The chemical that most commonly abuses my body.
Histamine - From hist-, because it is composed of Histidine amino acid residues, and -amine, because it is of the Amine chemical group similar to ammonia.
Histamine is created by the body as an antibody immune response to a protein-based invasion, be that a harmful parasitic protozoa in the gut or harmless ragweed pollen in the sinuses. Histamine (H1 receptor) dilates blood vessels to facilitate access for white blood cells which also causes swelling and itching, mucus release pins the invaders down, and the smooth muscles such as those that line the bronchi contract so the invader can't get into the lungs.
For that poor little protozoa in the gut histamine means a quick death. If the trigger is pollen the body kind of panics in the biological equivalent of using a cannon to kill a flea. The antibodies are screaming that there is a major invasion of those pollen things in the respiratory system. Histamine floods the area causing a runny nose and itchy eyes while an asthmatic finds it nearly impossible to breath. Meanwhile the white blood cells wander around wondering what all the hubbub is about.
This is the time to take an anti-histamine.
In the brain histamine has a non-immune system task by stimulating wakefulness as part of the brain's danger response system. In other words, histamine wakes you up when you have to run like hell.
This is why taking an anti-histamine makes you want to sleep away the day.