This reminds me of this: http://www.preeclampsia.org/forum/viewtopic.php?t=31602

Basically in this (small) study they found lower levels of an enzyme in immune cells of normal pregnant patients compared to patients pregnant with pre-e and compared to non-pregnant subjects.

They are speculating that this enzyme influences cytokine production, which signal the immune system to go on attack during an infection or auto-immune disorder. If (and this is a big if) this theory is correct, then this mechanism could be the target of future research to help moderate immune system response and reduce inflammation.

so what does this mean?

"Why do certain diseases go into remission during pregnancy?

Now, scientists have described a biological mechanism responsible for changes in the immune system that helps to explain the remission.

The expression of an enzyme known as pyruvate kinase is reduced in immune cells in pregnant women compared to non-pregnant women....
The study... also reports that expression of the enzyme is lower in pregnant women compared to those with pre-eclampsia, a condition with inflammatory components"

Article:
http://www2.med.umich.edu/prmc/media/newsroom/details.cfm?ID=1632

Abstract:
http://onlinelibrary.wiley.com/doi/10.1111/j.1600-0897.2010.00881.x/abstract

This is only one small study but the possibilities are very interesting.