James and others,
  I wish I had money for each person who walked into my office with poison
ivy dermatitis and said "I'm immune to it".  The problem with immunity is
that it may not be life long.  Every now and then, those who thought that
they were immune became sensitized and voila, they got the rash with their
next exposure.  I hope the immunity continues but it is not a given, at
least I know of no studies that indicate a genetic basis for never
developing PI dermatitis.
  Take care

Charlie
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Charlie Sturm, Jr
Research Associate - Section of Invertebrate Zoology
                     Carnegie Museum of Natural History, Pittsburgh, PA
Assistant Professor - Family Medicine

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