Brain immune cells undergo significant shifts during middle age

New research suggests that the immune environment of the human brain undergoes significant remodeling during middle age. Studies conducted by researchers at Stanford Medicine, the University of California, San Diego, the New York Genome Center, and the University of California, Irvine, indicate that the brain may not function as a closed system as previously believed.

Findings published in the journal Nature reveal that between the ages of 50 and 75, there is a gradual decline in microglia—the brain’s resident immune cells responsible for maintaining a healthy environment and removing damaged material in the hippocampus. Simultaneously, a different group of immune cells, which show stronger signs of inflammation and resemble cells typically found in the blood outside the brain, begin to enter the brain tissue.

This shift suggests that as people age, immune cells from elsewhere in the body flood into the brain. This discovery could reshape the scientific understanding of brain aging and provide new insights into why aging is a major risk factor for neurological disorders such as Alzheimer’s disease.