How did you get MCAS?

I keep seeing people say their autoimmune disorders started after a major physical health event like a severe infection or chemo, so I wanted to see how it was for yall? I'm no scientist but I do want to figure out why this happens because I'm a nerd.
Personally I was born with it, but I did have a fever right after I was born causing them to give me loads of antibiotics in case I had meningitis so maybe that? I've also heard of people who needed antibiotics as newborns growing up to have food allergies, so I don't know.
To me it feels like one's immune system can get ptsd like your brain, but I don't know if that's a real pattern or if my brain made it up. However I'm a highschool student who's got mystery hives on my wrist rn and can't conduct a more secure survey than a reddit poll.

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