Trisha Radley wrote on 1/11/02 7:15 AM :

>Actually, that is a good example of how the body develops an allergic
>reaction/response (like beestings). The first time you are exposed, you have
>no allergic antigens to the offending substance, and your body develops
>these so that the next time you are exposed, your body has developed a
>"memory" in its immune system and attacks the substance...hence the allergic
>reaction. Not sure I explained that very well, but its early!

Good explanation, but it's a little more complicated than that.  The 
oils/chemicals from the poison ivy are not harmful to the skin, do not 
cause any symptoms, and do not cause antibodies to be produced.  The 
invading chemical is "digested" (bad word), on the skin, and reacts with 
the skin proteins.  The resulting components are manifested as "foreign" 
to the body's immune system.  This usually takes hours, so there is time 
to wash the junk off (if you know it is there.)

Now the immune system sends white blood cells to the "foreign" area, 
where they end up attacking everything in the area.  This is what causes 
all the damage.

I don't remember the mechanism that explains "immunity," and why repeated 
exposure results in greater damage.

-Jim