Use of antibody which binds to AILIM (alternatively called JTT-1 antigen, JTT-2 antigen, ICOS and 8F4) or a part thereof for the preparation of a pharmaceutical composition for preventing, treating or prophylaxis of graft versus host reaction, wherein the antibody has an suppressive effect comparable with the effect of hCTLA-Ig.Use of antibody which binds to AILIM (alternatively called JTT-1 antigen, JTT-2 antigen, ICOS and 8F4) or a part thereof for the preparation of a pharmaceutical composition for preventing, treating or prophylaxis of graft versus host reaction, wherein the antibody has an suppressive effect comparable with the effect of hCTLA-Ig.