Microglia are very long-lived immune cells that only occur in the brain. They have a protective effect on the brain by eliminating waste and toxic substances through phagocytosis and pinocytosis. Therefore, they are counted among the tissue macrophages. Microglia are suspected to play a central role in neurodegenerative diseases. Scientists from the Hertie Institute for Clinical Brain Research (HIH), the German Center for Neurodegenerative Diseases (DZNE) and the University of Tübingen have collaborated with colleagues to investigate whether these immune cells change over time due to external factors in an animal model what effect this has on the health of the brain. “Epidemiological studies have shown that infectious disease or life-span inflammation can affect the severity of Alzheimer’s disease that occurs much later. We therefore wondered if an immune memory in the long-lived microglia could mediate this risk, “explains study leader Dr. med. Jonas Neher (HIH). So far, it was unclear whether microglia remember a previous inflammation. The working group was able to show this in laboratory mice. The scientists triggered an inflammation outside the brain. Depending on how often they repeated this process, they could cause two different states in the microglial cells of the brain: “training” and “tolerance”. By the first inflammatory activation the immune cells were trained – at the next they reacted more strongly. After four activations, the researchers noticed a tolerance, the microglia barely responded.