For immunities you may not play cards with the specified trait, use powers that would add that trait to the check, or roll dice with that trait during the encounter. If you somehow play a card before encountering the card (Toxic Cloud) we just pretend Toxic Cloud was never played. If you play multiple cards that add traits to the check the previous traits are not replaced. So you cannot override the ice trait by playing something else that adds a different trait to the check.