You haven't seen it used for organ transplants either, as is obvious from your post...
HLA matching is merely a name for the type of matching used, not the "injection." It's a courtesy for their long term health - google it - Again, the right thing to do is not engage in idiotic risk/reward scenarios as the staff clearly has (most likely out of ignorance).
http://ajs.sagepub.com/content/42/10/2410.abstract
I'd wager that what you "do in practice" is rooted in some idiotic youtube guidance. You have kids do squats? You're wrong. You have them bench for anything other than pre-combine workouts? You're wrong. Do you monitor their VO2 max, sleep patterns, and nutrition (intake and output*) as close as you do their traditional "performance metrics?" No??? You're wrong.