Crossing borders in Alzheimer’s disease: A T cell’s perspective

•A decline in functional properties of the brain barriers in AD allows T cells to enter the brain parenchyma.•Levels of effector memory and terminally differentiated T cells are increased in the blood of AD patients.•Potential protective T cell phenotypes are observed in cognitively healthy centenarians.•Longitudinal studies show that alterations in peripheral immunity are associated with cognitive decline in dementia.•Animal studies emphasize the value of targeting peripheral immunity to combat AD pathology.