The CD52 antigen, which is targeted by alemtuzumab, is highly expressed on mature T lymphocytes, monocytes and monocyte-derived dendritic cells as well as on mature B cells. Due to its more promiscuous effect on immune cells, alemtuzumab not only targets antibody producing B lymphocytes as does rituximab, but also targets alloreactive T lymphocytes and dendritic cells that also contribute to the complex pathogenesis of chronic GVHD.