Histamine is a substance present in cells of the immune system. It is released when tissue is damaged and during an allergic reaction. Anaphylaxis, hay fever and hives are common reactions involving histamine. Though it is associated with itching, runny noses and other annoyances, histamine is an important part of the body's defenses.
Histamine is a product of the amino acid histidine. Two kinds of immune cells release histamine: mast cells in the tissue, and basophils in the blood. Histamine has three main actions on the surrounding cells: dilating small blood vessels and increasing capillary permeability, causing redness and swelling at the area of injury or reaction; contracting the smooth muscle of the bronchial tubes, causing difficulty breathing and a tight feeling in the chest; and increasing gastric secretion.
Histamine and Anaphylaxis
Anaphylaxis is a life-threatening condition in which exposure to an allergen triggers release of histamine into the circulation. This causes blood vessels to dilate all over the body while increased capillary permeability allows fluids to leak out into the tissue. A severe drop in the peripheral blood pressure results, limiting return of blood to the heart. At the same time, constriction of the bronchial tubes causes respiratory distress. Anaphylaxis must be treated immediately with epinephrine or a similar drug.
Histamine in Hay Fever
Hay fever is a local allergic response triggered by allergens coming into contact with the nasal tissue. Histamine release causes the blood vessels of the nose to dilate and the capillaries to leak, resulting in swollen, secretory intranasal tissue--commonly known as runny nose and congestion.
Histamine in Hives
Hives, also known as urticaria, are a localized reaction caused by allergens coming into contact with specific areas of the skin. Local release of histamine causes a red flare (due to dilating blood vessels) and a well-defined swelling (due to fluid leaking from permeable capillaries). Heat and itching are other common effects. Hives can be a result of topical (e.g., contact with a nettle plant) or systemic exposure (e.g., eating certain foods) to an allergen.
The release of histamine and its effects on the tissue are part of the body's defenses. Dilating blood vessels near the site of an injury reduces blood flow, and therefore blood loss, in the area. Capillary permeability allows white blood cells to pass into the tissue, where they can begin the healing process. Bronchial constriction reduces the volume of air inhaled, thus limiting exposure to an airborne toxin. Histamine also triggers the release of stomach acids, a vital part of the digestive process.