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Histamines are chemicals released by cells of the immune system during the inflammatory response, which is one of the body's defenses against infection. For instance, the inflammatory response helps neutralize bacteria that enter the body when the skin is accidentally cut with a knife. In addition, the sneezing, runny nose, and itchy eyes of allergies are actually "small-scale" inflammatory responses initiated by allergens such as dust, mold, and pollen. Histamines play a prominent role in both kinds of reactions.

Histamines are contained within two types of immune cells, basophils and mast cells. Basophils are free-floating immune cells, while mast cells are fixed in one place. When basophils and mast cells are activated by other immune cells—such as in response to invasion of the body by bacteria—they release histamines into body tissues.

Once histamines are released into the tissues, they exert a variety of effects. Histamines dilate blood vessels, stimulate gland secretion, and prompt the release of proteins from cells. These effects, in turn, help the body rid itself of foreign invaders. The dilation of blood vessels increases the circulation of blood to the injured area, washing away harmful bacteria. The release of proteins from cells attracts other immune cells to the area, such as macrophages, which engulf and destroy bacterial invaders. In response to these activities within the body, the injured area becomes red, swollen, and painful. These symptoms of inflammation signal that the body's inflammatory response is activated.

Histamines also play a role in allergic responses. Instead of responding to bacterial or viral invaders, mast cells and basophils bind to allergens and then release histamines and a special kind of antibody called IgE. Histamines released from mast cells in the nasal passages, lungs, and throat in response to allergens prompt inflammatory responses in these organs, leading to allergic symptoms, such as a running nose, coughing, sneezing, and watery eyes.

An effective way to control allergic symptoms is to disable the histamines with antihistamines which prevent the allergen from exerting their effects on the tissues. Antihistamines are the active ingredients in many allergy medications, and work by binding to the released histamines, effectively inactivating them. Until recently, anti-histamines had an inconvenient side effect: they caused drowsiness in a small percentage of the population. Newer antihistamines do not cause drowsiness, and most people can tolerate these antihistamines without side effects.

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