Immune to Pain effectively blocks mist elven iron damage (if it makes 1 point/turn), however, the message is still displayed on each turn.
The message is technically correct, since PC just ignores the pain, but it still spams all over the place.

Moreover, such situation blocks extended move commands (w5 etc.) as well as eating (adding a prompt on each turn).

I suggest removing the message and side-effects (mentioned extended moving and others) in case of no damage.