Immune system: how it works and what are your enemies

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After hearing so much about vaccines, antigens, antibodies and immunity, it would be opportune to talk a little about what our immune system is and how it works.

What is immune system

The most common definition we have for the immune system is that it exists to defend us against external or potentially disease-causing agents. This definition assumes that the system must distinguish what is part of its own body (from the English, “self”) and what is strange and must be combated (“nonself”).

Here is the first challenge for us to understand the immune system: over hundreds of millions of years, animals and plants have developed a system to recognize what they do not know? To understand, it would be like assembling an army to fight like an unknown enemy, without knowing what weapons they are sensitive to.

There are major differences in the mechanisms that govern the protection systems between animals and plants, and even between animals closer to the evolution line. For the purpose of this conversation, we will consider only the immune system of humans, but it is worth mentioning that much of our knowledge about our immune system came from animal models, especially mice.

How it works?

Before talking about the immune system, let’s talk about the concept of antigen. An antigen is any substance capable of stimulating the cells of the immune system. Most of the time, the antigen is a protein, which can exist in the external or internal portion of cells, bacteria, viruses, etc. But, other chemicals can also stimulate the immune system, such as DNA and more complex sugars.

Back in the immune system, its main function is broader than defending the invader’s organism. The function of the immune system is to maintain the balance of our organism. In the first years of life, our immune system goes through an education process to select the effector cells of the immune response.

During this process, all of our cells are presented with a (practically) infinite number of antigens. The cells that react very strongly against these antigens (the correct concept is avidity or strength of the connection with the antigen) are eliminated. Likewise, those cells that are unable to react against any presented antigen are also eliminated.

The action of the immune response

At the end of the process of education of the immune system, we are left with a repertoire of cells capable of recognizing an almost infinite number of antigens against which we can mount an immune response, either by the production of antibodies or by the activation of the cellular response, by means of cells. defense system called T lymphocytes.

Here is the explanation for our “unknown enemy” challenge. We have a repertoire of cells that have already been selected to react against defined antigenic structures. Throughout our lives, these cells will eventually encounter these structures and, once stimulated, they will proliferate, fight and eliminate the cell, virus or bacteria that carried that antigen.

Thus, upon encountering an antigen for the first time, our immune system has cells to mount a specific immune response. But putting this answer together takes time, which can vary from 2 to 4 weeks. During this time, the disease can settle before the body finishes the immune response.

How vaccines work

Vaccines present the immune system with antigenic structures that stimulate it to mount a specific response. This speeds up the body’s response after the body’s exposure to an antigen, preventing the disease from setting in.

The immune system recognizes the signal as a potential imbalance, the cells are activated, multiply and resolve that signal, generating memory cells. That way, the next time the body comes into contact with this antigen, it will be ready and will be able to respond more quickly, regain balance or mitigate the manifestation of the disease.

The intriguing conclusion of this immune system education mechanism is that, even though it has never been introduced, for example, to the coronavirus, antigenic structures of the vaccine’s viral proteins are sufficiently similar to the antigenic structures against which our cells were trained during the process of selection.

* Content prepared with the contribution of Dr. Mariane Amano, PhD in Immunology and researcher at Hospital Sírio-Libanês.

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