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Immune Cells Enter Brain

Posted by Finnegan Flynn on 01-09-2026

Healthy Life

The brain is protected more tightly than most organs in the body. One of its main defenses is the blood-brain barrier, which prevents many cells and substances circulating in the bloodstream from entering brain tissue.

Because of that protection, scientists have long questioned where microglia — the brain’s specialized immune cells — come from in humans. New research suggests that, with age, a substantial portion of these cells may originate outside the brain.

The findings indicate that immune cells formed in bоne marrow can enter the aging brain and become part of its immune system.

Tracing Where the Cells Came From

Most immune cells are produced from stem cells in the bоne marrow. As people grow older, these stem cells gradually accumulate unique mutations. Those genetic changes are then passed on to the blood and immune cells they produce. In some cases, certain mutated stem cells begin multiplying faster than others. This process is known as clonal hematopoiesis.

Researchers used these naturally occurring mutations as biological markers. If cells found in the blood and brain carried the same rare mutations, the most likely explanation was that both had come from the same bоne marrow stem cell.

What Researchers Found

The team examined postmortem brain tissue from 20 older adults. In every case, they found brain cells carrying the same mutations that had been identified in the individuals’ blood cells.

This provided strong evidence that cells produced in bоne marrow can enter the human brain. The cells also resembled microglia in several important ways and made up a substantial proportion of the immune cells found in the brain tissue. Older individuals appeared to have more of these bоne marrow-derived cells than younger people.

This suggests that the movement of peripheral immune cells into the brain may gradually increase over the course of a person’s life.

A Transplant Provided More Evidence

The researchers also studied tissue from a person who had previously received a bоne marrow transplant. Microglia-like cells in the brain carried the same genetic markers as the transplanted marrow cells. That result offered additional support for the idea that immune cells originating outside the brain can eventually become established inside it.

According to study co-leader Julia Belk, the research points to a much larger influx of blood-derived immune cells into the aging human brain than scientists had previously recognized. She also noted that the discovery could eventually create opportunities to engineer peripheral immune cells as a way to deliver treatments into the brain.

A Possible Link to Alzheimer’s

After confirming that bоne marrow-derived cells contribute to the brain’s immune population, the researchers explored whether clonal hematopoiesis was associated with Alzheimer’s disease risk.

They analyzed genetic data from tens of thousands of people. Unexpectedly, most forms of clonal hematopoiesis were associated with a lower risk of Alzheimer’s disease. That does not mean clonal hematopoiesis directly protects against the condition. Different mutations and cell populations may have very different effects, and the relationship still needs further investigation.

At this stage, the finding shows an association rather than a proven cause-and-effect relationship.

Why It Could Matter for Treatment

The blood-brain barrier is essential for protecting the brain, but it also creates a major obstacle for medicine. Many potentially useful therapeutic molecules cannot easily reach brain tissue.

If bоne marrow-derived immune cells naturally cross into the brain, researchers may eventually be able to use that route therapeutically. For example, immune cells outside the brain could potentially be modified to carry beneficial molecules and then transport them into areas that are otherwise difficult to reach.

A New View of Brain Aging

For years, microglia were largely viewed as a self-contained population maintained within the brain throughout life. This study suggests the picture is more dynamic in humans. As people age, the brain’s immune system may increasingly include cells that were originally produced elsewhere in the body.

The broader implication is that the aging brain may be far more connected to the body’s peripheral immune system than previously assumed.

Understanding that connection could improve knowledge of normal brain aging and may eventually help researchers design new approaches for neurological and neurodegenerative diseases.