1/C

A lot of COVID-19 contrarians abuse the idea of "cross-reactivity" to make SARS-CoV-2 (the virus that causes COVID-19) look less dangerous than it really is. Many of them do this to avoid policies they dislike, like lockdowns.

So let's get into that

https://twitter.com/AtomsksSanakan/status/1288600865788637184

2/C

Some basics:

Immune cells known as T cells and B cells have receptors that recognize viruses.

Think of the receptors as a lock, + portions of the virus as a key; i.e. the lock (receptor) binds to a specific key (virus region), + not to other keys

https://twitter.com/AtomsksSanakan/status/1309309434867453952

3/C

Even if you've never been infected with a virus, bacteria, etc., you almost certainly have T + B cells that recognize it.

When you're first infected, those cells (especially B cells) take a few days to increase in number (and activity) + generate their full immune response.

4/C

But if you're re-infected, T + B cells reach their full response quicker + better. That's what makes the T + B cell response *adaptive*; it improves w/ re-infection.

Vaccines typically work by mimicking a 1st infection, so u respond better later

https://www.slideshare.net/Pratheepsandrasaigar/introduction-to-immunology-63981687

5/C

Sometimes two different viruses, bacteria, etc. are similar enough that the same T cell receptor or B cell receptor recognizes both of them.

In other words: 1 lock recognizes more than 1 key.

This is known as "cross-reactivity".

https://www.frontiersin.org/articles/10.3389/fimmu.2019.02631/full

6/C

So imagine a coronavirus that causes a cold infects u.
Then suppose SARS-CoV-2 (a different coronavirus) later infects u.

If u have cross-reactive cells that recognize both coronaviruses, your immune system could treat SARS-CoV-2 as a re-infection of the 1st coronavirus.