Histamine is found in almost all bodily tissues, mainly stored inside the granules of mast cells (a special type of white blood cell that is responsible for inflammation and allergic reactions). Mast cells are abundant in tissues that are prone to injury such as the extremities and blood vessels. Blood cells called basophils also contain some granules that have histamine inside them.
Histamine is released from mast cells and basophil cells in response to an antibody called immunoglobulin E (IgE). This antibody may be secreted in response to an invading pathogen such as a virus or bacteria, an allergenic substance such as pollen or in response to injury caused by toxins.
Whatever, triggers the release of IgE, the response is a flood of histamine into the blood stream that has various different effects depending on the histamine receptor it comes into contact with. Some examples of these effects include contraction of smooth muscle in the lungs, stomach or womb; the dilation and increased permeability of blood vessels; reduced blood pressure; accelerated heart rate and increased gastric acid secretion.