Guardians of the Brain

Many types of immune cells cannot cross the blood-brain barrier, but others patrol the borders and can send in signaling molecules.

The blood-brain barrier protects brains from pathogens and any other potentially harmful agents that aren’t supposed to be there. It also separates the brain from much of the mundane goings-on in the rest of the body – including, it had long been assumed, the immune system. But research conducted over the past 10 years or so overturns that assumption, indicating that immune cells surround the brain and communicate with neurons.

The brain has its own special immune cells, called microglia. Most researchers thought that this was the extent of the interaction between the brain and the immune system. But in 1999, Michal Schwartz...