A Stanford Medicine study published in Nature claims that blood immune cells brain ageing is not the isolated, one-way story textbooks have described. Blood-borne immune cells enter the human brain in substantial numbers as people age, where they appear to transform into microglia, the brain’s own specialised immune cells. If the finding holds up under further scrutiny, it would overturn a foundational assumption about how the brain’s immune system works.
The claim and what the evidence actually shows
The central assertion is this: the brain is not, or at least is not always, the immunologically closed system it was long assumed to be. Julia Belk, a postdoctoral scholar in pathology at Stanford Medicine and first author of the study, put it plainly. ‘We usually think of the brain as a closed system,’ she said. ‘What we found is that actually a lot of immune cells enter the human brain during aging.’
To establish this, the researchers needed to trace where the immune cells inside brain tissue had actually come from. The approach they used was genetic. Because blood stem cells accumulate random mutations over a person’s lifetime, and because immune cells produced by those stem cells inherit the same mutations, matching mutation signatures between blood and brain samples can serve as a marker of shared origin. Belk compared the logic to a consumer ancestry testing service. ‘If we see the same mutations in the blood and in the brain’s microglia, then we can be very confident that immune cells in the brain are descendants of those immune cells in the blood,’ she said.
The team worked with paired blood and brain samples from two sources: the Stanford Rapid Autopsy Center, led by co-author Jody Hooper, a professor of pathology at Stanford Medicine, and the University of Washington’s Alzheimer’s Disease Sequencing Project. When they compared the genetic signatures, they matched. The results indicated that peripheral immune cells had entered the brain, with the process beginning as early as middle age. Those cells then appeared to convert into microglia.
According to NIH, the study was published in Nature on 30 July 2026. The work received support in part from the Knight Initiative for Brain Resilience at the Wu Tsai Neurosciences Institute.
How blood immune cells and brain ageing came to intersect
The path to this finding was not straightforward. Siddhartha Jaiswal, a senior author and associate professor of pathology at Stanford Medicine, had previously examined genetic data from thousands of people, some followed for decades. That earlier work found that people carrying certain clones of immune cells produced by mutated blood stem cells were much less likely to develop Alzheimer’s disease. The mutations in question are associated with a condition called clonal haematopoiesis of indeterminate potential, found only in a minority of people.
The result was unusual enough to prompt a broader question. If these atypical immune cells could interact with the brain, could ordinary blood-derived immune cells be entering the brain more generally, as a routine feature of human ageing? ‘Unlike most immune cells, which are continuously replenished by blood stem cells from the bone marrow, immune cells in the brain were presumed to renew themselves throughout the lifespan without contribution from outside the brain,’ Jaiswal said. ‘Our first study showed that this might not always be the case.’
Co-senior author Howard Chang, the Virginia and D. K. Ludwig Professor of Cancer Research and a professor of genetics at Stanford Medicine, joined Belk and Jaiswal in investigating why peripheral immune cells appeared to increase resilience to Alzheimer’s. Before that question could be addressed, however, the team needed to establish the more basic point: whether peripheral immune cells could replenish microglia at all. That is what the current study set out to test.
One detail the researchers flag is that this pattern of peripheral immune cells converting to microglia does not appear to occur in mice or non-human primates. Belk described it as ‘a uniquely human feature of aging that we had no idea about.’ That distinction matters methodologically, since much of what neuroscientists know about brain immunity comes from animal models.
What the researchers say could follow
The team is careful to frame the therapeutic possibilities as speculative, not imminent. Belk suggested that, now the entry of peripheral immune cells into the brain appears established, it becomes possible to consider engineering such cells to perform useful functions, for instance targeting the amyloid and tau aggregates associated with neurodegenerative diseases. Such an approach might eventually be used preventively, before those aggregates accumulate. Jaiswal added a broader point: ‘Our findings suggest that the life history of blood stem cells could influence the risk of brain diseases by altering the microglia.’
These are possibilities the team raises, not conclusions from the current study. The study itself demonstrates the migration and transformation of peripheral immune cells in human brain tissue. Whether that process can be harnessed therapeutically is a separate question, and one the data here does not yet answer. The methodology, using somatic mutation signatures as lineage tracers in post-mortem human tissue, will now face scrutiny from other groups working in brain immunology.
