I know antibodies don't selectively enter peripheral neurons, but they do enter them right? as they're traveling through the body?

Backwards!: Antibodies _are_ selective, but mostly work by attaching to surfaces of cells rather than entering them. This may cause interior damage to cells or leave them vulnerable. Peripheral nerve disorders from antibodies often attack the 'insulation' cell around the actual nerve process making signal transmission inefficient. Each antibody type recognizes its target by a sort of 'lock & key' specificity.