On Tue, 08 Jan 2013 23:58:18 -0500, Wes Groleau wrote:

One source suggested that 15% are unaffected;
another said thirty.

We're talking different things, so let's clarify.

1. Nobody is immune to type IV CMI
2. But not everybody gets the rash under normal circumstances
3. Yet, almost everyone will get the rash if they get exposed to enough
urushiol.

It gets complicated to explain in a USENET post, but let's briefly take
these in turn - but this isn't the place for detailed discussions.

I. Nobody is immune.
Delayed contact dermititis is a type IV CMI (cell mediated immune
response), which nobody is immune to forever. Get exposed frequently
enough, and you WILL get it. It's the way your body works.

II. Not every gets the rash all the time
This is highly dependent on dosage! Remember the oil is NOT anywhere on
the outside of the plant! The oil is protectively ensconced INSIDE the
cells. Of course, chain sawing the plant in half tends to allow the oils
to leak out ... like this video I just took today of just that:
http://www.youtube.com/watch?v=qYcJslc6ymE

III. Give any human enough pure urushiol, and they ALL will get the rash!
(See #I and #II above.)

Anyway, while we're quoting figures, Wikipedia says the following on
percentages:
http://en.wikipedia.org/wiki/Urushio...act_dermatitis

Verbatim:
"Approximately 80% to 90% of adults will get a rash if they are exposed
to 50 micrograms of purified urushiol. Some people are so sensitive, it
only takes a trace of urushiol (two micrograms or less than one ten-
millionth of an ounce) on the skin to initiate an allergic reaction
(Epstein et al., 1974).[6]"

Now, if you're exposed to less urushiol, all bets are off - but - given
this picture below, I would safely say anyone exposed to this much
potential urushiol had better take some safety precautions (as I do).
http://www1.picturepush.com/photo/a/...g/11917834.jpg