According to the conventional PANDAS theory, due to biological mimickry antibodies against Step A bind to neurons and attack them, so there are antibody tests that focus on Strep A that are used as diagnostic tools in PANDAS, such as antistreptolysin O (ASO), anti-DNAse B (ADB), and streptococcal carbohydrate (anti-CHO).
However, there a lots of kids diagnosed with PANDAS who have high levels of those antibodies at some times but not at others, and there are kids with the exact same history and presentation who never show high levels of those antibodies at all. So it could be that there are actually a wide variety of situations in which antibodies that the body creates end up attacking neurons (see anti-NMDA encephalitis), and only some of them are even known about yet.
However, there a lots of kids diagnosed with PANDAS who have high levels of those antibodies at some times but not at others, and there are kids with the exact same history and presentation who never show high levels of those antibodies at all. So it could be that there are actually a wide variety of situations in which antibodies that the body creates end up attacking neurons (see anti-NMDA encephalitis), and only some of them are even known about yet.