Histamine is a biogenic amine and signalling molecule that plays a central role in immune responses, allergic reactions, gastric acid secretion, and neurotransmission. It is produced by mast cells and basophils (immune cells), enterochromaffin-like cells in the stomach, and neurons in the brain. In the context of allergy, histamine is the primary mediator released when IgE-bound mast cells are activated by an allergen.

Histamine intolerance is a condition in which the body accumulates excess histamine from dietary sources or internal production, combined with reduced capacity to break it down (due to deficiency of the enzyme diamine oxidase, DAO). This can cause symptoms including headaches, flushing, urticaria, nasal congestion, itching, palpitations, and gastrointestinal distress, mimicking allergic reactions but without IgE involvement.