The number of required patients will be established by comparison of the percentage of gut GVHD in the patients with or without complement activation. Based on our preliminary results, we hypothesize that 2/3 patients will not have complement activation among whose 20% will develop acute gut GVHD. We expect an increase of acute gut GVHD up to 60% of the patients with complement activation that would represent 1/3 of the cohort.