[Terr_]

Not the same as large-scale transplantation, but that reminds me of some work in Chimeric Antigen Receptor (CAR) T-Cell therapy, where the dangerously-useful cells either have a manual suicide-switch or else another method to temporarily deactivate/reactivate them.

So still sci-fi at this point, but I'm imagining some kind of "bone marrow in a box" implant which cages the borrowed immune-system (copied, rather than genetically-engineered) within a barrier that somehow allows only roaming/non-reproducing cells to exit.