Study Evaluating Allergic Reactions To Benefix In Hemophilia B Patients

Inclusion Criteria: Study Group: * Written informed consent or assent, as applicable. * Subjects with moderate to severe Hemophilia B (FIX:C \<5%), who have reported class II or class III allergic manifestations within 24 hours of a BeneFIX infusion. Control Group: * Written informed consent or assent, as applicable. * Subjects with moderate to severe Hemophilia B (FIX:C \<5%) Exclusion Criteria: Study group: * Subjects who had no reaction when rechallenged with BeneFIX, in the absence of prophylactic immunomodulating therapy. * Subjects whose most recent allergic manifestations with BeneFIX occurred \>36 months prior to providing written informed consent for this study. * Subjects who have taken antihistamine, steroidal, or other immunosuppressive therapy within 72 hours of the scheduled visit for this study. * Subjects with immune disorders. * Subjects unable to comply with a minimum 5-day FIX washout requirement. Control group: * Subjects with documented evidence of prior allergic reaction to any FIX product. * Subjects with documented evidence of prior or current FIX inhibitor (BU \>0.6). * Subjects have taken antihistamine, steroidal, or other immunosuppressive therapy within 72 hours of the scheduled study blood draw. * Subjects with immune disorder. * Subjects unable to comply with a minimum 5-day FIX washout requirement.