--people with autoimmune diseases can see the effects, especially nerve effects, from several separate mechanisms, each acting individually or in concert.

A specific chemical attack on tissue, as kiwi has been describing, obviously can cause nerve damage. The body erroneously attacks its own tissue through chemical mechanisms by misidentifying it as foreign, and this process can literally dissolve functional parts of nerve. Often this is referred to as a "molecular mimicry" process--in vulnerable people, a pathogen of other foreign body may kick things off in that the body will mount an immune response to the pathogen, but since this is, on the most fundamental level, based on molecular shape (the body produces antibodies that fit into parts of the cellular structure of the pathogen, like manufacturing a key for a specific type of "lock"), if any body tissue have a similar shape, the now-activated immune system attacks anything that looks like such a "lock" and an autoimmune process ensues.

But, in addition to this, autoimmune attack typically results in swelling and fluid accumulation at the sites where this is ongoing. That swelling in itself can cause compressive forces on nerve, and nerves do not function well when constricted. So damage may occur in this way, in addition to and/or independently of the chemical damage.

This has sometimes been referred to as a "double crush hypothesis" in medical literature, with the symptomolgic effect more than what one would expect from the sum of the parts.