And so, in order to ensure that none of these harmful things actually end up inside our body and cause harm to our body, the lungs have to have some type of protective capability. And this is what we're going to focus on in this lecture. So our lungs have several important methods by which they can actually protect themselves from these different types of agents that enter our body when we breathe in air. So number one is mucus membrane. So inside our passageways, we basically have specialized types of cells known as goblet cells. And these goblet cells can produce and secrete a special type of sticky and slimy material or substance that essentially forms the mucous membrane.