With all the talk around COVID-19, what does science have to say about the way our bodies respond to this threat? We have a course about what it does in your body here, but if you are up for a read, follow along!

SARS-CoV-2, the virus behind the COVID-19 disease, enters your body via your respiratory tract: your nose or your mouth. Once it gets to the lungs, it binds to a specific type of cell: alveoli.

Alveoli are important structures inside the lungs. Created in BioRender.

But why? Why alveoli and not other cells?

To answer this, let’s look at the virus itself and its name. “Corona” is the Latin word for “crown”. A coronavirus has this name because of a structure that looks like a “crown” encircling the virus, as you have probably noticed by multiple pictures and illustrations of the virus everywhere. This “crown” has key proteins that bind the coronavirus to alveoli cells by means of a very specific “bridge” – ACE2 receptors.

The coronavirus binds to ACE2 receptors in alveolar cells. Created in BioRender.

This type of receptor is thought to be the main bridge between the virus and human cells. ACE2 receptors are proteins in the cells’ membrane. Not all cells have these type of receptors, or. ACE2 proteins on their surface, but research has shown that they exist in many places in the human body, especially in the lungs and small intestine.

When the coronavirus enters the body, it binds to these proteins, enters cells and starts multiplying. Since the alveoli in your lungs have so many ACE2 receptors, the virus more. easily binds and enters these cells in particular.

When this happens, an intruder has entered your body, and your immune system now starts to respond to the invasion. 

How does your body react to a virus?

Before we dig into how our immune systems fight the SARS-CoV-2 specifically, let’s take a look in how it fights a common virus.

There are 3 main “guns” your body uses to fight an undesired virus: 1) cytotoxic cells, 2) interferon and 3) antibodies.

1 – Cytotoxic Cells

Cyto means cell. The word “cytotoxic” describes itself clearly. Some cells are able to kill other cells, and we call those cells cytotoxic.

When a virus enters a cell, it infects it. The infected cell will then display a protein on its surface called MHC I. This MHC I is like a “flag”, and tells surrounding immune cells that the cell has been infected with a virus.

T cells are immune cells that usually circulate in the bloodstream. A special type of T cell, called cytotoxic T cell, has a protein that recognizes MHC I. It’s like cytotoxic T cells have binoculars to see these “flags”. Once they recognize it, they release substances to kill the infected cell – cytotoxic factors. (We might also refer MHC I as a “spy” inside of your cells that alerts your immune system when a virus gets inside.)

Besides cytotoxic T cells, our bodies also have Natural Killer cells. These cells are highly effective in killing and destroying threats.

When a virus infects a cell, other immune cells are able to recognize it and kill the infected cells. Created in BioRender.

2 – Interferon

Infected cells make interferons. “Interferons” are small proteins that interfere with a virus’ ability to replicate. They also act as signaling molecules to warn nearby cells that there’s a virus around.

3 – Antibodies

You’ve probably heard this word before, especially since news outlets and people are now talking about antibody testing for COVID-19. We won’t get into describing the tests here, but we will get into understanding antibodies.

When some foreign substances enter your body, antibodies are able to recognize them. These substances can be viruses or bacteria, for example, and are called pathogens. Antibodies are proteins that recognize and stick to pathogens in specific parts called antigens. When an antibody binds to an antigen, it starts an immune system fight against it. There are various phases of this fight:

  • Agglutinating the virus

Antibodies sometimes act as “glue” and make viruses glue to each other. This way, immune cells can target viruses more easily. 

  • Activating phagocytes

Phagocytes are cells able to “eat” stuff. In this case, since the body aims to defeat a virus, it could be useful to use these cells to eat the virus and thus, destroy it. When antibodies bind to antigens (for example, a virus), phagocytic cells become able to recognize those antigens as a threat. If these cells recognize viruses as threats, they can then phagocyte (fancy word for “eat”) the threat and destroy it!

  • Directly neutralizing the antigen (whether it is a virus or bacteria)
  • Activating the complement system

The complement system is a series of different proteins. In this system, one protein activates another and so on in a cascade of reactions. These proteins are usually inactive. However, with specific molecular signalings (such as, for example, when cells recognize a virus), they are activated and start performing molecular reactions that help the immune response. 

How does your body react to SARS-CoV-2?

Now that we roughly understand how our immune system fights a virus, let’s get into understanding the immune response against the SARS-CoV-2. It’s important to mention that what we write here is based on the most updated and trustworthy research found on the topic. However, as discoveries lay ahead, let’s keep an open mind. Science may soon discover new responses that complement what we already know.

As you may imagine, science is still to fully understand how our bodies fight this virus. It’s difficult to find a description that fits all the immune responses because it gradually changes from person to person. However, scientists believe that the way our body fights the virus could undergo two stages.

Stage 1

  • The virus has entered your body and your immune system will fight it as best as it can. If you are healthy and with no underlying health conditions, the virus won’t damage your body much. However, you could still be sick for around 2 weeks, while your immune system responds to the virus.(Or, you could have few to no symptoms.) Immune cells would be flowing through your bloodstream and inflammation might be intensified during this period.

Stage 2

  • According to researchers, this second stage is when things start to get more complicated. If in the first stage of the immune response, the virus wasn’t eradicated, it will now start to spread more and destroy cells – especially the ones with ACE2 receptors on them. As these cells are destroyed, they will signal the immune system to act and respond to the virus more and more. Of course, the immune system is great at fighting pathogens. However, this battle has a downside to it. “Overreaction” immune responses lead to enhanced inflammation in the respiratory tract. This may lead to big amounts of mucus in the airways, which then can lead to acute respiratory distress syndrome (ARSD). This is the number one threat and is usually what leads to death due to SARS-CoV-2.

Here you can see a Healthy vs Acute Respiratory Distress Syndrome lung. As you move the grey slider, increased inflammation leads to increased mucus in the lungs. Consequently, breathing becomes a difficult task. Patients under this circumstance could need the help of a ventilator.

We have yet to understand fully how the human body reacts to this coronavirus. It’s still curious how people have so many different symptoms. However, if one thing is for certain, it is the fact that your immune system will always fight it the best that it can.

The main challenge exists for people with compromised immune responses who are more fragile and susceptible to suffer from COVID-19. So, what we could advise you to do is to keep up with a healthy lifestyle, even while indoors – you will preserve not only your health but also the health of the ones around you!