Immune cells flood into aging brain, Stanford finds

Immune cells from the rest of the body routinely enter the human brain as people age, Stanford scientists reported on August 14, 2026, in Nature, challenging the long-held view that the brain's immune system is self-contained.

The peripheral immune cells migrate into the brain and transform into specialized microglia, a process that appears to begin as early as middle age. Julia Belk, a postdoctoral scholar in pathology at Stanford Medicine, led the study; senior authors are Siddhartha Jaiswal, an associate professor of pathology, and Howard Chang, the Virginia and D. K. Ludwig Professor of Cancer Research and professor of genetics.

Researchers used paired blood and post-mortem brain samples from the Stanford Rapid Autopsy Center, led by Jody Hooper, and the University of Washington's Alzheimer's Disease Sequencing Project. By comparing DNA mutations in immune cells from blood and brain tissue, the team traced cellular ancestry and confirmed matching genetic signatures in blood and brain microglia. This migration does not occur in mice or non-human primates, making it a uniquely human feature of aging.

Earlier work by Jaiswal and colleagues showed that people carrying certain mutated blood stem cell clones were much less likely to develop Alzheimer's, suggesting blood immune cells influence brain health. The new findings imply that changes in blood or bone marrow could affect the brain. The researchers said engineering peripheral immune cells to target amyloid and tau aggregates may offer a preventive strategy for neurodegenerative conditions.