Histamines were substances produced by an individual in response to an allergic reaction.
When Doctor Mendoza was infected with Ferengi pyrocytes in 2366, his immune system responded with a system-wide histaminic reaction. (TNG: "The Price")
An early symptom of infection by the Tarchanne was an allergic histamine response. (TNG: "Identity Crisis")
In Iresine syndrome, patients exhibited electropathic residue and lowered histamine count. The absence of lowered histamine levels permitted the aftereffects of Ullian telepathic memory invasion to be distinguished from Iresine syndrome. (TNG: "Violations")
The Star Trek Encyclopedia, 4th ed., vol. 1, p. 338 defined histamine as, "Biochemical substance produced by the breakdown of histidine, an amino acid in humanoid tissues. Histamine is produced by the body when it comes in contact with substances to which the body is sensitized, and is a primary factor in the humanoid allergic response."