A leading scientist's guide to the way our immune system protects us—but only most of the time. What is our immune system, and how does it work? A vast array of cells, proteins and chemicals spring into action whenever our bodies are damaged, but immunity is not something you can see, touch, or feel. It can fight off malicious bacteria and viruses, locate cancerous growths, and even rewire our brains—but sometimes our own tissues can get caught in its crossfire, with catastrophic consequences. Humans may be the most disease-ridden animals on the planet. Professor John Trowsdale shows how the immune system protects us, and how our bodies invest huge resources to keep it running. Immunity influences how we age, and controls how we learn to fight off recurring diseases, and how our bodies respond to chronic conditions such as heart disease and dementia. But, in the case of allergies and autoimmune conditions, it can also easily get things wrong. What the Body Knows is an account of a fascinating phenomenon—one which, for good or for ill, impacts every aspect of our lives.