> There's also no reason to think that the immune system is so easily split up between systems that respond to parasites, bacteria, and virii.
Well, sure. We essentially have multiple "immune systems", with overlapping goals. (Sort of like having both an anti-virus and anti-spyware scanner installed, as well as a system integrity checker, secure-boot verification logic, etc.)
Some immune responses are good at stopping all sorts of things (e.g. xenoreactive antibodies, which attack both parasites, bacteria, and cancer cells.) I wouldn't suggest doing anything to these. Conveniently, these processes don't tend to cause many long-term health problems, save in edge cases (e.g. xenograft implantation.)
Other immune responses are more specialized. And — also conveniently — it's the reactions/systems that specialize in responding to things we see almost none of in "city living", that seem to do bad things for longevity.
Parasite-specialist immune cells (mast cells), for example, cause a lot of chronic inflammatory problems. We've been preventing mast cells from doing their jobs for a long time now — that's the point of taking antihistamines — with very little negative to show for it. I don't think it would be much of a stretch from there to a treatment for allergies that's just a siRNA that "NOPs out" the entire immune-response functionality of mast cells. (Or even a "permanent" treatment for allergies, via CRISPR gene edit to rub out the definition of mast-cell antigen recognition receptors from the body's vocabulary.)
And I would also say that it's not much of a stretch to hypothesize that people who had this done, would, under longitudinal studies, show increased longevity / QUALYs. "Never having an [even sub-clinical] allergic reaction" would do really good things for the body. At the cost of — for most people reading this — just having to avoid sashimi.