For decades, the prevailing dogma in neuroscience held that the human brain was an immunological fortress, a "closed system" largely insulated from the immune defenses operating throughout the rest of the body. Protected by the formidable blood-brain barrier—a specialized network of vessels that restricts the movement of substances and cells from the bloodstream into the delicate neural tissue—the brain was thought to rely exclusively on its own dedicated immune cells, known as microglia. These resident cells, established during early development, were believed to be self-sustaining, remaining as a static population that renewed itself throughout an individual’s entire lifespan without any outside assistance.
New research from Stanford Medicine, however, is fundamentally challenging this long-held picture. In a study recently published in the journal Nature, researchers discovered that, contrary to established belief, large numbers of immune cells from elsewhere in the body actually infiltrate the human brain as people age. This unexpected migration suggests that the brain’s immune landscape is far more dynamic than previously imagined, a discovery that could reshape our fundamental understanding of brain aging and open entirely new frontiers for treating devastating neurological conditions. The research was supported in part by the Knight Initiative for Brain Resilience at the Wu Tsai Neurosciences Institute.
"We usually think of the brain as a closed system," says Julia Belk, a postdoctoral scholar in pathology at Stanford Medicine and the first author of the new study. "What we found is that actually a lot of immune cells enter the human brain during aging."
An Unexpected Path Into Brain Research
The discovery is the culmination of an interdisciplinary journey that bridges computer science, immunology, and neuroscience. Belk’s path to this finding was far from conventional; she began her academic career as a graduate student in the Department of Computer Science at Stanford Humanities and Sciences. Her training was further enriched by the Sarafan ChEM-H’s Chemistry/Biology Interface Predoctoral Training Program, an experience she credits with fostering a unique approach to research that synthesizes the principles of basic science, computational analysis, and medicine.
This interdisciplinary background proved vital when Belk began collaborating with Siddhartha Jaiswal, a senior author of the study, an associate professor of pathology at Stanford Medicine, and a member of the Institute for Stem Cell Biology and Regenerative Medicine. Their partnership grew out of previous, foundational research where the team analyzed genetic data from thousands of individuals, some of whom had been monitored for decades. In those earlier studies, the researchers made a striking observation: people carrying specific clones of immune cells—derived from mutated blood stem cells—appeared to be significantly less likely to develop Alzheimer’s disease.
This correlation sparked a provocative question: Were these unusual immune cells simply acting on the periphery, or were they interacting directly with the brain? Further investigation revealed evidence that some of these mutant cells could indeed cross the boundary and enter the brain tissue itself. The mutations in question are associated with a condition known as clonal hematopoiesis of indeterminate potential (CHIP), which, while present in only a minority of the population, served as a "smoking gun" for the research team. If these specific mutant cells could infiltrate the brain, the team reasoned, perhaps it was not an isolated event, but rather a standard, yet previously overlooked, feature of human aging.
"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 explains. "Our first study showed that this might not always be the case."
Challenging a Longstanding View of Microglia
For years, the scientific community operated under the model that microglia were a fixed population, born in the brain and remaining there permanently. Under this paradigm, it was widely accepted that immune cells originating in the body’s peripheral system would never migrate into the brain to integrate into the microglial population. However, Belk and her colleagues began to consider a more fluid reality. If peripheral immune cells could enter the brain in some individuals, the team hypothesized that the process might not be an anomaly at all. Instead, they suspected it might be a routine, natural feature of the human aging process.
The suggestion that blood-based immune cells could play a tangible role in brain health—particularly in the context of Alzheimer’s disease—was both groundbreaking and initially met with skepticism. In 2022, seeking to push the boundaries of current neurodegenerative research, Jaiswal and his colleagues sought support from the Knight Initiative for Brain Resilience. This initiative specifically funds high-risk, high-reward research intended to rethink how scientists approach brain resilience and the biological mechanisms of neurodegeneration.
With the aid of a Knight Initiative Innovation Award, Belk, Jaiswal, and co-senior author Howard Chang, the Virginia and D. K. Ludwig Professor of Cancer Research and a professor of genetics at Stanford Medicine, embarked on a rigorous investigation to determine why peripheral immune cells seemed to enhance resilience to Alzheimer’s. Before they could solve that puzzle, however, they first had to prove the foundational premise: that immune cells from the blood truly could, and do, replenish the brain’s resident microglia.
Tracing Immune Cells From Blood to Brain
To investigate this phenomenon, the team turned to human brain tissue, utilizing samples from the Stanford Rapid Autopsy Center, led by co-author Jody Hooper, as well as tissue from the University of Washington’s Alzheimer’s Disease Sequencing Project. These programs are invaluable to researchers because they collect both blood and post-mortem brain tissue from the same individuals, allowing the team to conduct a direct, comparative analysis of immune cells circulating in the blood versus those residing in the brain tissue after death.
The primary challenge was one of lineage. To determine if the immune cells inside the brain were indeed "immigrants" from the blood, the researchers had to trace their cellular ancestry. They needed a way to distinguish cells descended from the original microglia population—present since birth—from those that had arrived later from the bone marrow. They accomplished this by using a sophisticated genetic technique: comparing DNA from immune cells in the blood with DNA from immune cells in the brain, looking for shared mutations as biological markers of ancestry.
As people age, blood stem cells naturally accumulate random mutations. Immune cells produced by these stem cells inherit these same genetic markers. By comparing these "genetic signatures," the team could treat these mutations like a genealogical map. "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," Belk notes.
Using techniques they refined during their 2023 research, the team compared paired samples of blood and brain tissue. The results were conclusive: the genetic signatures matched. Not only were peripheral immune cells entering the brain, but this migration was occurring as early as middle age. Furthermore, the researchers discovered that once these peripheral immune cells crossed into the brain, they underwent a transformation, effectively turning into specialized microglia. Interestingly, the team noted that this process appears to be a uniquely human development, as it does not seem to occur in other species, such as mice or non-human primates.
A Possible New Route for Brain Immunotherapy
The implications of this finding are profound. By shattering the idea that the brain is an isolated, closed system, the researchers have opened a new door for potential medical interventions. If immune cells from the body can be recruited into the brain, they could potentially be harnessed as a therapeutic tool.
"Now that we know that these immune cells actually can get into the brain, we can think about all kinds of new engineering strategies to have those peripheral immune cells do useful things," says Jaiswal.
One visionary possibility is to engineer these peripheral immune cells to specifically target and clear the amyloid and tau protein aggregates that characterize neurodegenerative diseases like Alzheimer’s. If scientists could successfully program these cells to recognize and degrade toxic buildup, such therapy could potentially be administered to patients preventively, long before the onset of cognitive decline.
Furthermore, this discovery underscores the critical importance of the systemic environment. Because it now appears that many microglia in the aging human brain originate from blood stem cells, the health and "life history" of an individual’s blood stem cells may have a direct, causal link to their risk of developing neurological disease. Anything that impacts the blood or bone marrow could, by extension, influence the brain’s ability to defend itself.
For Belk, the significance of the study lies not just in the potential for future treatments, but in the revelation of a fundamental human trait. "I think this is exciting because this is also a uniquely human feature of aging that we had no idea about," she says. As research continues, the team’s findings promise to shift the focus of neurodegeneration studies from the brain alone to the complex, systemic interactions between the blood and the central nervous system.
