Are immunities only working if you want them to?

Question

For example, a Necropolitan Dread Necromancer just leveled, earning herself the class ability Negative Energy Resistance. However, she has always been using her Charnel Touch ability to heal herself. Is this no longer possible, or can she choose when she wants to be immune or not?

Or, the same Necropolitan is in a party with a Bard, using his Inspire Courage to buff her allies. However, this is a mind-affecting spell, which the Necropolitan is immune to. Does she not get the buff, or can she "disable" her immunity for this buff?

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Sources:
Bard is in Player Handbook, page 26.
Dread Necromancer is in Heroes of Horror, page 84.
The Necropolitan template is in Libris Mortis, page 114.

Answer 1

Immunity or Resistance can only be suppressed if the text specifically mentions it.

Here is an example of an immunity:

Cold Immunity: A creature with cold immunity never takes cold damage.

As you can see above, there is absolutely no mention that you can suddenly not be immune to cold whenever you feel like it.

Here is an example of resistance:

Spell Resistance: Spell resistance is the extraordinary ability to avoid being affected by spells. A creature can voluntarily lower its spell resistance.

As you can see above, there is specific mention that you can lower your resistance; you probably wouldn't want to resist a restorative spell being sent your way - unless you wanted to.

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Regarding an Undead Dread Necromancer specifically...

First, lets look at the ability in question:

Charnel Touch (Su): This touch heals undead creatures...

Negative Energy Resistance: A dread necromancer gains a bonus on saving throws made to resist negative energy effects...

Now, lets look at the Necropolitan template in question:

Size/Type: The creature's type changes to undead...

As mentioned in previous answers, there is no saving throw to apply the resistance to. The Charnel Touch would heal the Necropolitan, since the Necropolitan is an undead creature - there is no saving throw - it just happens.

Now, regarding the Undead's:

• Immunity to all mind-affecting effects (charms, compulsions, phantasms, patterns, and morale effects).

This is overcome by one feat, that the bard should have taken knowing there was an undead presence in the party (and on his/her side):

Requiem (Libris Mortis, p. 29): You can extend the effects of your mind-affecting bardic music abilities so that they influence the undead.

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Before someone asks, "Is Inspire Courage a Bardic Music Effect?"

Yes, it is. See below:

Bardic Music states: "While these abilities fall under the category of bardic music... they can all be activated by reciting poetry, singing lyrical songs... Each ability requires both a minimum bard level and a minimum number of ranks in the Perform skill..."

Inspire Courage requires 3 or more ranks in a Perform skill, and uses song or poetry.

Answer 2

Your question title and question elaboration are not the same.

In general, immunities/resistances cannot be switched on/off. Unlike spell resistance, which is specifically mentioned to have such a switch.

The Dread Necromancer's Negative Energy Resistance grants a bonus to saving throws, not actual resistance in the general sense. Now, characters can always choose to fail a saving throw. In this case, the Dread Necro doesn't want to resist her own Charnel Touch, not that it offers a save anyway. ;)

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Wow, because this got derailed into nit-pickery on saving throws (and, correctly enough, there is nothing to cite about voluntarily failing any saving throw, although it is fairly obvious, just like there is nothing to cite about a character choosing to be Helpless against a sword attack 'coz she wants to commit Suicide-by-Town-Guard), I'll try to scope it back to point by focusing on saving throws and how they relate to the situation detailed in the question.

Voluntarily Giving up a Saving Throw: A creature can voluntarily forego a saving throw and willingly accept a spell’s result. Even a character with a special resistance to magic (for example, an elf’s resistance to sleep effects) can suppress this quality. (PH, 177)

This means that even a non-undead Dread Necromancer 9+ can always elect to be affected by negative-energy effects, even though she's resistant to it, whether by the Negative Energy Resistance ability or some other source. Why? Maybe she's got rider abilities contingent on a negative-energy effect. In any case, it's her choice.

Now, take note that the citation also mentions "special resistance to magic" and the Elf's "resistance to sleep effects".

Elf Racial Traits: Immunity to magic sleep effects, and a +2 racial saving throw bonus against enchantment spells or effects. (PH, 16)

The Elf's "resistance to sleep effects" is actually an immunity, so we can infer that resistances and immunities are essentially the same when it comes to voluntarily accepting magical effects.

Undead Type: Immunity to all mind-affecting effects (charms, compulsions, phantasms, patterns, and morale effects). (MM, 317)

Because it is worded just like the Elf's immunity to sleep effects, it should be safe to say that an Undead creature can choose to be affected by a mind-affecting effect, such as the Bard's Inspire Courage. Neat! ^_^

BUT WAIT! THERE'S MORE! Rules Lawyers will argue that willingly accepting the spell's result is contingent on foregoing the saving throw. They'll argue that since there is no saving throw offered, there is no saving throw to forego, and thus the "willingly accept" part cannot be triggered (it is an AND clause, after all). Also, this mentions a "spell", which Inspire Courage, being a supernatural ability, is not. From this point of view, all Undead cannot be affected by Inspire Courage.

Take that as you will. I'm just providing different POVs. ^_^

Answer 3

Generally, I would say not. A fire elemental can't suddenly decide to be harmed by fire, just the same as how an ice-creature of some form can't suddenly be harmed by cold-damage if it wants to.

A creature of necrotic or radiant energy is harder to picture in this context (those being ill-defined/indistinct forms of energy), but the same rules apply. Remember that any rule that applies to your necromancer's immunity would apply to every other creature with immunity.

In any case, there are no rules to support this, as far as I'm aware.