Instead, our immune system is spread out throughout the entire body. It is found in many different regions of the body. Now, as we'll see in the next several lectures, what the immune system does is it utilizes a series of internal defense mechanisms that ultimately protect the cells of our body from pathogens. And a pathogen is any type of agent, it can be living or non living that brings harm to the cells of our body. So a bacterial cell, a virus, a chemical that can harm our cells, all these different things are considered to be pathogens. Now, for a pathogen to actually cause damage, cause harm to the cells of our body, these pathogens must enter our body.