How does the immune system keep tabs on the brain?

Alzheimer’s disease, multiple sclerosis, autism, schizophrenia and many other neurological and psychiatric conditions have been linked to inflammation in the brain. There’s growing evidence that immune cells and molecules play a key role in normal brain development and function as well. But at the core of the burgeoning field of neuroimmunology lies a mystery: How does the immune system even know what’s happening in the brain? Generations of students have been taught that the brain is immunoprivileged, meaning the immune system largely steers clear of it.