On Mon, 01 Jul 2013 07:47:19 -0400, Pat Kiewicz wrote:

http://www1.picturepush.com/photo/a/...g/11947484.jpg
it's still automatic for me to ID poison ivy, oak, or sumac
I am amazingly good at spotting it.

I understand what you mean, as I am always identifying it for the
grandkids, who wander by oblivious to all harm.

Nobody is immune to cell-mediated immune responses (since the T-Cell
are randomly generated and passed by the Thymus), but some people just
haven't (randomly) gotten it yet. Or, they haven't gotten enough of a
dose that their T-Cells wandered by a urushiol quinone which has
bound to a receptor site on a Langerhans cell in their skin.

NOTE: Contact dermatitis is NOT mediated by humoral antigen/antibody
responses, so, all the conventional wisdom of "being immune" goes out
the door.

In the case of the amount of sap in that picture, almost nobody on
earth would not respond to that amount, were it to touch skin (keeping
in mind, the urushiol oil is known to stay active over 100 years in
dendrology drawers) and probably 10 years in my relatively dry climate.