Even if there is no acute rejection event, long-term accumulation of damage from the host's immune system attacking the graft at a low background level is still

Even if there is no acute rejection event, long-term accumulation of damage from the host's immune system attacking the graft at a low background level is still very likely. Immune suppression has to walk a tightrope that in addition to being very thin is also constantly moving around. Each time you get sick, if your immune suppression is too aggressive, you might just die. If it's not aggressive enough, the pro-immune signalling generated in response to the infection might override the immune suppression and trigger rejection of the graft. Any other event that triggers or suppresses an immune response also has similar risks associated with it. It's kind of similar to how diabetes patients need to constantly monitor their blood sugar and manually adjust their insulin levels to compensate, but on a longer time scale (generally weeks to months rather than minutes to hours). But with diabetes, at least you have the potential for almost direct control over your blood sugar levels (by regulating food intake). There is no such direct control for your level of immune activity, nor is there a simple and quick blood test to measure whether immune suppression is at the right level. In short, anyone with a transplant is walking a wobbly tightrope of immune suppression for the rest of their life, and they sometimes don't even know they've fallen off until they hit the ground. (Source: I have previously worked in a lab studying transplant rejection and trying to, among other things, develop such a simple and quick blood test: https://www.ncbi.nlm.nih.gov/pubmed/24725967 https://www.ncbi.nlm.nih.gov/pubmed/24725967)