Allergic reaction is a combination of symptoms resulting from an immune hypersensitivity to environmental substances such as food, pollens… or to non-environmental such as medication.
During the first steps of allergy occurs an immune reaction involving a subset of T cells and antigen presenting cells that present the allergen and resulting in IL-4 production. B cells are stimulated by an interaction with those activated T cells in conjunction with IL-4 signaling. Those B cells begin to secrete IgE, a type of antibody, that are specific of the allergen. Circulating IgE binds to a specific receptor (FcεRI) that is located on the surface of mast cells and basophils, that become sensitized to the allergen. Those cells are actors of the acute inflammatory process.
When the organism is exposed again to the allergen, it can be bound by the specific IgE that will activate the mast cells and the basophils and those cells will release a large quantity of inflammatory mediators among which histamine is preponderant. Symptoms such as vasodilatation, smooth muscle contraction, mucus production… This acute response us usually followed by a late phase response caused by the recruitment of other immune cells by the cytokines produced by mast cells.
Histamine is a natural amine present in many animal and human tissues and apart from its role in inflammation, it also has functions in the gut and in some neuronal circuits.
When high doses of histamine are ingested, it can cause a food poisoning called scombroid poisoning.