The truth is I don't really know. I have been attempting to build a page on this but can not find enough credible references to how and why. I think my wife (betsy) is closer to finding the answer on this than me.

I will answer it this way. Systemic is a catch all description when the over response from the immune system attacks various parts of the body where the oil did not come in contact. In other words, you begin breaking out on your back when oil touched your toe. :-) In fact, some observers say that when you have a systemic reaction you will break out on previous places that you had the oil from past exposures.

There is general agreement that systemic can happen when the oil enters a wound such as a cut or the oil becomes airborne because of burning, etc. New information now indicates this can happen when large amounts of the oil are absorbed through thin areas of the skin. So some speculate that the oil moves through the body.

Zanfel Laboratories seems to offer more on this than others I have seen. According to a zanfel funded research grant by Ponciano D. Cruz, MD in April, 2003 (J.B. Shelmire Professor of Dermatology), they mention memory T-cells which could explain this past exposure phenomenom.