Biology key to understanding virus
Ever wondered how something too small to see could send our nation into lockdown? The answer lies in the form and function of the miniscule virus pseudo-organism.
What exactly is a virus?
When compared to other infectious agents such as bacteria or yeast, a virus is small and fairly simple. Viruses consist of two basic parts: the viral genome and the protein capsid, according to Neal Rote in the textbook “Understanding Pathophysiology.” The virus’s two-part design is very different from the cell, which is the basic unit of life.
Viruses are non-cellular infectious agents. A cell composes all other unicellular and multicellular organisms including the infectious agents of bacteria and yeast.
The viral genome, its genetic information, is composed of nucleic acids, which are the same types of molecules that make up the genomes of other organisms such as humans or plants. Viral genomes uniquely can be made of DNA or another type of nucleic acid called RNA unlike cellular organisms which are all DNA.
Surrounding the virus is a protein capsule that protects the viral genome. More specifically, this is called the “capsid.” Fortunately, the capsid can be dissolved with soap and water. The viral capsid is a layer of protein that surrounds the genome. In addition to protecting the viral genome, the capsid also has specific protein molecules displayed on its surface, according to Gerald Karp in his book “Cell and Molecular Biology.”
Before the virus enters a cell in the infection process, it is called a virion and is basically a package of very specialized biomacromolecules arranged in a specific order. Because virions are not cells, they lack the ability to metabolize or reproduce and consequently are not considered living organisms, according to Karp. However, they are able to enter other cells and highjack the cellular machinery to perform these reproductive and metabolic functions for them. The cell suffers from a viral infection when it is highjacked by a virion in this manner.
How does a virus infect organisms?
Understanding a viral infection requires understanding the life cycle of a virus. In order to reproduce, a virus must first identify and enter a target cell. The receptor proteins on the virus connect to the receptor proteins on the target cell like a lock and key. This protein-receptor binding is very specific and dictates what kind of cells a virus is able to infect, according to Rote.
Once the virus has identified its target cell, it can enter the cell either by fusing to the cell’s membrane or by wrapping its membrane around the virus in a process called endocytosis. Either way, once the virus has entered the cell, it proceeds to either immediately infect the cell or to integrate the information stored in its genome into the genome of the host cell. If a virus integrates into a cell genome, it can remain latent there for extended periods of time before a specific stimulus causes it to become active, according to Karp. This is why diseases such as HIV and other viruses can be asymptomatic, sometimes for decades.
When a virus immediately infects the cell, its proteins and nucleic acids take over the cell’s metabolic machinery. Instead of allowing the cellular proteins to carry out their normal metabolic activities that keep the cell alive and functioning, the virus uses these cellular proteins to reproduce itself, making copies of its own viral proteins and replicating the viral genome, according to Rote. This is the process used by the coronavirus SARS-CoV-2 which causes COVID-19.
As these viral molecules are produced, they begin to reassemble into more viral units. Eventually these newly minted virus units leave the host cell and seek other cells to infect and repeat the process, leaving a path of cellular damage and death in their wake. In SARS-CoV-2, the path of cellular damage in the lungs has been compared to a “ground glass” opacity appearance in chest imaging.
How does the human body fight a viral infection?
The human body has three main levels of resistance against infections, viral or otherwise. First, the body uses physical and biochemical barriers to infection, according to Rote. Physical barriers include the skin, which forms a tough, impenetrable barrier to most infectious agents. Biochemical barriers include digestive enzymes in the mouth and an acidic environment in the stomach, both of which can kill infectious agents that enter the gastrointestinal tract.
However, often these barriers are not enough. The human body’s second level of protection involves the inflammation response. This response to infection includes white blood cells migrating to the site of infection where they perform a variety of different functions including attacking infectious agents. Other cells involved with the inflammation response can activate healing processes, cause swelling, or elevate temperature, according to Rote.
Often a virus is able to evade these first two levels of protection in the body and infect specific types of cells within the body. Once this has happened, the third line of defense, called adaptive immunity, becomes involved with fighting the infection.
Adaptive immunity involves two steps: clonal diversity and clonal selection, according to Rote. Clonal diversity involves specialized immune system cells called B cells. Different B cells produce many different, unique receptor proteins on the surface of their cell membranes, according to Karp. These receptors are able to bind to a wide variety of different proteins, including many proteins that may be present on the surface of infectious agents such as bacteria or viruses. In contrast with the first two lines of defense, adaptive immunity is only induced by the presence of an infection, according to Rote.
During the process of clonal selection, the diversity of receptor proteins produced by B cells allows one of the B cell receptors to bind to the proteins expressed on the surface of a virion. Once a B cell has bound to the virus, it is able to recognize the virus. The B cell then begins to multiply into several different types of cells and activate other immune cells, all of which focus on fighting the infection caused by the specific virus recognized by the original B cell. Interestingly, early observations of COVID-19 show females are fighting the virus better than males, for reasons not yet fully understood.
How does a vaccine help build immunity to viral infection?
Vaccines seek to induce the body’s adaptive immune response without creating a full viral infection, according to Rote. Inserting into the body a deactivated, harmless form of the virus allows the body to recognize the virus and build up a specific immune response. In this way, the body is able to better fight the actual virus when it is encountered.
The virus causing the COVID-19 pandemic is in the Coronavirus family. Coronaviruses primarily infect the tissues of the respiratory tract, according to Rote. According to an article published in “Nature,” SARS-CoV-2 is very similar to corona viruses known in bats, and thus the human population has low immunity to it. This has led to a rapid spread of the virus and many national and local leaders putting social distancing measures in place to slow the transmission.
For more information, The Centers for Disease Control and Prevention guidelines can be found on their website.
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