The human immune system is the most complex biological system we know, after the human brain, and yet, most of us never learn how it works. Or what it is. Your immune System consists of hundreds of tiny and two large organs, it has its own transport network spread throughout your body. Every day it makes hundreds of billions of fresh cells. It is not some sort of abstract entity. Your immune system is YOU. Your biology protecting you from the billions of microorganisms that want to consume you and from your own perverted cells that turn into cancer. It's so manifold that it is impossible to cover in one video, so we’ll make a series looking at different aspects of it. Today, what happens when your body is invaded and your first lines of defenses are engaged in a fight for life and death?