Most PN is not autoimmune. There are MANY different causes of PN and autoimmune does seem to have a good size piece of the pie but it's still far behind conditions like diabetes, B12 deficiency, trauma, alcoholism, medications, etc.

That being said, you are quite correct in saying that boosting your immune system when you have an autoimmune condition can make things worse. AI diseases are, in fact, typically treated with immune suppressors for that very reason. When you boost the immune system, you risk increasing the autoimmune activity.