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Virus & Immunity

Nathan Clumeck Professor Emeritus of Infectious Diseases - CHU Saint-Pierre – ULB July 2020

We are all now suffering the consequences of the uncontrolled spread of a virus, SARScov2, which emerged from its animal reservoir host, the bat (with which it coexisted) – and went on to infect humans.

But what is a virus?

A virus is a sequence of information (genes) enveloped in a shell that allows it to attach and penetrate a target cell, take control of it at the level of its genetic code (DNA or RNA) and thus enable it to multiply to infect other target cells. In biology, a virus is the equivalent of a computer virus: a series of coded sequences programmed to take control and reproduce to the detriment of the infected medium. SARScov2, through its surface protein (the "Spike") binds and penetrates human cells expressing a receptor called ACE2. These cells are widely distributed throughout the body in vital organs such as the lungs, heart, and blood vessels. The destruction of target cells of various significance causes clinical manifestations which can ultimately lead to death in patients infected with SARScov2 - in particular the elderly or those weakened by a deficient immune system.

But what is an immune system?

The immune system is a very complex interactive system that protects the body against aggressors (viruses, bacteria, fungi, parasites, etc.). It recognizes and distinguishes the "self" (what is constitutive of the organism) and destroys the "non-self". It is the equivalent of a country’s army - and like any army, it is subdivided into different lines of defences and various means by which to fight off and destroy the invader.

Basically, the first line of defence, the one that occurs within minutes of the assault, is called "innate immunity". It is mainly made up of large circulating cells with phagocytic capacities (macrophages) swallowing and destroying the pathogen. They produce pro-inflammatory factors (interleukins) that mobilize and attract white blood cells to the site of the attack.

The second line consists of humoral immunity (measured by serological tests), which is characterized by the production of specific antibodies neutralizing the infectious agent (the equivalent of the production of anti-tank, anti-tank missiles, planes etc. in the army). This production takes several days to be effective and may be affected by the age and general condition of the host. Once the infection is cured, the body keeps a "template" of the aggressor, which going forward, would help trigger an effective humoral reaction within a few hours if the aggressor were to reappear (this is the principle of vaccination).

The third line of defence consists of so-called "cellular" immunity. In the case of certain infections by pathogens that survive inside infected cells, what this immunity does is to activate a subclass of white blood cells (T lymphocytes) as "killer" cells that destroy infected cells.

As with the army of a country at war, it is the coordinated and synergistic mobilization of all lines of defence that makes it possible to defeat the aggressor. It is when these defences are overwhelmed by a particularly virulent pathogen and a specific treatment is absent, that the patient will die.

"Good health" is characterized by an efficient immune system that successfully stores in its "hard disk" (T-memory lymphocytes) information acquired during previous infectious episodes or as a result of vaccination, thus allowing rapid neutralization of the infection.

As there is no effective treatment against SARScov2, this is the reason why a future vaccine is much anticipated.

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