Allogeneic bone marrow transplantation (BMT) involves transfer of primitive multipotential lymphohematopoietic stem cells from donor to recipient. In clinical transplantation, however, the aspirated marrow also contains substantial numbers of viable lymphoid cells, some originating from the marrow and some inadvertently collected by contamination of the marrow inoculum with peripheral blood. These lymphoid cells are immunocompetent and may respond to host factors with a series of activation responses including blastogenesis, cell proliferation, elaboration of proinflammatory cytokines and cytotoxic injury to host tissues. While the inciting host antigens are not fully defined, they may include major histocompatibility antigens encoded by the major histocompatibility complex (MHC), or minor (non-MHC) histocompatibility antigens. Alternatively, normal host antigens altered by interaction with mucosal pathogens or by injury from chemoradiotherapy conditioning may appear as neo-antigens capable of inciting an alloreactive immune response from the host lymphoid cells. In addition, the effector cells mediating this donor-host lymphoid interaction