Introduction

I received my first dose of Pfizer's COVID vaccine today. Thanks to my job, I counted towards Phase 2. However, back when the vaccines were first distributed and administered, I had my doubts. I felt that the vaccines were sent out too quickly and the side effects made me hesitant. But after doing a bit of research on how the vaccine works, combined with its high efficacy rate and my knowledge of how the immune system works, I felt comfortable enough to give it a try.

The mRNA Vaccine

Pfizer's vaccine is different from the "traditional" vaccine that may be familiar of. The type that people are more familiar with are vector vaccines where a modified, harmless version of a virus is injected into the patient. That modified virus serves as a recipe for the immune system to translate the spike proteins and make antibodies.

mRNA vaccines work differently in that rather than the whole virus, only the messenger RNA that codes for the spike protein is inserted. In a cell, the mRNA will bind to a ribosome which will synthesize the spike proteins. This, too, will lead to the formation of antibodies.

But how, exactly do we go from spike proteins to antibodies?

MHC-1 and MHC-2 Proteins

Our immune systems are extremely complex. It is composed of many different types of cells dedicated to different tasks: phagocytes, macrophages, cytotoxic T-cells, helper T-cells, and B-cells to name just a few. When we get infected with a pathogen, the MHC-1 and MHC-2 proteins play a major role in our adaptive response.

Adaptive immunity is where our immune systems makes the antibodies.

When we are infected, the virus will try to hijack our cells by injecting its DNA into the cytoplasm. Two things can happen. If it enters the lytic cycle, then the DNA will be transcribed into mRNA where it will be translated into viral proteins. If the virus likes to do it nice and slow, then in the lysogenic cycle, its DNA will be incorporated into the host cell's genome. The host cell will replicate via mitosis and eventually, the cells will enter the lytic cycle.

Basically, the host cells become virus making machines. This where the MHC-1 protein comes in. When infected, a host cell will present a piece of the foreign viral protein with the MHC-1 complex at the cell membrane. An effector cytotoxic T-cell will detect the foreign protein presented by MHC-1 and bind to the host cell before killing it. This will effectively put an end to the virus's infiltration.

However, it is the MHC-2 protein that we are more interested in in the context of the mRNA vaccine. MHC-2 is specific to what are called antigen-presenting cells (APC). What an APC does is that it detects, engulfs, and presents the antigen at the cell membrane to inform the immune system of an infection. A helper T-cell will detect the antigen and bind to it.

This activates the helper T-cell as it makes several copies of what are called memory T-cells and effector T-cells. The memory T-cells are the ones that will bind to more antigens presented by the MHC-2 complexes from antigen-presenting cells. Meanwhile, effector T-cells will release cytokines which serve as chemical alarm bells to activate the B-cells.

When B-cells activate, they divide into memory B-cells and effector B-cells. The latter serves as an antibody-making machine which helps our immune system fight off the infection. The memory B-cells will help our immune system respond to the same infection more quickly.

The plasma cells are the effector B-cells, by the way.

Tl;DR Version

1. Our cells' ribosomes will bind to the mRNA from the vaccine and translate it into the spike proteins.
2. The antigen-presenting cells will engulf the spike proteins and present pieces of them at the cell membrane via the MHC-2 complex.
3. The helper T-cells will bind to the MHC-2 proteins presenting the antigen.
4. The helper T-cells will proliferate into memory T-cells and effector T-cells. The latter releases cytokines, a chemical "alarm bell", to activate the B-cells.
5. The B-cells will divide into effector B-cells and memory B-cells. The former churns out antibodies for our immune systems to immediately fight the virus. The latter will allow our immune systems to react to the same infection more quickly if the virus invades our bodies again.

Closing Thoughts

As long as the recipe for the spike proteins is accurate, then the vaccine should be very effective. Initially, I was skeptical as there were a few accounts of very adverse reactions and side effects. But as time went by, I heard less of these types of stories. After thoroughly understanding the mechanisms of the mRNA vaccine, I became more comfortable and decided to get my first dose today.

Fingers crossed that the second dose will go without any issues.