What is the immune system? How does it work?

Céline Giustranti
When faced with attacks from the outside world (viral or bacterial infection), the body defends itself by activating the immune system. Often compared to an army, this system is very complex. It is able to mobilize several types of cell and produce molecules to defend the pour défendre notre organisme.

The immune system comprises two lines of defense. The first is innate immunity, which has no form of memory and is constantly on the lookout for abnormal cells, cancer cells and virus-infected cells. The second is adaptive immunity, which takes longer to develop and specifically targets the enemy. It requires a “learning” phase of five to seven days, during which the T and B lymphocytes – and particularly the body’s killer cells, the DC8+ T lymphocytes – learn to recognize the target to be eliminated.

This learning enables the "profile" of the enemy to be memorized, and the body reacts promptly at the next encounter. Thus, over time, efficient adaptive immunity develops, which explains why young children are particularly vulnerable to infection. They gradually acquire an adaptive memory and thus the ability to react to infectious agents. Vaccination is based on this ability of the immune system to memorize an enemy in order to react promptly.


The long learning process

To activate this adaptive immune response, the enemy must first be identified, and a characteristic fragment – an antigen – must be isolated from it. The dendritic cells perform this role. After having detected a potentially dangerous cell, which is infected by a virus, for example, they partially ingest it and cause it to decompose.

The antigen that will be used to characterize the virus and be recognized by the immune system is found among these “pieces.” It is generally a protein fragment. The antigen is then conveyed to a cellular compartment (endoplasmic reticulum), where it combines with carrier molecules (MHC: Major Histocompatibility Complex). It is then brought to the surface of the dendritic cells to be introduced into the immune system.

Once they have this characteristic piece of the intruder, the dendritic cells then migrate to the lymph nodes, the headquarters of the immune system, where the T lymphocytes are located.

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

Our immune system can thus eliminate any foreign body, since its primary function is to fight microbes (viruses and bacteria), which it is quite able to do, since in 48 hours it is able to eliminate a virus with no outside help. It sometimes encounters difficulties and fails. This is the case with tumor cells.