Our brains are protected by our skull. But that is not the only protection it has. A natural barrier around our brains prevents germs and other damaging substances in the blood from entering the brain. Doctors call this protection the blood-brain barrier. However, it does more than protect. This barrier also stops medicines from entering the brain. Doctors have trouble using drugs to treat brain tumors, Alzheimer’s and other diseases affecting the brain. So researchers have been trying to find safe ways to get drugs through the barrier. At Imperial College, London, researchers have been performing experiments with mice. Doctor James Choi is a leader of this research effort. He describes the blood-brain barrier as a secure entry point -- what he calls border patrol. This keeps out harmful germs or pathogens.