New research reveals that around age 50, the brain starts replacing its long-standing immune cells with more inflammatory ones. This discovery offers insight into how normal aging might lead to dementia, particularly Alzheimer’s disease.

Honestly, this surprised me, Furhan Qureshi, a board-certified internal medicine physician, shared with Newsweek. He explained that medical training taught that the brain’s main immune cells are present before birth and remain, rejuvenating throughout life. The study suggests otherwise, indicating that the hippocampus, a critical brain area for memory, swaps these cells for inflammatory ones entering from the bloodstream.

Qureshi mentioned that the study could change our understanding of Alzheimer’s origins. He noted that while inflammation in Alzheimer’s cases is well-known, the study suggests it might be a result of normal aging, occurring long before visible symptoms. The brain’s immune environment is quietly changing under the surface for decades first, he said.

What the Study Found

The National Institutes of Health funded the research, focusing on the hippocampus. Experts from the University of California, San Diego, the New York Genome Center, and the University of California, Irvine, examined postmortem hippocampal tissue from 40 healthy adults aged 20 to 95 using advanced single-cell techniques.

They discovered that microglia, the brain’s main immune cells, decline from age 50 to 75. As these cells decrease, they are replaced by more inflammatory cells resembling monocytes, which are typically found in blood.

This finding challenges the belief that microglia develop during embryonic stages and then continually renew in the brain.

Researchers combined traditional gene activity measurements with modern tools that map the three-dimensional genome structure and its chemical modifications, uncovering changes in cell identity and origin. Additionally, early signs of decline were noted in cells maintaining the blood-brain barrier.

What This Means for You

Aging is the strongest risk factor for neurodegenerative diseases, and the hippocampus is especially vulnerable. It’s estimated that 42 percent of Americans over 55 will develop some form of dementia.

Previous studies had linked aging with increased inflammatory gene activity, but gene expression alone didn’t fully explain the decline.

Ankit Chawla, a longevity and functional medicine physician, sees the study aligning with observations in his field. From a longevity perspective, the immune system is critical in aging processes, particularly in the brain, he said.

The study further shows that aging deteriorates the brain cell types’ 3D genomic structure and a marked decline in astrocytes, which support brain signaling and metabolism.

For further details, Newsweek reached out to the study’s authors.

Reference: Zemke, N. R., et al. (2026). Epigenetic and 3D genome reprogramming during the aging of the human hippocampus. Science. https://doi.org/10.1126/science.adt8307.