Immunology is one of the hardest and most complicated processes to understand. 27 people have won Nobel Prizes for explaining parts of it.

I'm going to hazard explaining how pandemic coronavirus infection and disease relate to immunity using everyday analogies in a few tweets.

The villain of our story is, of course, a diminutive criminal mastermind and sneaky thief, SARS-CoV-2, which I'll refer to henceforth as pandemic coronavirus. The setting for his dastardly violence is a populous metropolis- the human body.

Pandemic coronavirus really loves himself. He's a man of few possessions and a bit of a hobo. He has no fixed address. What he does have is a key that unlocks wealthy houses in the city. A blueprint for his body. And a rare device that let's him hack 3D-printers inside the houses

There are only a few ways the villain can get inside the city, which is guarded by a massive wall (of skin).

Basically he's got a chance sneaking in through a couple of gates. Inside there are other defensive barriers such as mucus and pokey hair, but those don't work as well.

Inside the city, are houses that go down a very long boulevard- the respiratory airway. His criminal uncle, SARS, had a key that opened locks to houses way down in the lungs. His key is better: he can do that, but can also open houses (in the nose), near the gates to the city.

Now, most cities are very well managed and have very efficient administration. Essential services are phenomenal. The police and fire department (the immune system) are highly professional. But in some cities they can be a bit weak or trigger happy. We will get to that later.

Remember that the police and fire department have never seen this particular villain, pandemic coronavirus, before. But they're seasoned pros. They have a lot of experience in handling these kinds of threats. Their memory goes back to the founding of the city.

Pandemic coronavirus is going to try to find a house that has the lock his key opens. Once he finds one, he's going to bring the blueprint and tool inside and run riot. He'll completely trash the house, and make copies of himself using stuff he finds, and break down the walls.

As you might imagine, the best time to apprehend pandemic coronavirus is before he's broken through a lot of houses and made a lot of copies of himself. And so on and so forth.

The good guys know this and it's a game of cat and mouse early on.

The houses do have home alarm systems that pick up on viral trash (viral RNA) and send alarms (signalling cascades). Unfortunately, they don't have cameras so the guys back at the station don't know what pandemic coronavirus looks like.

The cops on duty that rush out are the proud men and women of Team Cytokine that are part of the formidable force known as Innate Immunity. There are many, but you should remember one group in particular, IFN-I (Interferon type I)

If IFN-I can find and get to pandemic coronavirus early, the game is over before it even starts. But pandemic coronavirus also knows this. He hides his RNA in places in the house. He makes them look like they belong there (human mRNA).

And once pandemic coronavirus is caught by the home alarm, he doesn't stop. He tries to prevent IFN from even leaving the station before he can make copies of himself. There are a lot of tricks and that's a story for another day.

But remember this: the TIMING of when IFN leaves the station and gets to the attacked houses is key. If IFNs get there early they can find the infiltrators (now plural). But if they're late then it is a full-blown gunfight on tissue avenue- like a narcowar.

This inflammatory reaction is going to leave a lot of destruction in its wake (and that's what is seen with some massive lung damage in some of the worst cases).

Now if you could send in weak bad guys before pandemic coronavirus hit the city, then the cops would already be vigilant and out on the streets looking for hoodlums for a few months.

(That's the concept behind prepriming using a live attenuated vaccine like BCG or polio vaccine)

More threads on immunity (T and B cell responses; antibodies; protective immunity and vaccines) later, but in closing...

In severe cases of COVID-19, there have been abnormal records of IFN-I activity, compared to in milder cases.