J ---- back to your original question..... Many many people have received IVIG for an immune related neuropathy (myself included) in which the insurance company has covered.
If your Dr. feels you need it due to the antibody you have, then ask him to be more specific in the medical coding-----they should have given you a reason for why they denied the first two requests.
Dealing with the insurance companies can be frustrating, it's easier if you have a good Dr. who will call and work it out with them for you. Good luck.., don't get discouraged.....and I hope you are able to get the treatment you need.