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).

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.

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.