They only ignore the effects, and take full damage. MM1 is ambiguous on the issue, but MM2 makes it pretty clear.Some monsters (see the Guard Drake and Needlefang Drake Swarm in the Monster Manual) are immune to fear. Does this mean they take no damage from attacks with the fear keyword, or they only take no additional effects from attacks with the fear keyword?
MM2 said:immune: A creature that is immune to a damage type (such as cold or fire), a condition (such as dazed or petrified), or another specific effect (such as disease or forced movement) is not affected by it. A creature that is immune to charm, fear, illusion, poison, or sleep is not affected by the nondamaging effects of a power that has that keyword. A creature that is immune to gaze is not affected by powers that have that keyword.
Damage types are always noted in the description of the power, e.g. 1d10+5 fire damage, or 2d10 damage + 5 cold damage. So, a creature immune to poison ignores the effects of a power with the poison keyword, and any damage that power deals with the poison type. For example:Thanks! That makes it pretty clear for fear, but now I'm confused about poison. Poison is a damage type as well as an additional effect, so does this mean that nothing can be immune to poison damage, only poison effects?
Hypoisthetical Power (Standard, encounter) * Poison
+15 vs. Fortitude, 2d10 damage + 10 poison damage and the target is dazed until the end of your next turn.
If this power hits a creature immune to poison, the creature takes 2d10 damage, ignores the 10 poison damage, and ignores the dazed condition.