The cells in the mouth and nose do not have the tough protective barrier that skin on the outside of the body has. They are instead protected by mucus.
Mucus is water made thick and sticky by the addition of proteins that have simple sugars attached. These are called glycoproteins. They help the mucus to stick to the walls of the mouth and nose, and they trap bacteria, and particles so that they don’t get into the lungs. The lungs also have a mucus layer to deal with anything that gets past the mouth and nose.
Anywhere we don’t have skin there is a layer of mucus that protects the cells from bacteria, fungi, and viruses.
Mucus lubricates the food we eat, so it is easier to chew and swallow. When you see, smell, or even think about food, your salivary glands start to produce fluids in anticipation of the need to moisten and lubricate the food you are about to eat.
The cells that line your nose and throat contain cells that constantly move mucus towards the back of the throat, where it can be swallowed, along with any particles and bacteria it has trapped.
Besides trapping bacteria, mucus contains enzymes that break open bacterial walls. The enzyme lysozyme and the protein lactoferrin are part of what is called the innate immune system. This is the system that works against any bacteria that comes along. The adaptive immune system recognizes germs so we don’t get the same disease again.
Lactoferrin has oxidized iron in it (rust) that makes it look red when purified (but the amount in saliva is so small it does not have a color). Lactoferrin binds to bacterial cell walls, and the oxidized iron produces peroxides that break down the walls and cause the bacteria to leak.
Lactoferrin also attracts white blood cells to the bacteria, so they get eaten up. Its main function elsewhere in the body is to ferry iron around to where it is needed. But it has many other functions. In saliva, it binds iron that bacteria need to live, so it is not available to them, it binds to the cell walls, it intrudes inside the bacterium, it releases peroxides, and it interferes with the enzymes in the bacteria or fungus.
Lactoferrin also binds to the same sites on cells that viruses do, so it blocks viruses from entering cells. It also binds directly to some viruses, and it can prevent viruses from growing inside cells, by attracting natural killer cells and macrophages to come along and kill the infected cell.