Is there a plausible mechanism for histamine to affect gut-health? I thought histamine was just an airway thing.

Histamine is a general inflammation-inducing chemical. That pain and rash you get when you touch a stinging nettle - that's caused by the stinging nettle literally injecting histamine into your skin.

Histamines affect a broad spectrum of body systems. One time I was really stressed about work, went to bed and started itching unbearably and had welts. Apparently I had broken out into hives, which I didn't recognize at first, then I took an antihistamine and within half an hour everything was back to normal.

Mast Cell Activation Syndrome would be the main one. Mast cells are an integral part of the intestinal epithelial barrier. When they become over-sensitive, even small amounts of dietary histamine (present in a large number of foods) can trigger GI and systemic symptoms. H2 blockers have shown some efficacy, as well as DAO supplements (which break down dietary histamine).

I believe it has been found that the new h1 inhibitors like Allegra can inhibit TNF-a, so it could technically work on several inflammatory diseases.

This is what some biologics do.

I have spoke to people who had improvement after doing an antihistamine protocol. In the long covid/cfs space.

I'm constantly eating beef and occasionally cheese with no issue, so I haven't been thinking about histamine. I don't seem to react to cheese, other fermented milk or fermented meat or canned fish.