Video: ARNm vaccine
All vaccines are based on the same principle: stimulate our immune system to prepare it for a pathogen before it infects us.
Injecting an inactive portion of the virus, or the complete inactivated virus, stimulates sentinel cells and then B lymphocytes to produce pathogen-specific antibodies in large numbers. The body is thus prepared in case of infection.
Let's see how it works.
In the case of the SARS-CoV-2 virus, responsible for COVID-19, we know that the "S" protein of its envelope is able to trigger an immune response inside our bodies.
Where many vaccines inject this S proteins in our body, m-RNA vaccines rely on a genetic approach to vaccination. m-RNA, developed in the laboratory, contains the genetic instruction to teach our cells how to make this S protein.
Messenger RNA strands are encapsulated in tiny vesicles: lipid nanoparticles. Their function is just to transport their cargo of messenger RNA to their target: muscle cells.
The lipid membrane of nanoparticles is similar to that of our cells. They will therefore fuse according to a natural mechanism called endocytosis and release their precious content into the cytoplasm of the cell.
The genetic information of messenger RNAs is decoded by small units of protein synthesis: ribosomes.
Encoding one amino acid after another, the S protein fragment lengthens as the ribosome advances until the protein acquires optimal conformation. This phase corresponds to the translation.
The proteins thus synthesized are released to be taken care of by specialized cells of our immune system: the follicular dendritic cells.
These cells play the role of sentinel patrolling the tissues of the body. They are able to recognize foreign elements.
Follicular dendritic cells digest these foreign elements. They possess all the enzymatic equipment to cut the S protein into small antigenic fragments. These fragments are expressed on the membrane surface of the follicular dendritic cells.
The next step will involve other cells part of the immune system: B lymphocytes.
They are able to recognize the antigenic fragments expressed by follicular dendritic cells.
This recognition is based on the principle of a key in a lock.
When the complementarity is perfect, the B lymphocyte is activated.
It then undergoes multiple cell divisions forming a clone. Some of the cells will differentiate into memory lymphocytes while others become antibody-producing cells called plasma cells.
The plasma cells secrete a large number of antibodies into the bloodstream.
If, later, the adequately vaccinated subject is in contact with the virus, it will be immediately recognized and neutralized by antibodies.
Vaccination will have played its role in preparing the body against a particular infectious agent.
