The human body is like an exceedingly well-fortified castle, defended by billions of soldiers – some live for less than a day, others remember battles for decades, but all are essential in protecting us from disease. This hidden army is our immune system and without it we could not survive the eternal war between us and our microscopic enemies. Immune explores the incredible arsenal that lives within us – how it knows what to attack and what to defend and how it kills everything from the common cold to the plague bacterium. We see what happens when the immune system turns on us and conversely how impossible life is without its protection. We learn how diseases try to evade the immune system, how they exploit vulnerabilities and even subvert it to their own advantage and we discover how scientists are designing new drugs to harness the power of the system to advance medicine in the 21st century