AD and PD are accompanied by impaired intestinal barrier (Forsyth et al. 2011), lipopolysaccharide (LPS) and pro-inflammatory factors produced by pathogens can enter the human body through the damaged intestinal barrier, destroy the integrity of the blood–brain barrier (BBB) and increase the brain’s uptake of Aβ and α-syn, thus activate microglia by LPS/toll-like receptor4 (TLR4)/nuclear factor-kappa B (NFκB) pathway to induce the immune response, which ultimately leads to the loss of neurons (Dutta et al. 2008; Lund et al. 2011). Here, TLR4 is linked to Alzheimer disease.