It turns out that sinuses aren't totally hollow after all; they're lined with a pinkish membrane that manages to produce mucus all day and all night. In fact, your respiratory tract produces about a quart of slime each day. Most of it disappears without a second thought down your throat (yum!), while some glides down through your nose. This mucus is mostly water, though it's also loaded with glycoproteins and enzymes that attack germs.
Obviously you use your nose to breathe, and this is where mucus dons its germ-fighting costume. Breathing invariably means you're sucking up garbage that you definitely don't want in your body, including viruses, bacteria, and fungus -- not to mention dust, dirt, pollen, and other tiny foreign particles.
Fortunately, mucus plays an important role in fighting the filth. In the nose, it acts as a filter that traps smaller particles. These particles are either eventually swallowed, meaning your digestive system makes short work of them, or they're forced out via a cough or a sneeze.