Immune response 101

Think of immune response is antigen-antibody interaction.  When a foreign thing (bacteria or viruses) invade our body, our body treats it as antigen and attacks it.  If the battle is won, that means our body has produced an antibody specific against this antigen.  So next time when the same bacterium or virus (antigen) invades again, the antibody (Y-shaped) will neutralize and destroy it.

Virus is composed by either DNA or RNA inside wrapped by a protein coat.  SARS-CoV-2 (the virus that causes Covid-19) has many proteins, the most important one is spike protein, which attaches to a receptor in a human cell, thus enter the cell to wreak hovac. 

People infected with Covid-19 and recovered, that means they have developed many antibodies in response to each protein of the virus. Of these antibodies the most important one is the antibody against spike protein (antigen) [antigen is either virus or components of virus]  Having this antibody will protect us from getting sick once we are exposed to SARS-CoV-2, this is why the antibody against spike protein is called neutralizing antibody.


The purpose of getting Covid-19 vaccine is to have our body produce antibody against spike protein without being infected by SARS-CoV-2 .  There are many ways to induce our body to produce antibody against spike protein.  BioNTech/Pfizer and ModeRNA use new technology called messenger RNA (mRNA), that will be the next topic.



The above diagram shows spike protein specific antibodies (Y shaped) attach to spike proteins on the surface of SARS-CoV-2, so that the spike proteins cannot bind to the receptors of human cells to do harm.


The antibody against spike protein can be obtained from Covid-19 vaccine or from getting Covid-19 and recovered. Then why it is recommended that those already got Covid-19 still get vaccine.  Two reasons: (1) the antibody that is produced by natural infection may not last too long, both BioNTech/Pfizer and ModeRNA vaccine last at least 6 months, how long will it last is still an ongoing monitor.  Pfizer CEO just announced (4/16/2021) that it is likely that we may need the third shot in 6-12 months.  (2) the antibody level obtained through vaccine is much higher than that from natural infection. The higher the antibody level, the better protection it provides.


Pfizer has filed for Emergency Authorization Use to FDA, so that their Covid-19 vaccine can be used for those age 12 to 15.  It may not be a bad idea that you have basic understanding about the vaccine you’ll soon get.

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