Your First Line of Defense Against Viral Infections
By. Dr. Michael Murray – March 19,2020
The worldwide pandemic due to the novel COVID-19 coronavirus has brought focus to prevention – both in terms of reducing the risk of infection through “social distancing” and good hygiene as well as measures to bolster the immune system. But even before our immune system gets in contact with a coronavirus or any microorganism, there are natural barriers to infection.
For any virus to infect the throat, sinuses, airways or lungs it must first pass through or enter the body through the mucous membrane. It is the first barrier to infection; the immune system is the second line of defense. There are two routes for COVID-19 to enter the lungs and cause serious damage. The primary route is through the respiratory tract, the other is through the gastrointestinal tract.
The Importance of the Mucous Membranes in Our Airways
The respiratory tract mucous membrane that lines our airways is the first line of defense against COVID-19. It consists primarily of cells known as ciliated epithelial cells. These cells have external surface covered by hair-like structures called cilia and act like brushes to move the respiratory tract secretions, microorganisms, and debris up and eventually out the nose or mouth.
On top of the ciliated epithelial cells are two layers of mucus. The mucus is produced by another type of epithelial cell called a goblet cell. The mucus is composed of mucin, which refers to a network of proteins complexed with sugars.
In the lungs, there are only very thin epithelial cells, connective tissue, and blood capillaries all designed to perform the function of delivering oxygen to the blood and exchanging it for carbon dioxide. When particulate matter or microorganisms make it to the lungs it is a very serious situation as there is very little protection there.
The importance of the health of the mucus and the lining of the airways in preventing COVID-19 infection cannot be overstated as conditions associated with poor functioning of this line of defense is associated with an increased risk of more serious infection.
Preventing the Gastrointestinal Route of Infection
The secondary route of COVID-19 entering the body is through the gastrointestinal tract. Within the GI tract, there are a number of protective factors beyond the mucus lining. The most notable additions are digestive secretions such as stomach acid and digestive enzymes. The immune system structure in the gut is also much larger. If COVID-19 is able to avoid these protective factors and infect the GI tract it is able to enter the bloodstream and also infect the lungs.
What Determines the Severity of COVID-19 Infection?
The difference between a mild vs. a severe COVID-19 infection appears to be based on a few things. Most important the viral load that a person is initially exposed to. If exposure is a small dose of COVID-19, in most cases in healthy individuals it will lead to mild or moderate symptoms. If the viral load a person is exposed to is quite high, then it greatly increases their risk for a more severe infection. That is why healthcare workers are especially vulnerable.
Another factor that determines the severity of COVID-19 may be the ability of the virus to travel down the respiratory tract into the lungs. A respiratory tract viral infection generally starts in the nose and travels down the airways. The deeper it goes, the more serious or severe the infection. Remember the lung cells have little protection. During a viral infection of the lungs, not only are the lung epithelial cells damaged by the infecting virus, but they are also damaged by the body’s immune response to the infection. If the response and clean up by the immune system are quick, the infection can be contained and cleared in a few days. But if the immune response is either insufficient or overly aggressive, it can lead to significant damage.