Is histamine only involved in allergies?

Histamine is something your own body makes, not something that arrives from outside. The raw material is histidine, one of the amino acids, which are the loose pieces that proteins in food and in the body get built from; change one part of that piece and what's left is histamine. That's why it's called a biogenic amine: a small molecule, born from an amino acid, made inside a living thing.

What histamine does isn't written into the molecule itself. It depends on where it lands. It has four receptors, called H1, H2, H3, and H4, and those are the locks it fits into to give the order. Think of a master key in a building: it's the same key, but it opens different things depending on the floor, because each floor has different locks laid out in a different way. That's how its receptors sit in the body's defense tissues, in nerve tissue, and in the digestive tract, in different proportions. So the same molecule is caught up in allergy and inflammation, and at the same time has a hand in how much acid the stomach makes and in how signals get passed along inside the nervous system.