Histamine is an organic nitrogenous compound involved in local immune responses as well as regulating physiological function in the gut and acting as a neurotransmitter. 

A molecular model and chemical structure of histamine. image: wikipedia

Histamine is involved in the inflammatory response. As part of an immune response to foreign pathogens, histamine is produced by basophils and by mast cells found in nearby connective tissues. Histamine increases the permeability of the capillaries to white blood cells and some proteins, to allow them to engage pathogens in the infected tissues.

How is it stored and released?

Most histamine in the body is generated in granules in mast cells and in white blood cells called basophils and eosinophils. 

Mast cells are especially numerous at sites of potential injury — the nose, mouth, and feet, internal body surfaces, and blood vessels. 

Non-mast cell histamine is found in several tissues, including the brain, where it functions as a neurotransmitter. Another important site of histamine storage and release is the enterochromaffin-like (ECL) cell of the stomach.

The most important mechanism of release is from of mast cell and basophil histamine release is immune response. These cells, if sensitized by IgE antibodies attached to their membranes, degranulate when exposed to the appropriate antigen.

Source:  Histamine. (2015, January 5). In Wikipedia, The Free Encyclopedia. Retrieved 06:01, February 7, 2015, from http://en.wikipedia.org/w/index.php?title=Histamine&oldid=641097535