Histamine is regarded as a neuro-immuno-endocrine system mediator and is produced by many cell types, including basophils and mast cells. These cells are responsible for releasing endogenous histamine in response to immunological and nonimmunological stimuli. Histamine plays many key roles in the human body; it functions as a bronchoconstrictor in the lungs, a key mediator of wheal formation on the skin, a controller of gastric acid secretion, and a neurotransmitter in the brain. Another main source of histamines is food. The fresher the food, the lower the probability of histamine formation.

There are two main enzymes responsible for the degradation of histamine in the human body, Histamine N-Methyltransferase (HNMT) and Diamine Oxidase (DAO). Genetic polymorphisms in the genes encoding these enzymes can lead to a disproportionate amount of histamine in the body resulting in selective hypersensitivity to certain drugs, migraines, atopic dermatitis, and histamine intolerance which is characterised by gastrointestinal, skin, and respiratory problems, following the ingestion of histamine-rich foods.

There are many inconsistencies in the literature, around the association of histamine with various other allergies such as asthma and allergic rhinitis. This could be due to several reasons:
• Gender-related differences observed in the activities of the two histamine-degrading enzymes
• Interactions with unknown genetic polymorphisms
• Racial heterogeneity
• Various environmental factors which interact with an individual’s genetic architecture