These Y-shaped proteins recently made headlines again, because the antibodies generated by the vaccines are not as effective against the Omicron variant as against previous versions of the virus - at least not without a booster dose.

To fight the virus, antibodies attach themselves to the tip of its spike protein, preventing it from using it to get into cells and thus make a person sick.

But they are not the only ones fighting.

In reality, it is "a complex and coordinated response, really beautiful from an evolutionary point of view," according to Harvard immunologist Roger Shapiro.

"Bomb carpet"

In the minutes and hours following the intrusion of the virus, proteins sound the alarm to recruit the big arms of the so-called "innate" immune system.

The first to kick in are neutrophils, which make up 50 to 70% of white blood cells, and are quick to go into battle but also to die.

Other white blood cells, macrophages, swallow pathogens and reject small parts of them to drag other sharper colleagues: those menacingly called natural killer cells, and dendritic cells, tasked with gathering information. .

"It's like lining the area with bombs, hoping to do as much damage as possible to the invader," compares John Wherry, an immunologist at the University of Pennsylvania.

"And at the same time call the headquarters for the specialized units to prepare."

B and T lymphocytes: spies and assassins

If the attacker is not repelled, the adaptive immune system kicks in.

A few days after infection, the B cells have identified the threat and begin to produce antibodies.

Vaccination also helps to train the B lymphocytes upstream - especially inside the lymph nodes in the armpit, near the site of the bite - so that they are ready.

Roger Shapiro compares them to intelligence agents, with crucial information on the enemy.

The strongest type of antibodies, called "neutralizers", are like chewing gum placed on the end of a key: they prevent the virus from unlocking the door to our cells.

Other types of antibodies don't stick as well, but still help by redirecting the virus to cells of the immune system, or calling for help to intensify the response.

Key partners of B lymphocytes, T lymphocytes can be divided into two main categories: the "helpers" and the "killers", known as cytotoxic.

The latter "are like assassins, they attack cells which are infected", explains Roger Shapiro.

But these assassins also inflict collateral damage.

The auxiliaries play the role of "generals", continues the expert, gathering the troops and directing them towards the enemy, while encouraging the B lymphocytes to increase their production of antibodies.

Prevent severe cases

Because of the large number of mutations on the spike protein in Omicron, this variant can more easily escape neutralizing antibodies, developed by a vaccine or a past infection.

The bad news is that it makes you more likely to get sick.

The good news is that it's harder to trick T cells.

These can identify, inside infected cells, the different components of the virus during its replication cycle, details John Wherry.

They are therefore much better at recognizing the adversary, even if the disguise donned by the virus allows him to go unnoticed by the antibodies.

The "killer" T cells carry out commando missions, digging a hole in cells and causing them to burst, triggering reactions that allow inflammatory proteins called cytokines to be included in the fight.

Depending on the speed of the immune response, a person who is vaccinated but still becomes ill may develop mild symptoms similar to a cold, or moderate such as the flu.

But the risk of developing a severe case of the disease is drastically reduced.

This does not prevent a booster dose from being useful: it skyrockets the production of all types of antibodies, and further entrains B and T cells.

“Omicron is worrying but the glass is still half full,” positive John Wherry.

"He's not going to totally escape our immune response."

© 2021 AFP