The immune system is the part of our body which protects us from infection. Let’s imagine the cells that make up the immune system are a large and diversified police force. In a perfect world the policing of the streets would work perfectly with a high level of law and order, and whatever crime there was would be dealt with swiftly and efficiently. Even larger disturbances would not unduly affect the rest of society because of efficient policing. Law-abiding citizens would be left alone by the police to live in peace.