Histamine is an endogenous short-acting biogenic amine synthesized from the amino acid histidine and is widely distributed throughout the body. Histamine is an important mediator of immediate-type-allergic reactions. Normally, there is a minimal amount of histamine circulating in the human body. An allergic reaction can occur when a substance enters the body (i.e. food, chemicals, insect bites, oils from plants). In response to the molecule, the body will then release larger quantities of histamine, which starts a cascade of events and begins an immune response. The body balances the histamine release by producing Epinephrine (adrenaline) which can help modulate the effects of histamine. When histamine is released, part of the cascade of events is inflammation. Antihistamines work by blocking the action of histamine and the resulting inflammation, amongst other symptoms, to provide relief.

Anaphylaxis occurs when there is a hypersensitive response. Reactions can range from mild to severe and, in extreme cases, can be fatal. Symptoms of hypersensitivity to histamine include hives, tingling feeling in the mouth, difficulty breathing, stomach cramping and diarrhea.

Elevations in plasma and tissue histamine levels have been reported during anaphylaxis and experimental systems investigating the allergic responses of the skin and airways. Because of its potent role in the immune response, histamine in the body is present in a very transitory state and can only be measured within minutes of release. The half-life of histamine in a biological system is four minutes before conversion to n-methyl histamine. N-methyl histamine is a major product of histamine metabolism, is present in urine and has a longer half-life than that of histamine. This assay reacts equally to N-methyl histamine and histamine, making it an excellent choice for measuring histamine levels in urine.

Histamine is also in a class of neurotransmitters called “Small Molecule Neurotransmitter Substances”. This group includes molecules such as Serotonin, Epinephrine and Dopamine. Additionally, histamine plays a role in gastric acid secretion, assisting in the induction of acid production.