In fourth and fifth editions, Immunity is a critter power that grants Essencex2 as an Hardened Armor against an attack or affliction that deals damage (except for Immunity to Age) ; Natural Immunity is a quality that make a character effectively immune "to a single disease or toxin" or one "disease, drug, or poison". Note that the actual "Toxins" header in the rules is separate from the "Drugs and brainbenders" or "Drugs and BTL".
At no point does Shadowrun addresses the frontier between characters and critters (dragons for instance have skills listed above the maximum rating for skills that supposedly apply to all characters), and whether immortal elves ought to be treated as characters, critters, or both, and which immunity they should get.
Harlequin stats as published in Street Legends Supplemental listed "Immunity (Age, Disease, Pathogens, Toxins)" - on the other hand, Lugh Surehand stats in Street Legends has none of those (not even age!).
As far as the rules go, the Immunity critter power only applies when there is a Damage Value. "Toxins" require a Toxin Resistance against Power, instead of a DV. Only for some toxins does it translate into stun or physical damage. "Drugs" have none of those, and as far as I remember the one that can actual deal damage do so unresisted.