Understanding your immune system

Our immune system's mission is to protect our body against foreign or dangerous invaders. These invaders can be:
  • microorganisms (usually called germs, such as pathogenic bacteria, viruses or fungi)
  • parasites (such as worms)
  • cancer cells
  • transplanted organs and tissues
In order to defend our body against these invaders, our immune system must be able to distinguish:
  • What belongs to the body (self)
  • What is not part of it (exogenous or foreign)
Antigens are substances that the immune system can recognize and which stimulate an immune response . If antigens are perceived as dangerous (when they can cause disease), they can stimulate an immune response in the body. Antigens can be located inside or on the surface of bacteria, viruses, other microorganisms, parasites or cancer cells. In other cases, they are independent substances, such as food molecules or pollens.


What is the immune system?

When we are faced with external attacks (viral or bacterial infections), our body defends itself by activating our immune system. Often compared to an army of small soldiers, this device is very complex. Our immune system is capable of mobilizing several types of cells and producing molecules to defend our body. Although it is invisible to our eyes, it nevertheless keeps watch, day and night, whether to help us cure a cold, an ear infection or cancer... the immune system is essential. The majority of cells are not found in our blood but in a set of organs called the lymphoid organs.

What are the organs of immunity?

The major organs of our immunity are the bone marrow and the thymus, these are the organs which produce immune cells (lymphocytes), the spleen, the lymph nodes, the tonsils and the clusters of lymphoid cells located on the mucous membranes of our tracts. digestive, respiratory, genital and urinary. It is generally in these peripheral organs that our cells are called upon to react.

A rapid response from our immune system in the event of an “attack” is extremely important. This is based, among other things, on the effectiveness of communication between the various actors involved. The cardiovascular system remains the only pathway that connects the lymphoid organs.
Science today allows us to affirm that there are important interactions between the immune system, the nervous system and the endocrine system.

What are the stages of our immune response?

Our immune system has two lines of defense.
The first lineage is the “innate” immune response, which is said to be non-specific, acquired since our birth, devoid of memory, it constantly watches to detect abnormal cells, tumors or infected by a virus.

The non-specific immune response only takes into account the microorganisms that it fights using various defense barriers. The first barrier is physical. Indeed, our skin and our mucous membranes are the first natural barriers that attackers encounter. The skin is the largest organ in the body and provides incredible protection against infections. In addition to constituting a physical interface between the environment and our vital systems, our skin provides a hostile environment for microbes: its surface is slightly acidic and rather dry, and it is covered with “good” bacteria. The mouth, eyes, ears, nose, urinary and genital tracts still provide pathways for microbes. These routes also have their own protection system. For example, cough and sneeze reflexes expel microorganisms from the airways.

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Then, inflammation is the first barrier that microorganisms encounter after crossing our body envelope. The goal of inflammation is to inactivate the attackers and initiate tissue repair (in the event of injury). Here are the main stages of inflammation.

  • Vasodilation and greater permeability of the capillaries in the affected area have the effect of increasing blood flow (responsible for the redness) and allowing the arrival of those involved in inflammation.
  • The destruction of pathogens by phagocytes: a type of white blood cell that is capable of engulfing pathogenic microorganisms or other diseased cells and destroying them. There are several types: monocytes, neutrophils, macrophages and natural killer cells (NK cells).
  • The complement system, which includes around twenty proteins which act in cascade and directly destroy microbes. The complement system can be activated by the microbes themselves or by the specific immune response.

Finally, interferons come into play in the case of a viral infection. Indeed, these are glycoproteins which inhibit the multiplication of viruses inside our cells. Once they are secreted, they diffuse into our tissues and stimulate our immune cells. The presence of toxins of microbial origin can also trigger the production of interferons.

The second line is the “adaptive” or “acquired” immune response, which is called specific, it involves the recognition of the pathogenic agent and the memory of this event, it is specifically directed against the enemy. This is where the lymphocytes (T and B) come into play. This specific response takes longer to put in place, it requires a “learning” phase of 5 to 7 days during which the T and B lymphocytes – and particularly the body's killer cells, the T lymphocytes CD8+ – learn to recognize the target to be eliminated.

Thanks to this learning, the "profile" of the enemy is kept in memory and the body is ready to react during a second attack. It is therefore over time that effective adaptive immunity develops, which explains why young children are particularly sensitive to infections. Gradually they acquire memory and therefore the ability to react to infectious agents. Thus, our immune system remembers the particular bacteria and viruses that it has already encountered in order to make the second encounter much more efficient and rapid. It is estimated that an adult has in memory 10^9 to 10^11 different foreign proteins. Which explains why we don't catch chickenpox and mononucleosis twice, for example. It is interesting to note that the effect of vaccination is to provoke this memory of a first encounter with a pathogen.

The long process of learning our adaptive immunity

To trigger our adaptive immune response, we must first identify the adversary and isolate a characteristic fragment, an antigen. It is our dendritic cells that provide this dual role. After detecting a potentially dangerous cell, for example infected by a virus, they partially ingest it and break it down.

Among these “pieces” is the antigen which will be used to characterize the virus and to be recognized by our immune system. The antigen is then transported to a cellular compartment (endoplasmic reticulum), where it associates with transport molecules (Major Histocomputing Complex, MHC). It is then brought to the surface of dendritic cells to be presented to the immune system.

In possession of this characteristic piece of the intruder, our dendritic cells then migrate to our lymph nodes, the headquarters of our immune system, where the T lymphocytes are located.

The antigen is used to teach T lymphocytes to recognize the enemy that they must eliminate. The meeting between a dendritic cell and a T lymphocyte, and the recognition of the antigen nestled in an MHC molecule and the T lymphocyte receptor will trigger the multiplication and activation of the T lymphocytes. Once informed, the latter trigger targeted hostilities, in order to rid the body of bacteria, tumor cells or cells infected by a virus.

Our immune system can thus eliminate any foreign intruder in our body, since its primary function is to fight microbes (viruses, bacteria, etc.). Which he knows how to do very well, since in 48 hours he is able to eliminate a virus without any outside help. He sometimes encounters difficulties and fails. This is particularly the case with tumor cells.

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