IVIG is a big dose of IgG. Yet, when introducing it to the immune system with autoimmune activity it's seems to help modulate it and help it to function better. They really don't know why it works so well. This actually isn't uncommon for a medicine to work and they not know why. The use of Neurontin and other anti-seizure meds for neuropathic pain began as an accident of sorts. They just noticed that seizure patients receiving it start to have less pain. So they start using it for PN. They now know more about how it works but they didn't at the start.