Whether white blood cells can be found in the brain has been controversial, and what they might be doing used to be a complete mystery. In a seminal study published in Cell, an international team of scientists led by Prof. Adrian Liston (VIB-KU Leuven, Belgium & Babraham Institute, UK) describe a population of specialized brain-resident immune cells discovered in the mouse and human brain, and show that the presence of white blood cells is essential for normal brain development in mice.