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The immune cells of the brain help reduce fear after repeated safe exposure to a once-threatening place.

Microglia monitor neurons respond to activity and help reshape brain circuits.

The findings may help explain why fear memories persist in conditions such as PTSD.

Fear protects us by teaching the brain what to avoid. After a frightening experience, a sound, place, smell, or image can become linked with danger. That link can save a person from future harm. But when the danger has passed, the same system can become painful. In posttraumatic stress disorder (PTSD), reminders of trauma can trigger fear even when a person is physically safe.

A new study in Nature Neuroscience offers a closer look at how the brain learns that a feared cue no longer signals threat. The study focuses on microglia, a type of immune cell that lives in the brain. Microglia act like caretakers of the nervous system. They scan nearby tissue, respond to inflammation and injury, communicate with neurons, and help shape the connections between brain cells. Although scientists once viewed them mainly as defenders against disease, newer work shows that microglia also help regulate learning, memory, and behavior.

The study examined fear extinction, the process through which fear decreases after repeated safe exposure to a once-threatening situation. In therapy for anxiety and trauma,........

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