Creatures can be immune to effects or damage types.
A creature immune to a damage type doesn't take damage from that type of damage. Additionally, if a creature is immune to fear, poison or illusion, it isn't affected by the non-damaging effects of power with those keywords. A creature immune to a condition|condition or another effect (such as being dazed), is not affected by the stated effect.
Otherwise, being immune to one part of a power doesn't make a creature immune to other parts of it; a creature immune to thunder hit by a power dealing thunder damage and pushing the creature, will still get pushed.[RC:225]