>wouldn't it be antibodies to random goat antigens, not ATryn?<
This is not a consequential distinction, IMO. Since the active ingredient in ATryn (antithrombin) does not cause an immune response, the only component of ATryn that could cause an immune response is an impurity.
“The efficient-market hypothesis may be the foremost piece of B.S. ever promulgated in any area of human knowledge!”