Thank you for contacting Wizards of the Coast game support!
Damage is not an "ill effect" as per the intended rules definition. (The literal argument that damage is a bad thing, therefore it should be considered an "ill effect" doesn't apply.)
Resistance and Immunity are not different degrees of the same thing.
Resistance applies only to damage.
Immunity, however, applies to two things. First, the "ill effects" of powers that have the specified keyword you are immune to. Second, if the power deals damage of a type that you are immune to, then you are immune to the damage as well. (This is because some keywords are also damage types.)
Example 1: A power with the Lighting and Thunder keywords that deals 10 lightning and 10 thunder damage and stuns the victim for one round:
Answer: Immunity to thunder negates the stun effect, negates the thunder damage, but does not negate the lightning damage.
Example 2: A power with the Lighting and Thunder keywords that deals "20 lightning and thunder damage" and stuns the victim for one round:
Answer: Immunity to lightning (or thunder) negates the stun effect and negates all of the mixed "lightning and thunder damage."
Example 3: Banish to the Void, Warlock 27 Encounter Power with the Fear and Teleportation keywords (deals untyped damage and teleports the victim, then makes the victim attack others.)
Answer: Having an immunity to either fear or teleportation negates the teleportation and forced attacking effects of the power, but does not negate the untyped damage.
Hopefully this clarifies things for you!
Take Care and Good Gaming!