A groundbreaking study from Stanford University has challenged a long-standing assumption about the brain’s immune system. Researchers found that as people age, substantial numbers of immune cells from elsewhere in the body infiltrate the human brain. For years, many believed that the brain’s specialized immune cells (microglia) were established at birth and remained a self-sustaining population throughout life. However, the Stanford team used a novel approach: they compared immune cells from paired blood and brain samples, looking for shared mutations that trace back to a common origin. They discovered that immune cells from the blood did enter the brain, with this process beginning as early as middle age. Furthermore, once inside, these peripheral immune cells transformed into specialized microglia. The researchers noted that this process does not appear to occur in other species, such as mice or non-human primates. They suggested that the life history of blood stem cells could influence the risk of brain diseases by altering the microglia.
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