Rashes are a common
IVIG reaction. Web up the ‘prescribing information’ and read up on the side effects. It can list every dermatology ‘word’ for what’s essentially an allergic reaction. The rash itself is usually eased by topical steroids and can be avoided by pre-medications of Benedryl or the like. It’s really more common than most docs are aware of and all you have to do is print out a copy of the side-effects highlighting these effects.
In the meantime it’s a poison ivy or hive-like itching that drives you crazy and don’t scratch yourself raw.