Is there a mechanic for an ally to jump in front of an attack?

@JeremyECrawford
Is there a mechanic for an ally to jump in front of an attack? (take hit/ reduce damage to an ally(& prevent their death))

— Mathew Michalak (@Trogaf) November 12, 2016

@Trogaf @ChrisPerkinsDnD @JeremyECrawford I'd allow it as a one off thing, maybe also req spend inspiration #wotcstaff

— (((Mike Mearls))) (@mikemearls) November 13, 2016

I allow this often. If one character wants to take a hit for another, it makes for good drama. But I might maximize the damage. #WOTCstaff https://t.co/MNjVNTNdPK

— (((Chris Perkins))) (@ChrisPerkinsDnD) November 13, 2016