An unexpected role for the brain's immune cells

An important part of the brain's immune system, cells called microglia constantly extend and retract "branches" from their cell body to survey their environment. Think of an octopus, not moving its body, but reaching its tentacles in every direction. That's how microglia operate. In the span of an hour, each cell will have covered the entire three-dimensional space that surrounds it. And then, it will start all over again.