The cosignaling network mediated by the herpesvirus entry mediator (HVEM; TNFRSF14) functions as a dual directional system that involves proinflammatory ligand, lymphotoxin that exhibits inducible expression and competes with HSV glycoprotein D for HVEM, a receptor expressed by T lymphocytes (LIGHT; TNFSF14), and the inhibitory Ig family member B and T lymphocyte attenuator (BTLA). To dissect the differential contributions of HVEM/BTLA and HVEM/LIGHT interactions, topographically-specific, competitive, and nonblocking anti-HVEM Abs that inhibit BTLA binding, but not LIGHT, were developed. We demonstrate that a BTLA-specific competitor attenuated the course of acute graft-versus-host reaction in a murine F(1) transfer semiallogeneic model. Selective HVEM/BTLA blockade did not inhibit donor T cell infiltration into graft-versus-host reaction target organs, but decreased the functional activity of the alloreactive T cells. These results highlight the critical role of HVEM/BTLA pathway in the control of the allogeneic immune response and identify a new therapeutic target for transplantation and autoimmune diseases.