I've heard of this a few times but I still don't quite get it..

It's just the only hard enough part of the body that won't be rejected but won't the autoimmune issues persist?

It's really incredible this has been around since the 60's and I would love to have a chat with the first person who said "let's try a tooth"

File this question under "intrusive thoughts": does his body's natural mechanisms manage to keep the tooth moist? Does it end up feeling cold?

This is wild read and worth a pool upgrade. He now has about 20/40 or 20/30 vision in that eye, but needs to wear (normal I think) sun glasses.