One of the most interesting examples of feedback loops is the one that controls cell replication. Let's consider this - right after conception cell replication occurs very quickly, after birth it still occurs and continues to add new cells to the body but at a slower pace. After adulthood cells are not added any more but they are replaced with new cells being created and old ones commiting suicide. At old age the process of replacement stops. We know how the last part of the loop works, the ends of the chromosomes get shorter at each replication and thus serve as 'counters' for when replication is to stop. This of course is totally symbolic, there is no purpose to those counters other than as counters.
Naturally, I went searching right away for more information especially WRT the p21 aging gene. I found this article you might also enjoy: Perspectives on Systems Biology (pdf)
I'm also searching for more information on the self/non-self determination. That is a classic conditional that requires symbolization.