From Darren Shan Wiki

Saliva from vampires and vampaneze is know to have healing powers. It can completely heal minor wounds in a matter of seconds and can speed up the healing process of more severe cuts, bruises and breaks. In Hunters of the Dusk, when a vampaneze cuts off Darren's thumb, Vancha stitches it back on and rubs his and Larten's spit around the join to quicken the healing process.