A healthy person’s nose has to deal with several hundred milliliters of snot every day. But we don’t have a runny nose every day. Where does so much of it go? A small part evaporates, a small part dries up and becomes boogers, but most of it, is swallowed by us. The mucous membrane of the nasal cavity is covered with cilia, and these cilia will swing from front to back, and the snot will be sent backward to the pharynx. Because the nasal cavity and the esophagus are connected, most of the snot is swallowed by us unknowingly. This sounds disgusting, but it is not harmful to the human body. In addition to water, nasal snot consists of proteins, carbohydrates, salt and some shed cells. The protein in nasal snot is mainly mucin, which is a glycoprotein wrapped in a “sugar coating” made up of carbohydrates, which allows it to absorb a lot of water. Other proteins in nasal snot include antibodies and lysozyme, which are capable of killing bacteria and viruses. These components are digested and absorbed by the stomach and intestines as nutrients. Of course, nasal snot also contains stuck dust, pollen and microorganisms, but these impurities can be dealt with by stomach acid and will not cause trouble to the body. Some of the snot is actually tears. The tear ducts in the eyes also produce tears to moisten the eyes all the time, and the reason why we don’t have teary eyes all day is that these tears flow from the tear ducts connecting the eyes and the nasal cavity into the nose and become part of the snot. If you cry a lot, part of the tears flow out from the corner of your eyes, most of them still pour into the nasal cavity, making your nose “sob”, and you have “a snotty nose and a tear”. But most of the snot is secreted by the nasal mucosa itself. The nasal mucosa contains cells shaped like a goblet – hence the name goblet cells. The cup-shaped cells produce a lot of mucin, which is released to the outside of the cell and absorbs a lot of water, expanding 600 times in size. The cup cells only need to produce 1 ml of mucin a day, which is enough to meet the normal needs of the nasal cavity. If the nasal cavity is irritated or infected, the production of nasal snot will surge, which is natural because one of the main functions of nasal snot is to remove the inhaled impurities. For example, if a cold virus invades the nasal cells, or if an allergic person inhales pollen or dust, the immune system will create the appropriate antibodies to try to destroy these antigens. The antibodies are distributed on the surface of the mast cells in the nasal cavity, which contain a large amount of an active substance called histamine, and the combination of antigen and antibody stimulates the mast cells to release histamine. Histamine in turn stimulates the cupped cells to make more mucin, which also produces more nasal discharge. At the same time, histamine also causes vasodilation and increased permeability, water leaks out of the blood, and white blood cells follow to destroy pathogens. This not only further increases the amount of nasal mucus, but also leads to nasal blockage. Part of the excess nasal mucus flows out, and part of it gets blocked in the back. So a stuffy, runny nose is actually an allergic reaction to the discomfort that the immune system creates for us. Histamine needs to bind to the histamine receptors on the cell surface to have these effects, so if you can keep histamine from binding to its receptors, you can reduce the symptoms of a stuffy, runny nose. Anti-allergy and cold medications often use these types of histamine antagonists, such as chlorpheniramine maleate (also known as paracetamol), which compete with histamine and compete to bind to its receptors, so that histamine binding is not possible and the allergic reaction is suppressed. Histamine antagonists are often used together with decongestants like pseudoephedrine, which constrict the blood vessels in the nasal cavity, thus reducing nasal congestion. Mixing histamine antagonists, decongestants, and antipyretics (such as acetaminophen, also known as paracetamol) and cough suppressants (such as dextromethorphan) together makes a very effective combination cold remedy. The composition of the famous cold medicines on the market (such as Tylenol, white plus black) are basically the same, super not these four ingredients. Normal nasal discharge is colorless and transparent, which is also called clear nasal discharge. At the beginning of the cold, the runny nose is also clear, after that it becomes thicker and becomes white. Later on, the nasal discharge may be green and thick, which looks like pus, especially if there is a secondary bacterial infection. Why does the snot become green? It is the same as pus because it contains a large number of neutrophils. Although neutrophils are white blood cells, they are green in color. Neutrophils are the first to arrive at the battlefield when the immune system finds out that a pathogen has invaded the body and starts mobilizing its troops. Neutrophils are sent by the blood, but they run to fight outside the blood vessels. It is a phagocytic cell that fights by “eating” the bacteria and killing the pathogens with various weapons inside the cell. One of these weapons is the release of hypochlorous acid (the main component of bleach), a disinfectant, into the bacteria. Hypochlorous acid is made by myeloperoxidase in neutrophils. Myeloperoxidase has a structure in common with chlorophyll, both contain a dihydroporphyrin ring, and this particular structure determines their color: green. Therefore, the thick nose will be green because it contains a lot of neutrophils, which in turn contain a lot of green myeloperoxidase.