Allogeneic haematopoietic stem cell transplantation (HSCT) often remains the only curative treatment for haematological malignancies. The anti-leukaemic effect of allogeneic HSCT, called the GvL (Graf-versus-Leukemia) effect, is often associated to the development of an immune response against healthy recipient cells leading to a graft-versus-host disease (GvHD) in 20 to 70% of allogeneic HSCT. Acute GvHD, that usually targets the skin, the gastrointestinal (GI) tract and the liver, is an important cause of morbidity and mortality after allogeneic HSCT, particularly in the case of GI GvHD. The main goal of the research in the field of allogeneic HSCT is to determine strategies that could decrease the risk of GvHD without affecting the GvL effect. According to GVHD experimental models, it is likely that GvL but not GvHD may occur in the absence of inflammatory signals induced by the transplant-associated conditioning. Based on this hypothesis, we have chosen to analyse the role of Complement system in patients who received allogeneic HSCT. Indeed, Complement system is a major actor of inflammation and in the generation of tissue destruction, both of which are involved in the physiopathology of GVHD. Furthermore, it might be a potential target of some available inhibitory drugs (purified C1-Inhibitor, anti-C5 antibodies) in a preventive or curative manner in such patients. Preliminary data obtained from 34 allografted patients in our institution suggest that Complement activation by the classical pathway is correlated to the occurrence of GI GVHD. The goal of our current project, in order to confirm these preliminary results in a larger series, is to explore Complement system activation in patients who received allogeneic HSCT in three Adult Hematology departments in Paris fot two years and to correlate the biological results to the clinical events occurring after HSCT.