"Engineer a replacement for the immune system" sounds insanely hard. Problem statement: "constant incoming stream of incredibly diverse unknown bacterial and viral invaders that you have to recognize and kill before their replication overwhelms your systems, but make sure you don't attack any of your own extremely diverse tissues, oh also those bacteria and viruses are constantly evolving to bypass your defenses". Natural immune systems are incredible biotech and it's a miracle we're not all dead.
> I don't think I would have independently invented the idea of killing and consuming an animal if other's hadn't taught it to me.
If you were really hungry I think you'd figure something out.
I completely agree on your first point. I was going for a bit of understatement, but to be clear, doing any of that is firmly on the other side of many revolutionary breakthroughs in our understanding of biology. But that being said, the standard for success isn't to have an immune system that can protect a complete animal for its entire life. The standard is to put up a nonzero amount of resistance to the reactor getting colonized by opportunistic bacteria (yeast etc), and not attack the one specific cell type that you care about. It's about pushing the requirement for sterility down from 100% to 'only' 99.99%.
> I don't think I would have independently invented the idea of killing and consuming an animal if other's hadn't taught it to me.
If you were really hungry I think you'd figure something out.