A case-control study is conducted comparing prevalent hemophilia patients with inhibitors identified in the HMS Study with three age-matched controls without inhibitors to determine clinical and laboratory characteristics associated with inhibitor development. A histocompatibility study is conducted identifying patterns of HLA Class II antigens by molecular typing in hemophilic subjects identified in HMS, with and without inhibitors, which may be linked to inhibitor formation. An animal tolerance study is conducted in the hemophilia A murine model, evaluating blockade of T cell costimulatory pathways by CTLA4-Ig and anti-CD40L mAb, to prevent inhibitor formation.