The brain’s borders teem with an army of immune cells that monitor and protect it.

Diana Kwon

The brain is the body’s sovereign, and receives protection in keeping with its high status. Its cells are long-lived and shelter inside a fearsome fortification called the blood–brain barrier. For a long time, scientists thought that the brain was completely cut off from the chaos of the rest of the body — especially its eager defence system, a mass of immune cells that battle infections and whose actions could threaten a ruler caught in the crossfire.

In the past decade, however, scientists have discovered that the job of protecting the brain isn’t as straightforward as they thought. They’ve learnt that its fortifications have gateways and gaps, and that its borders are bustling with active immune cells.

A large body of evidence now shows that the brain and the immune system are tightly intertwined …