The Science of Vaccines

Sara Humes

With the recent outbreaks of measles across the United States and an outbreak of mumps right here at the University of Florida back in May, it’s time we share a post on vaccines and how and why they work. Before you close this post and decide you aren’t interested in what can be a very controversial topic, let me make a disclaimer: this post will be based solely on the science of how vaccines work – we’ll be focusing exclusively on what happens in your body after receiving a vaccine, specifically what your immune system does in response to the vaccine and then future encounters with that pathogen. Though there is plenty of science to back up concepts like herd immunity and vaccine schedules, we will not address that here. Let’s get started!

The best way to understand the human immune system is to imagine that your body is a country and your immune system is your army. This immune system army can be broken down into two divisions – innate immunity and adaptive immunity. Your innate immunity is always ready to fight anything foreign that enters your body. It isn’t specific – it can fight anything and everything, and it acts fast – within just a few hours or days. Your adaptive immunity, on the other hand, takes some time to mobilize. It waits for the message that something foreign has entered your body before it organizes and rises to fight, but once it starts fighting, it uses very specific tactics for each enemy (pathogen) that it faces, and this targeted approach makes it more effective against that pathogen. The adaptive immunity division can be further divided into humoral immunity and cell-mediated immunity. The main soldiers of cell-mediated immunity are T cells, while humoral immunity utilizes B cells and their weapons, antibodies. The coolest thing about the immune system is that all these different divisions and groups communicate and work together to protect you. Neither one works in isolation; in fact, their success is dependent on their ability to cooperate with one another. 

When something foreign enters your body like the bacteria that causes strep throat or the influenza (flu) virus, the innate immune system recognizes the foreign invasion. Generally speaking, the part it recognizes is called an antigen, and this is the signal that tells your immune system army to fight and protect you. Antigens are specific to each pathogen; for example, the measles-causing rubeola virus has different antigens than flu viruses. After recognizing an invader, the innate immune system quickly organizes and begins the fight to keep the pathogen from spreading throughout your body and making you sick. Sometimes, it can do this all on its own, but other times it needs back up from the adaptive immune system. Therefore, it will carry the antigen to where your adaptive immune system lives (usually in your lymph nodes) and present it to the cell soldiers, encouraging them to rise up and fight too. It takes your adaptive immune system a few days to get organized, but once it does, it chooses a strategy based on the information learned from the antigen. If the foreign invader can get inside your host cells, like a virus, cancer, or some bacteria, your T cells will rise up to lead the fight. T cells fight infection by directly killing infected cells and releasing cytokines (chemicals that signal to other cells), which tell other cells to kill the infected ones. If the foreign invader does not need to enter your cells to cause infection, like some other bacteria, then your B cells and antibodies will lead the way. B cells fight by releasing antibodies, which block the ability of a pathogen to infect host cells, attach to pathogens as a way to tell other immune cells to destroy it, and communicate with and recruit other immune cells to help fight. All of this is really hard work for your body, which is why even when your immune system is doing a good job, you might still feel sick or run a fever. It’s because your immune system is fighting a hard, microscopic battle! 

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So why do we use vaccines? Wouldn’t exposing your body to something foreign start an unnecessary battle for your immune system? Well, kind of! But the vaccine battle is much easier for your immune system army, and the results will protect you for a long time. The battle is easier because vaccines do not contain the same pathogen that you would be exposed to normally – for example, after coming into contact with a sick person. The pathogen inside a vaccine is either dead or attenuated (meaning that it’s missing some pieces, so it won’t work the same way and wage the same big battle as its unattenuated counterpart). A vaccine pathogen still shows your immune system its antigen, encouraging your immune armies to rally, but the pathogen from the vaccine cannot actually wage a real battle against the immune system. It would be like if one country invaded another and only sent one or two weak and harmless soldiers to fight against a country with a full, well-equipped army. It’s pretty obvious who would win that battle, right? In the case of vaccines, it’s not just about winning but training the body for any future fights against a specific invader’s entire, fully-equipped army. The immune system learns from its easy little battle with the vaccine, and now knows which tricks will work best if an entire pathogen army ever showed up at the border. This is called immunologic memory, or the ability of the immune system to respond much more swiftly and with greater efficiency during a second, or later exposure to the same pathogen.

So, this is why vaccination works! Exposing someone to a tiny bit of weakened pathogen, that is unable to cause illness, trains the immune system army against that pathogen, so if/when they are re-exposed to the real thing in full force, their immune system can swiftly organize and attack the invader, preventing serious illness. 

Reference: Owen, J. A., Punt, J., Stranford, S. A., Jones, P. P., & Kuby, J. (2013). Kuby immunology. New York: W.H. Freeman.