I've read this week that the spleen has been identified as a repository of monocytes - immune cells responsible for repairing damaged tissue (assuming that my very limited and brief understanding of the article is accurate). I deduce from this that part of Wolverine's mutant abilities is a hyperactive spleen.
Mokalus of Borg
PS - He may have more than one.
PPS - Wouldn't it be frustrating to attempt surgery on someone who heals that quickly?