Histamine is an important naturally-occurring neuro-transmitter compound and 'local hormone' involved in mammalian gut regulation, immune responses and many other vital processes.

Histamine levels are finely balanced, and variations have dramatic effects on a range of bodily processes. Because the compound is also found in a range of foodstuffs (cheese, fish etc.) it has been suggested that some people may be prone to developing excess histamine levels if their digestive system doesn't properly deal with the extra intake.

'Histamine Intolerance' has been suggested as a possible cause for a set of adverse reactions - but the syndrome is yet to be accepted as a 'real' disease by the mainstream medical establishment.

It has been widely investigated, and many research groups claim to have found solid science-based evidence for it.

Histamine intolerance arises from the imbalance of accumulated histamine and the ability to degrade histamine. In healthy people, exogenous or dietary histamine is eliminated primarily with the participation of intestinal diamine oxidases (DAOs). Insufficient expression of secretory DAO (due to genetic polymorphisms) or inhibition of their activity (xenobiotics or drugs) increases the risk of histamine toxicity to the human body."

Source :Nutrients, 13(9): 3207

Many clinical research groups, however, are adamant that the condition isn't medically valid.

Adverse reactions to food or food ingredients are more often perceived than objectively verifiable. However, reliable laboratory tests are often lacking. As a result, people with perceived adverse reactions to food often follow extensive elimination diets for years and unnecessarily restrict their diet, as in the case of the frequently suspected histamine intolerance. In this condition, laboratory parameters such as the determination of diamine oxidase in serum have been shown to be inconclusive. The lack of symptom reproducibility calls into question the clinical picture of adverse reactions to ingested histamine."

Source : Allergol Select. 5: 305–314.
_{( Editor's note: Diamine oxidase is an enzyme, the presence or absence of which can be used to gauge histamine levels )}