As stated in the title: is there any realistic way to either evolve (in our evolutionary past) or acquire via genetic engineering an immune system better than what we already have? I tried to do some research, but it looks like our current immune system is already pretty good compromise between "immunity" and "autoimmune disease", moreover any disease-specific gene mutations (like this one: https://en.wikipedia.org/wiki/CCR5#CCR5-%CE%9432) seem to have downsides, like increased susceptibility to other diseases.
One idea I had before was to replace adaptive immune system with whitelist-based approach, targeting every strand of DNA whose checksum fails to comply with anything on a fixed whitelist - but then we would need somehow to make humans independent from any symbiotic bacteria, and severely limit the population gene pool (because "too different" embryos would be killed in the womb). So, not really a good idea, right?
So, is there anything that could be improved? (and how?)
Edit: I am asking only about some inheritable biological traits, like (positive) mutations, not something that may be received during the lifetime, like drugs and vaccines.