So what, exactly, IS your immune system? Second only to the human brain in its complexity, it is one of the oldest and most critical facets of life on Earth. Without it, you would die within days. In this book, Philipp Dettmer, the brains behind the most popular science channel on YouTube, takes readers on a journey through the fortress of the human body and its defences. Each chapter delves deeply into an element of the immune system, including defences like antibodies and inflammation as well as threats like viruses, bacteria, allergies and cancer, as Dettmer reveals why boosting your immune system is actually nonsense, how parasites sneak their way past your body's defences, how viruses - including the coronavirus - work, and what goes on in your wounds when you cut yourself.