The vampires I have in mind are a sub-species of humans: they share a similar immune system with us, however they evolved fangs and are capable of injecting some kind of substance into their victims, usually a human being. 

This chemical not only prevents the blood from clogging, it can also paralyze the prey. When threatened my vampire can alter the potency of the chemical so that it can kill any person within minutes. 

How does my vampire acquire the chemical and seemingly not be affected by it?