 1992. To reach this general aim, five intermediary objectives have to be achieved: 1) The sentinel study of the methodology used to collect information about transfusion accidents, as well as their identification and their early clinical expression; 2) The analysis of the occurrence mechanisms of incompatibilities and transfusion accidents; 3) The up-date definition of post-transfusion alloimmunization, in particular regarding 3 parameters: a) the immunogenicity of the different erythrocyte antigens, that will have to be reassessed; b) the modes of occurrence of post-transfusion anti-erythrocyte alloimmunization; c) the different types of chronology in the appearance and the persistence of anti-erythrocyte antibodies. 4) The search for significant criteria in order to assess the immunopathological correlations of the consequences of antigen-antibody conflicts; 5) The elaboration of the principles for tests evaluation and identification of the techniques linked to blood groups and the study of anti-erythrocyte antibodies.