How do vaccines work?

Vaccines are especially relevant during the COVID-19 pandemic, as scientists are racing to develop a vaccine to combat the rampant virus. That prompts the question - how exactly do vaccines protect us from pathogens?

First, it is important to understand what a pathogen is. A pathogen is a disease-causing agent that is covered with antigens, molecules that trigger specific immune responses.

Vaccines mimic those disease-causing agents and stimulate our immune system to produce the necessary defenses to combat them. But don't worry. Vaccines appear just like pathogens, however they do not make our bodies sick like pathogens do. Think of it like this: vaccines "train" our immune systems to respond to pathogens so that they will be prepared if they encounter a pathogen in the future.

In order to better comprehend how vaccines affect our immune systems, it is important to understand how our immune systems deal with harmful, foreign agents.

Our blood contains red blood cells, which carry oxygen to tissues and organs, and white blood cells, which fight infections. There are three main types of white blood cells: macrophages, B-lymphocytes, and T-lymphocytes.

Macrophages swallow and digest germs while leaving behind antigens.

B-lymphocytes produce proteins called antibodies, which bind to specific antigens to counteract them.

T-lymphocytes attack cells that have already been infected.

Knowing the basics, let's delve deeper into what happens after a person has been vaccinated.

Antigen-presenting cells (APCs) locate the antigen from the vaccine, ingest it, break it apart, and display a piece of it on their surface. Helper T cells, which are specific to the antigen detected, alert nearby immune cells to the presence of the foreign antigen. B cells rapidly divide in response to the antigen. Some B cells transform into plasma B cells, which secrete antibodies that bind to the antigen, preventing it from entering a cell or marking it for destruction.

Attenuated vaccines contain reduced or weakened viruses. The attenuated virus might enter cells, prompting killer T cells to respond. Killer T cells find the virus and destroy it.

So what's the point of getting a vaccine without having a disease or virus? Well, the goal of vaccination or immunization is to produce memory of the vaccine antigen. In order words, if a person's immune system came across the antigen, it can quickly and effectively respond to it. This is where memory cells come into play. Memory cells can either be memory B cells, memory helper T cells, or memory killer T cells.

Now, let's suppose that in the future, you are infected by the virus the vaccine primed your immune system against. When the pathogen enters the body, APCs ingest it and display its antigen on its surface. Then, memory T cells, which were created during vaccination, recognize the antigen. Memory helper T cells release signals that alert the other immune cells to the foreign agent. In addition, long-living memory B cells, which can respond to specific pathogen, will become activated. In response to the antigen, memory B cells transform into plasma B cells and secrete antibodies at a faster rate than they did during the vaccination. Then, antibodies bind to the antigen, eliciting a response.

If there was a killer T cells response during the vaccination process, memory killer T cells quickly locate infected cells and destroy them, preventing the spread of the infection.

Once the pathogen is taken care of, some memory B and T cells remain, acting as guards should they encounter the same pathogen later on. These memory cells can remain a person's body for many decades!

So the next time you get a vaccine, think about the preparation your immune system is doing. Hopefully, that will make you appreciate the importance of regular vaccination!