A4. To expel the viruses invaded into our body.
Immune is a system to attack foreign substances invading into the body and it is white blood cells in blood. White blood cells patrol all over the body. When the white blood cells patrol the nose and find foreign substances, they secrete histamine. Histamine goes to the brain flowing in blood stream and orders the brain to secrete more nasal mucus.
A5. Inside of the nose becomes swollen because mucus membrane in the nose absorbs nasal mucus like a piece of bread soaked in milk. That makes the space in the nose narrower, resulting in stuffy nose.
If you rub your eyes and ears with the finger holding viruses, the viruses invade into your body.