TLR4 and neuritis: In this study, we also found that the HMGB1–TLR axis downstream signaling factors, MyD88, NF‐κB, and TNF‐α, were highly expressed in the sera of patients with PD, indicating a connection between the upstream and downstream signaling of the HMGB1–TLR4 axis that plays an important role in the neuritis of PD.