That was worded badly. The "immune system", if that is taken to mean
the population of antigens in the blood, does indeed develop after
conception. The particular histocompatibility keys of affected tissue,
though, are genetic and fixed. I should have said, then, that the bugs
evolve to adapt to your tissues, not to your immune system, which
adapts to them. The effect is still there, though: inbred organisms
do not fight disease well.
-- Lee Daniel Crocker <lee@piclab.com> <http://www.piclab.com/lcrocker.html>