When a person is unwound, every body part is harvested and used or saved for someone else to have. EVERY part. Well, 99.4% of the parts. The appendix isn't so much kept around.
There are all kinds of reasons teens could be unwound. If they were troublemakers and their parents didn't like them, if they were wards of the state who could no longer afford to feed them. One alarming reason for unwinding is the "tithe." Religious couples could choose to offer their children as a tithe to their church. Conor, Risa, and Lev fall into these categories respectively. Though these three are marked for unwinding, they end up on the run. Though unwinding is used to get rid of people who ususally aren't really wanted, it isn't seen as a bad thing. It is seen as living in another form. The person never really dies, he or she is just distributed around to other lives.
This book gave me a lot to think about and wince at. It is a fascinating story which was alarming and powerful.