Immune responses are often regulated by opposing receptor pairs that recognize the same ligand but deliver either activating or inhibitory signals. Paired immunoglobulin-like receptors (PIRs) expressed on B cells and myeloid cells comprise a major histocompatibility complex class I recognition system that regulates the responsiveness of these cells. Here, activating PIR-A and inhibitory PIR-B bound various mouse major histocompatibility complex class I (H-2) molecules, and in vitro H-2 tetramer stimulation of PIR-B on B cells or PIR-A on macrophages induced intracellular phosphotyrosine signaling. After transfer of allogeneic splenocytes into PIR-B-deficient mice, the mice showed exacerbated graft-versus-host disease, which was due to augmented activation of recipient dendritic cells with concomitant upregulation of PIR-A and increased interferon-gamma production. PIR-A-induced dendritic cell activation also led to increased proliferation of donor cytotoxic T cells. Thus, PIR-A and PIR-B are counteracting receptors that are essential for successful tissue transplantation and may regulate irrelevant reaction to autologous tissues in a constitutive way in physiological conditions.