How Immune System Produces Antibodies Against Coronavirus

(Courtesy of Annagen Biotech, Baltimore, MD, USA)

 Currently, we are going through one of the toughest times in human history. As of today, 6 million people are infected with coronavirus worldwide and around 368,000 people died. Most surprising thing is that the mortality rates of several countries from coronavirus infection are significantly high, showing more than 10%, whereas the average mortality rate is 5% worldwide. According to Johns Hopkins University, USA, the 3 Western countries with highest mortality are Belgium, France, and Italy corresponding to 16.2%, 15.4% and 14.3% respectively; whereas the 3 Western countries with lowest mortality are US, Switzerland, and Ireland corresponding to 5.9%, 6.2% and 6.6% respectively. Why do these big differences exist? One of the reasons we can think of is that the difference in the capability of producing antibody against coronavirus among individuals. It is well known fact that mortality of coronavirus among aged population is significantly higher that young population. Aged people are sometimes immunocompromised because of diseases and medication. They failed to raise proper immune response, including antibody production against a virus.  Location of the countries on the earth, including temperature also showed critical player for coronavirus infection and mortality. In this article, we like to discuss about how our bodies make antibody against a virus. Say for instance, someone is exposed with coronavirus (Figure 1). Virus are tiny creatures and could be airborne easily. This is one of the biggest challenges to prevent the virus infection, including the coronavirus. In a cramped bus or train, a virus infected sick person could infect all the passengers with a single sneeze because of the ability of viruses of being airborne. Once a person is exposed with virus particles, it enters into lungs through nasal passages and trachea by inhalation. While passing through trachea, virus can be trapped by mucous and regurgitated by coughing, otherwise it reaches to lung (Figure 2). If virus particles are removed by coughing, the person can evade the infection otherwise the person can be a victim of virus infection. From this point, all the later events in our bodies are complex and dramatic. The person becomes the host and virus behave like a parasite.  Once coronavirus enter in the lung, the host’s immune system is activated and try to kill the virus. Let us discuss what happens when coronavirus reaches the lung. It binds the lung cells through binding a particular protein, which is biologically known as receptor. Each virus has a unique receptor on the cell surface for binding. These receptors are uniquely expressed on a particular organ.  Therefore, lung virus can not bind to brain, or brain virus can not bind to the lung unless both organs express similar receptor. Once they bind to the receptor, they enter into the cells through a process known as endocytosis. Inside the cells, they multiply their number with billion and trillion copies.  The proliferated virus particles inside the cells create a pressure because of limited space; as a result, the infected cell burst. Newly born viruses again infect the neighboring cells. In this way, virus causes a wound in the local tissues and finally disseminate into blood through damaged tissues. Our lung is highly vascularized with artery, veins, and lymphatic ducts. Virus enters into the lymphatics ducts and come to the lymph nodes where it faces the wrath of our immune system. In the lymph node, the viruses are eaten up by special kind of cells of our immune system known as dendritic cells (Figure 3). Dendritic cells have finger like appendages that immobilize the virus and engulf. Virus are digested and viral proteins are processed inside the dendritic cells similar to food processing center.  Dendritic cells select the antigenic part from the mixture of proteins and flag it outside the cell surface for presentation to T cells. This is one of the critical parts of antibody production against the virus by our immune system (Figure 4). Remember, antigen is a part of a viral protein that is recognized by our immune system to produce antibody. T cells are the most critical and specialized cells in our immune system. You can say that T cells are the brain of our immune system. Dendritic cells donate the viral antigen, here the coronavirus antigen to T cells. T cells receive the antigen and make a replica (mold) of the antigen. T cells donate the replica (mold) of the antigen to the B cells for the mass production of the cast. Here the cast is nothing but the antibody (Figure 5). B cells are highly specialized type of immune cells that are capable of producing antibody using the antigenic replica (mold) of the virus created by the T cells. Using the antigenic replica, B cells can produce billion or trillion of copies of antibodies. Importantly, antibodies are Y-shaped molecules made of protein. All these antibodies come to blood and attack the coronavirus by binding the antigens that are expressed on the outer surface of the virus. Antigen-antibody binding are very strong. Antibody has special affinity to bind antigen. Therefore, once antibodies come to the blood, they immediately go to the virus and attack them.  Thus, viruses are encircled, immobilized and killed by the antibodies (Figure 6). This is an overall mechanism of how one’s immune system fights with the coronavirus and kill them. If the person’s immune system is weak or immunocompromised, he or she fails to develop the appropriate immune response, including the antibodies and may die from the virus. 


(The writer of the article, Dr. Manik Ghosh, MS, PhD is an Ex-NIH Fellow, USA. He has expertise in Immunology and currently a Lecturer in University of Maryland System.