This documentary TV episode opens our eyes to the inner workings of the immune system, using the powerful stories of four people to highlight our bodies' natural defenders.

Throughout human history, our bodies have been tasked with adapting to their environments in order to help the species survive.

And in a world full of potentially dangerous organisms, the immune system is still the human body's best defense.

Human: The World Within is a series that explores the ways in which different people live their lives and the complex biological processes that support them.

Directed by award-winning filmmaker Stephen David, known for historical documentaries like Sons of Liberty and The Men Who Built America, this docuseries uses personal stories and expert interviews to shed light on our species' unique physical makeup.

In this episode, we take a journey into the immune system, and the ways our bodies protect us.

Through the stories of a wilderness teacher, a pair of twins, a cancer survivor, and a doctor who managed to fight off ebola, we see how the immune system protects us in incredible ways.