What are vaccines?

This week we will talk about vaccines, something that we are all used to, but that we do not know or do not understand exactly.
Would you know how to define the term vaccine, what they contain, what they are used for or what are the existing types? If not, I recommend that you continue reading.
I could not write this post without telling you the curious way in which the vaccination story began. In 1768, the doctor Edwar Jenner studied the case of a farmer who could not suffer from the disease of smallpox, having previously suffered this same infection but in its vaccine form.

Almost 30 years later, he carried out an experiment that left no one indifferent as he injected the contents of a smallpox patient’s sore into a healthy child in the form of a “vaccine.” After that, he repeatedly infected him with the smallpox virus to verify the effectiveness of this new concept.

Luckily, the boy never fell ill, and the doctor was able to demonstrate the efficacy of vaccination, a name that comes from the word “cow” and that until a century later was not attributed to this novel technique.

Now let’s get down to business. What is a vaccine?
A vaccine is, according to the WHO, any preparation designed to generate immunity against a disease by stimulating the production of antibodies. To better understand this definition, we must first talk about our immune system.

The immune or immune system has a clear function in our body: defending ourselves against possible threats, both internal (cancer cells) and external (viruses, fungi, bacteria, harmful substances …).

But how does our immune system know what is a threat and what is not?
Thanks to antigens, components (almost always proteins) capable of developing a complete immune response (with the formation of antibodies), and which are present in the pathogenic microorganisms that infect us, in addition to cancer cells.

How does this whole process happen?
When a pathogen tries to invade our body, it meets a first barrier, in this case physical and made up of multiple components. Among them the skin, the oral mucosa, the lysozymes that we secrete in our saliva, the sweat, the acid of the stomach, the mucus that is in the trachea … All of them are a first obstacle that tries to prevent the entry into our body of the evil invader.

If this invader manages to cross this first stumbling block by entering our interior, phagocytic cells, natural killer cells and the complement system appear. The former phagocytose, that is, engulf and destroy pathogens. The latter kill infected cells and cancer cells. And the complement system is a cascade response that also has the purpose of eliminating the invading microorganism.

Both of these barriers constitute what is called innate immunity, that immunity with which we are all born and that attacks, unspecifically, those substances, microorganisms or cells with malignant characteristics.

If despite both systems, the pathogen persists in our body, the last strategy comes into play: acquired immunity and mediated by T lymphocytes, B lymphocytes and the famous antibodies or immunoglobulins. While T lymphocytes attack cells infected with the pathogen in question, B lymphoctia produce specific antibodies against a specific antigen for a specific pathogen.

This process is what is known as acquired immunity and is characteristic of each person, since it will depend on what pathogens you face throughout your life.

What is the great advantage?
That after producing an immune response against a specific pathogen, a series of T and B lymphocytes called memory lymphocytes will be saved. In this way, if the same pathogen were to infect us again, the immune response would be up to 3 times faster, thus preventing the development of the disease derived from its infection.

What is the function of a vaccine?
Generate a controlled immune response, in order to generate this memory, facilitating that after a subsequent infection, the response is much more effective, avoiding suffering the symptoms of the disease.

If we think about the case of the smallpox immune farmer, perhaps we understand it better: once you have an infectious disease, you become immune to it. Vaccination tries to simulate the first infection, without developing the disease, but causing immunization against it.

How is it achieved? Injecting the pathogen itself into our blood but in a harmless way.

What does a vaccine contain?
Any vaccine contains, as the main element, the microorganism or part of the microorganism against which you want to immunize. Along with the antigenic agent itself, we have adjuvants, substances that help our immune system to react better to this pathogen.

In addition, components such as preservatives and stabilizers are added to the antigenic agent and the adjuvants. While preservatives prevent the vaccine itself from becoming contaminated with other pathogens, stabilizers such as sugar or gelatin help keep the vaccine in good condition and can continue to work until it is used.

What types of vaccines are there?
Vaccines are mainly classified into two main groups: classic vaccines and new generation vaccines. And if we look closely, we will be able to verify that most of them, from one group to another, could be defined as biotechnological products by using microorganisms or parts of them.

CLASSIC VACCINES

Live attenuated vaccines, as the name suggests, use live but weakened micro-organisms. How does it weaken? By chemical methods, with mutagenesis …

The main risk of this typology is that the attenuation process is not strong enough to inactivate the microorganism and this can trigger the disease. For this reason, they are not recommended for patients with impaired or weakened immunity.

Examples? Measles, mumps, rubella, chickenpox and yellow fever vaccines, among others.

Inactivated vaccines use the own microorganism against which you want to immunize, but dead. In this way, they expose our body to the pathogen generating an immune response, but prevent it from causing the disease by being totally inactivated. The main advantage over the previous type is that in no case can they develop the disease.

However, the immune response they trigger is usually weaker, and therefore multiple doses are required. How does it weaken? With chemical substances, with physical means such as heat …

Examples? Hepatitis A, influenza, polio, and rabies vaccination.

And finally, subunit vaccines, which instead of using the whole microorganism, only use part of it, provided it is an antigenic part, that is, it develops a complete immune response.

Among the different options we have to use toxins produced by the pathogen, proteins from the surface of a virus, polysaccharides from the capsule of bacteria … In any case, the possibility of developing the disease is also avoided.

Examples? Hepatitis, papilloma, pertussis vaccine …

And at this point we can also mention conjugate vaccines, a subtype of subunit vaccines that consist of conjugating the antigenic part of the microorganism with a transporter protein that improves the immune response.

NEW GENERATION VACCINES

Apart from these three forms of classic vaccines, more recently and thanks to new technologies, many other alternatives have emerged that are currently being studied.

The first type are attenuated vaccines, similar to classical live attenuated vaccines but with the only difference that the weakening process has been carried out through genetic modification. This process consists of altering the genes involved in the pathogenicity of the microorganism, thus preventing it from causing disease; or modify genes of the microorganism that manage to develop a better immune response.

The next type is the recombinant protein and peptide vaccines. These consist of selecting the genes responsible for producing the antigenic proteins of the pathogens, and introducing them into plants or bacteria to massively produce these proteins, isolate them and use them as a vaccine.

On the other hand, we have DNA vaccines, sometimes also classified as recombinant, and which consist of directly using the genes of the pathogen in charge of producing the antigenic proteins. This genetic material can be used naked, or inserted into a harmless bacterium or virus but acting as a vehicle.

If the second option is chosen, the chosen microorganism will synthesize the proteins, triggering the desired immune response. One of the main advantages is that gene sequences belonging to different pathogens can be used and therefore, immunize against various microorganisms with a single vaccine.

In addition, we have another option, synthetic peptides and proteins. This type consists of chemically synthesizing the antigenic proteins of the pathogen, thus avoiding having to use the microorganisms themselves. The main drawback is that the synthesized proteins are not totally identical to the natural ones, and therefore, are not capable of developing a strong immune response.

And if that were not enough, we have anti-idiotype vaccines, consisting of using antibodies that recognize the microorganism against which we want to vaccinate, awakening an immune response exogenously. The main of its limitations is the need to administer repeated doses.

And finally, edible vaccines, probably the most surprising. These consist of genetically modifying edible vegetables so that they produce, by themselves, the antigenic protein of the microorganism. In this way, just eating the vegetable or fruit would be enough to immunize the patient.

The main problem with these vaccines is that they are quickly inactivated in our stomach. In addition, it is necessary, for the moment, to ingest large amounts of the transgenic food to develop a complete immunization.

Although as we have just verified, there are multiple alternatives among the new generation vaccines, today, most of those used are part of the group of classic vaccines. Can we say in a few years that these new vaccines are more effective, and therefore, a better option than the classic ones? It only remains to wait.

In addition, we have another option, synthetic peptides and proteins. This type consists of chemically synthesizing the antigenic proteins of the pathogen, thus avoiding having to use the microorganisms themselves. The main drawback is that the synthesized proteins are not totally identical to the natural ones, and therefore, are not capable of developing a strong immune response.

And if that were not enough, we have anti-idiotype vaccines, consisting of using antibodies that recognize the microorganism against which we want to vaccinate, awakening an immune response exogenously. The main of its limitations is the need to administer repeated doses.

And finally, edible vaccines, probably the most surprising. These consist of genetically modifying edible vegetables so that they produce, by themselves, the antigenic protein of the microorganism. In this way, just eating the vegetable or fruit would be enough to immunize the patient.

The main problem with these vaccines is that they are quickly inactivated in our stomach. In addition, it is necessary, for the moment, to ingest large amounts of the transgenic food to develop a complete immunization.

Although as we have just verified, there are multiple alternatives among the new generation vaccines, today, most of those used are part of the group of classic vaccines. Can we say in a few years that these new vaccines are more effective, and therefore, a better option than the classic ones? It only remains to wait.

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