With its sticky icky grossness, you may wonder why mucus exists in the first place. But this annoyance of cold and allergy season plays a very important role in keeping our bodies healthy. Dr. Michael R. Knowles, professor of pulmonary and critical care medicine at UNC Chapel Hill, explains the many attributes of this slick substance.
Q: Why do we need mucus?
Its main job is to protect the body from irritants like bacteria, viruses or allergens. Mucus is made up of water and the protein mucin, which is one of the most complex biological molecules on the face of the Earth. The body can put any of 400 sugars or moieties or side chains on the surface of the mucin protein backbone, creating an almost infinite variety of sites that can bind up your offending substance of choice.
Q: Where does all the mucus come from when you are sick?
First, the airways don't have pain fibers, but they do have sensory arcs, which are nerve connections that can respond to irritants or stimulants. So if you eat hot chili peppers, these sensory arcs cause your airways' submucosal glands to secrete mucus.
Mucus also comes from specialized goblet cells, which secrete tightly packed "dry" granules of mucin into the airway, which absorb liquid and expand a thousandfold in volume when an irritant comes along. The point of that expansion is so the airway can be coated with mucus, which sticks to anything irritating like bacteria or viruses or pollen.
Then the bodily function mucociliary clearance sweeps the contaminated mucus out of the airway (into the back of the throat where it is swallowed), protecting it from the irritants and infection.
Q: So why are there different colors for different types of illness?
If you have inflammation in the airway then immune cells called neutrophils or eosinophils or macrophages come into the airway, do their job and then die, leaving behind their DNA and cellular protein that is yellow. If in addition to that you had an infection with bacteria - Pseudomonas is the classic example, it makes a pyocyanin that is green - then the color of the mucus also reflects the products of the bacterial organisms.