The big culprit on faces, though, isn’t dirt so much as an internally-produced substance called sebum that makes the skin cells shed inside the pores. These dead cells, unable to flake away, collect until they become an irritation to the pore. At this point the body reacts in a miniature version of the way it deals with any other invasion: it sends white blood cells to the site to attack the invading body. The result is a mild inflammation at the scene of battle. That inflammation is the zit. So evolution, while having cleaned our hairy faces, gave us pimply skin instead.