Your sinuses also produce mucus that lubricate your nasal passages and keep your nose moist. They are integral to the process by which your nasal immune system gets rid of potential threats. [3] Small hairs called cilia help mucus travel through your sinus cavities, filtering out unwanted substances in the process. The mucus then drains into your nasal passages and down the back of your throat. [4] Once you swallow it, mucus and all its contents end up in your stomach, where bacteria and other pathogens are neutralized by stomach acid and cell debris is recycled. [5]