Vaccines: How do they really work?

Nowadays, talking about vaccines is trendy. Although we have a lot of information about the topic, most people still don’t understand how and why vaccines work (yes, they do!).

In order to understand it, it is necessary to know how does the immune system acts. In fact, the immunology as a discipline stems from the observations that led to the development of vaccines. Some people who recovered from an infectious disease did not get it again.

How does the immune system work?

The immune system acts in two different ways: one is faster but less specific, known as innate immunity; the other acts in a slower way but it is much more specific, the adaptive immunity. The second one is the responsible of the immunological memory that is generated in our bodies when we get vaccinated. More specifically, the cells involved are the lymphocytes.

Lymphocytes are the most specialized and efficient cells recognizing things as part, or not, of our bodies. Also, they can differentiate which of these external agents can be dangerous. Obviously, this requires a very careful selection and maturation. The lymphocytes, instead of recognizing external pathogens, recognize small parts of some molecules that they express, named antigens.

*In fact, lymphocytes can also recognize self-antigens from the body. They are able to distinguish them so they do not respond. If this process fails autoimmune diseases are produced such as diabetes or lupus.

This interaction can be imagined as if lymphocytes would have a key (receptor) which fits in a lock (antigen), and every microorganism has its own antigens.

Which immune cells can protect us?

There are different types of lymphocytes: B cells, T cells, and NK cells (Natural Killer). B and T cells have in their membranes (external part of the cell)  their own receptors (named BCR and TCR from B Cell Receptor and T Cell Receptor, respectively).

When a B cell recognize an antigen, it can specialize and proliferate. Then lots of those B cells start to produce copies of their BCRs. But these BCRs have a difference, B cells secrete them. These secreted “copies” is what we know as antibodies and they have several functions that are helpful for the immune system. One of the most important functions is the capacity of interacting with the bacteria in order to mark them, a process known as opsonization. Let’s go back to the “key-lock” system; the antibodies would be now the keys that can fit into the pathogen’s locks. So, they let them marked making them much more visible for the immune system (so they can be eliminated faster).


Actually, when the BCR recognize an antigen, it is not a perfect joining, but through a process named as affinity maturation, B cells are able to create antibodies that will fit better. What really happens is that some of them start to mutate and to produce BCRs that have small modifications (this process is known as somatic hypermutation). Those receptors that are able to recognize in a better way, will give a strong signal of activation to the B cells, while the others will die. Thus, in a selection process, only the B cells capable to produce better antibodies, will survive.

Moreover, some of these cells will keep in the body as memory cells for a while (in some cases during all of our lives). The immunological memory will let the immune system to attack a pathogen directly and faster the next time that the pathogen enter to our bodies.



Why do we have to get vaccinated, if the immune system works?

Some microorganisms are very efficient evading the immune system. They can never be detected or attacked, so the outcome may be death. In that case, we wouldn’t be able to create memory since when we got infected we wouldn’t survive. At this point is where vaccines have an important role.

There are different kinds of vaccines but all of them work based on the same principle. The capacity of the immune system to create memory if it can see the antigens before the pathogens enter to the body. Then it will create this “re-configuration” and will be efficient in case of infection. Different mechanisms can be used to introduce the antigens. It can be through the inoculation of the pathogens in attenuated forms (they cannot reproduce), in inactivated or dead forms, in a recombinant way (introducing only the antigens)…

So, for example, if we get vaccinated against the flu, what we are really doing is to introduce the attenuated flu virus. It won’t be able to infect our body, but the immune system will detect it and will create a memory. After that, if the real flu virus infects us, the body will attack it so fast that we won’t feel that we have the virus inside.

Vaccines in numbers:

  • Vaccines are saving between 2-3 milion lives every year.
  • Poliomyelitis is closer than ever to be eradicated, thanks to the vaccine. Nowadays it only exists in Afghanistan, Nigeria and Pakistan. In 1988 it existed in more than 125 countries.
  • Vaccines help to prevent the emergence of antibiotic resistances.
  • Vaccines do not cause autism.
  • Cases of infection that could have been prevented with vaccination: visit this interactive map.

The most important thing to remember is that vaccination is not a personal choice, but a collective one. There is a lot of people that can not be vaccinated. If the people around them are protected, they will also be. Another day we will talk about what is the collective vaccination and the myths of the vaccines.


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