How Immune System Produces Antibodies Against Coronavirus
(Courtesy of Annagen
Biotech, Baltimore, MD, USA)
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.