"John... You have to bite me. NOW." The saliva in Sherlock's mouth has anti coagulant properties, it could help to keep the two tiny wounds open, but only for a sort time. If John didn't bite him before they stopped bleeding, then he would Change. Humans would change into vampires, but he had no idea what would happen to John.

Sherlock leaned his head over, exposing his neck, even pointing out where John should bit. "Hurry. Bite me now, before it's too late. I need you to bite me."