Immunity

Summary
Immunity is a new feature within R2DA that allow an object to be resistant to certain aspects of the game.

Currently, there are 7 common types of immunity that can be found in-game.

Damage Immunity
This immunity was added in v0.6.1.

This can be triggered by: This grants immunity to most forms of damage except: All melees — except the Defibrillator and including the Hammer — supply damage immunity.
 * Punching.
 * Kicking.
 * Swinging a melee weapon.
 * Blocking.
 * Explosions.
 * Radiation.
 * Being caught by a Leaper.
 * Being caught by an Edgar.
 * A Brute's club.
 * Touching environmental hazards.
 * Falling out of the world.

During Rapid Infection, melee weapons do not grant their usual immunities. However, Punching and Kicking still do.

Forcefield Immunity
This can be triggered though: This grants immunity to most forms of damage except: Prominent examples of environmental hazards include the surrounding terrain in Terminal Decay and scaling the cliffs in Mogadishu Transit. In v0.9.9, this immunity was extended to protect you from being knock-backed and from being tripped.
 * Being a survivor
 * At the beginning of every round.
 * After being revived by a Defibrillator.
 * Being a zombie
 * As a Brute (or Charged Brute) by holding Q at the expense of 30 energy per second.
 * As a Stalker by holding E at the expense of 20 energy per second
 * As either
 * By ranking up at any point in the round.
 * Being caught by an Edgar.
 * Fire damage (if the effect is still there after the immunity wears off).
 * Falling out of the world.
 * Touching environmental hazards.

Chronos XI receives one whenever he uses Laser Storm and when the round initially starts. This prevents him from being killed too easily since he isn't directly attacking anything.

Helicopter/Bus Immunity
This immunity was added in v0.6.8.

By sitting down, you became completely immune to damage. However, you were still vulnerable to: At one point, this immunity was bugged allowing normal zombies to simply claw to kill survivors. This was fixed.
 * Radiation
 * An Elemental's touch
 * A Brute's club

With the new helicopter (and the Bus, when it was added), you are teleported to a stage where no zombies can reach you, basically making you invincible. This is visible by spectating someone inside the new helicopter stage.

Incendiary Immunity
This immunity allows these mobs to be resistant to incendiary damage and applies to: This immunity also applies to survivors with the Firevest
 * An Elemental
 * A Ticker
 * Lord Pumpkin Jr.
 * Mr. Macaroni (Flamethrower only)

An Elemental is actually healed by fire, and the Ticker is resistant to Flamethrower damage, but not Molotov damage.

This immunity can be disabled for the Ticker if they are knocked down specifically by a Fire Extinguisher. When done so, this renders them vulnerable to Flamethrower damage.

Knock-back Immunity
This immunity applies specifically to a Brute against a Fire Extinguisher. If used in an attempt to push the Brute back, it won't work. A Brute will also not get knocked-back by melee attacks when he is shielded.

This technically also applies to the Elemental, but it usually kills it before any knock-back is applied.

Trolley Immunity
This immunity was added in v0.6.4.

By sitting down, you become immune to a Brute's club. However, all other damage is still fair game.

A slight bug with this is that you will not win the round if you are sitting down. This has since been fixed

Admin Forcefield Immunity / God Mode
The only people who have access to this immunity are PlaceRebuilder and Jopede.

This immunity is only accessed by a command which will then put a forcefield on you. All damage will not affect you.

Heavy abuse of this in R2D 2014 led to the decline of the latter.

Video evidence can be seen here.

Explosive Immunity
So far, this immunity applies to: As explosives do high damage, this was added in so that they couldn't be defeated so easily.
 * Lord Pumpkin Jr. (2016)
 * Stone Golumns (Partially, doesn't apply for the RPG)
 * Chronos XI (Partially. This specifically applies to the Grenade and Pipebomb, but overall, it's just a glitch.)

With this immunity, you cannot damage these mobs with:
 * The M202.
 * The X-Mas Launcher.
 * The RPG.
 * A Grenade.
 * A Pipebomb.
 * An Airstrike.
 * A Jerry Can.
 * A Propane Canister.
 * A Firebox.
 * A Clockbomb.

Melee Immunity
So far, this immunity applies to: Melee weapons do not affect these bosses. You can still kick and punch, though.
 * Lord Pumpkin Jr.
 * King Crab
 * Chronos XI

Pipebomb Immunity
This is a scrapped immunity that was implemented on the Leaper before v0.6.8.

This allowed you to resist a Pipebomb and the way it forced you to go in its direction.

Idle Immunity
This was an unintentional immunity that applied to Idle mobs that existed from v0.9.3 to v1.0.2.

A mob, while in Idle mode, could not be stunned in any way, shape, or form. Knockback, however, still applied.