Given the role of B cells in the pathophysiology of chronic graft versus host disease (GvHD), the association between elevated BAFF levels post-transplant in abnormal B-cell homeostasis and chronic GvHD, and the efficacy of belimumab in the inhibition of soluble human B lymphocyte stimulator protein (BAFF) signaling, these proof-of-principle findings support the rational for use of belimumab as prophylaxis of chronic GvHD. The investigators propose a pilot and feasibility study to assess the safety and tolerability, as well as preliminary efficacy, of belimumab as prophylaxis of chronic GvHD following allogeneic hematopoietic cell transplantation (alloHCT). The investigators' central hypothesis is that belimumab will be well tolerated and have a favorable effect on incidence and severity of chronic GvHD.