To understand how COVID-19 impacts our lives, we must first address the basics of the virus that causes it — what it is, what it looks like, how it attacks our bodies, and how our immune system fights back.
The novel coronavirus, also known as SARS-CoV-2, is the specific virus that causes the disease COVID-19, which was first identified in late 2019. The name “coronavirus” refers to the virus’s structure: It has spikes that protrude off its surface, making it look like a crown. There are many different types of coronavirus, including the common cold and SARS-CoV, a similar coronavirus that spread in 2003.
SARS-CoV-2 has a very simple structure, yet it is highly infectious. It is an enveloped virus, which means that its genetic material, RNA, is encased in a membrane — a protective layer that separates the genetic material from the outside environment. The virus’s RNA encodes 29 proteins, four of which are structure-defining features of the virus that play a direct role in its function.
The spike protein, the envelope protein, and the membrane protein all protrude from the virus’s membrane and play important roles in invading cells and producing new viruses. The spike protein facilitates the virus’s attachment to host cells in the body, while the envelope protein and membrane protein are key in maintaining the structure of the membrane and, importantly, facilitating the assembly of newly-synthesized proteins for the creation of new viruses. The nucleocapsid protein encases the viral RNA, protecting the virus’s genetic material, and also plays a vital role in the formation of new viruses.
SARS-CoV-2 attacks the body when the spike protein binds to the ACE2 receptor, a protein expressed on human cells. ACE2-expressing cells line many tissues in the body and are found in particularly high concentrations in the lungs, kidneys and upper respiratory tract. The spike protein binds to the ACE2 receptor similarly to how a key fits into a lock. Upon binding, the virus is able to fuse with the host cell’s membrane and release its genetic material into the cell. At this point, the nucleocapsid protein coat that surrounds the RNA falls away, and the virus hijacks the host-cell’s reproductive machinery to make copies of its components (RNA and proteins) and form new viruses. When new viruses are finished forming, they fuse with the host cell membrane and are released into the rest of the body, where they can bind to and infect more cells.
Symptoms of COVID-19 are caused as the virus begins to spread throughout the body. In the first stage of infection, SARS-CoV-2 invades cells in the upper respiratory tract, where it begins to replicate. As the disease begins to spread to the lower respiratory tract and into the lungs, symptoms start to arise and the body mounts an immune response. The immune system recognizes the viral proteins and signals the body to attack and kill the virus as well as cells infected by the virus. Antibodies targeting the spike protein also block the virus from entering new cells and replicating by preventing the spike protein from binding to ACE2.
Unfortunately, the immune system is not always successful in fighting off the virus. If the body fails to mount an effective immune response, the virus can spread uncontrollably, damaging cells and resulting in dangerous conditions like pneumonia. However, sometimes, the body’s response to the virus is too strong, and results in potentially deadly over-activation of the immune system.
Despite the novel coronavirus’s relatively simple structure, the complex way it interacts with different systems within the human body renders it a highly infectious and dangerous virus. Therefore, understanding SARS-CoV-2 — how it infects our cells and how our immune system responds to infection — is vital as we develop methods to control the spread of the virus.