Based on the finding that anaphylactic reactions to Benefix ("FIX") are associated with a specific IgE, a Basophil histamine release assay was selected to evaluate and demonstrate subject sensitization to FIX.

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.