Pig Heart Valves and Rejection

[wolf tom]

This may seem like a strange question, but I do not know the answer and 
cannot find any written material on the subject.

Why are replacement heart valves from swine not subject to rejection 
problems when translanted into humans?  I have a student undergoing this 
procedure and nobody, including the medical people involved, have been 
able to provide a satisfactory answer.

Thanks for any help.

Tom Wolf