Histamine is the bane of all asthmatics, sufferers of allergic rhinitis and other such annoying allergic conditions.

Histamine is the good guy in helping the body fight parasite infestation, but in modern society is more often than not the bad guy as the initiator of a "type 1 hypersensitivity reaction" (aka acute allergic response).

Histamine is the reason people allergic to bee stings puff up like a balloon when they get stung, why some asthmatics gasp for breath when they inhale an allergen, and why hayfever really sucks.