Immune System and the Coronavirus

Ed Yong wrotes this excellent article explaining how the immune system works. First up, keep in mind that the word “immunity” means different things to different people. To the layman, it means “protected from”. To the immunologist, it just means the “immune system has responded to a pathogen”.

 

There are three phases to it. Phase 1 is called the “innate immune system”:

“(The body sounds) alarms, summoning and activating a diverse squad of white blood cells that go to town on the intruding viruses—swallowing and digesting them, bombarding them with destructive chemicals, and releasing yet more cytokines. Some also directly prevent viruses from reproducing.”

This part happens practically instantly, it’s ancient “technology” shared with most other animals, and is the same for almost every person. As you noticed, it doesn’t care about the details of the invader (non-human = dangerous):

“What the innate immune system lacks in precision, it makes up for in speed.”

Redness, inflammation, swelling are all signs of this phase at work.

 

Let’s say that didn’t work. It’s time to call in the specialists. Aka Phase 2. Messenger cells grab “small fragmants of the virus” and take it to the specialized white blood cells (called T-cells):

“The T-cells are selective and preprogrammed defenders. Each is built a little differently, and comes ready-made to attack just a few of the zillion pathogens that could possibly exist. For any new virus, you probably have a T-cell somewhere that could theoretically fight it. Your body just has to find and mobilize that cell.”

And then:

“When a match is found, the relevant merc(enary) arms up and clones itself into an entire battalion, which marches off to the airways.”

Some T-cells are killers, who “blow up the infected respiratory cells”. Others help by activating B-cells to produce antibodies that prevent the virus from latching onto their hosts. Summary: T-cells + antibodies = adaptive immune system. Since it involves finding the right tool for the job, it takes longer (even days).

 

Once the virus is wiped out, the T-cells and B-cells die off. Most of them. A few are retained for future use, should the need arise. This is Phase 3, aka memory:

“Memory is the basis of immunity as we colloquially know it—a lasting defense against whatever has previously ailed us.”

 

Ok, now we know the general theory of how the immune system works. But of course, the virus too has some tricks up its sleeve. Let’s focus on what we’ve seen with the coronavirus for the rest of the blog. It is good at delaying Phase 1:

“Those delays cascade: If the innate branch is slow to mobilize, the adaptive branch will also lag.”

But even with that delay, most people do recover. Others don’t. And a third set have T-cells that “underperformed” – they were mobilized but their levels receded before the virus was vanquished. In such people, the virus keeps going deeper and though the immune system can’t stop things, it doesn’t stop trying. And the fact that the immune system stays at work, says Yong, is also a problem:

“Immune responses are inherently violent. Cells are destroyed. Harmful chemicals are unleashed. Ideally, that violence is targeted and restrained… But if an infection is allowed to run amok, the immune system might do the same, causing a lot of collateral damage in its prolonged and flailing attempts to control the virus.”

In other words, you are now at danger from the virus and the immune system.

 

Normally, the immune system activates different cells for (1) Viruses and microbes within cells, (2) Bacteria and fungi outside cells, (3) parasitic worms. In some (not all) coronavirus cases, the body produces all three! It’s almost as if the immune system is confused, writes Yong. Again, this is just some cases. Which is why Yong warns us from jumping to conclusions. As the infected count increases, inevitably we will find some people with very different reactions:

“In other words, the worse the pandemic gets, the weirder it will get.”

So expect more weirdness, just don’t assume every new weird case is going to be the new normal.

 

Another set of studies shows between 20 to 50% of people never exposed to coronavirus still have T-cells that can recognize it. Is that because they were infected earlier by related viruses in the past, which cause common colds?

 

Then there’s the question of how long we stay immune from the coronavirus? When the T-cell count drops to zero within a few months of recovery, is that because the T-cells are gone (and with it immunity from a relapse)? Or is it because our instruments aren’t good enough to measure below a certain number? Then again, immunity isn’t just a matter of quantity alone. Quality matters too. But which qualities, we still don’t understand. In any case, repeat business isn’t unique to this virus alone: our immunity for many other diseases is also known to wear off with time. Which is one reason Yong argues that the hunt for the vaccine is necessary. Because we just don’t know enough: immunity might wear off.

 

I’ll wind up with two lines that Yong repeats throughout his article:

“You see, the immune system is very complicated.”

And:

“Immunology is where intuition goes to die.”