It is noteworthy that P. gingivalis was detected in the brain microglia and hippocampal capillaries of ApoE-/- mice from the P. gingivalis infection group, which means P. gingivalis invasion of ApoE-/- induced complement activation in mouse brains.24 Poole et al50 also showed that P. gingivalis was able to enter the ApoE-/- brain and contribute to the development of AD inflammatory pathology through mechanisms involving cytokines, acute phase proteins, and the complement cascade, in which neurons are attacked. Here, APOE is linked to Alzheimer disease.