Title: Inflammation and Histamine

Key words: inflammatory response, mediators, histamine, prostaglandins, interleukins, vasodilation, oedema, histamine, mast cells, hypersecretion

Date: Sept 2000

Category: 16. Food Intolerance/Allergy

Type: Article

Author: DJE Candlish





The inflammatory response is the normal response of the body to physical damage or injury, such as cuts and burns or bacterial or parasitic invasion. Inflammation is triggered by the release of chemicals known as mediators from damaged cells, especially mast cells and basophils. One of the most important of these mediators is histamine. Others include prostaglandins and interleukins.


The mediators cause the small blood vessels or capillaries in the affected area to vasodilate (increase in diameter). Vasodilation increases the flow of the blood into the area, bringing blood platelets and fibrin to start the clotting process and other blood cells that begin healing the wound.


The mediators also make the capillaries more permeable (leaky), which allows fluid and protein from the blood to leak into the injured tissue, causing swelling or oedema. Granulocytes, neutrophils, monocytes, lymphocytes and other phagocytic cells leave the leaky capillaries and enter the injured tissue, to clear away the damaged cells and any organisms that may have entered the wound. Some mediators have the specific function of attracting these cells.


The result of all this activity is that the tissue becomes red, swollen, painful and hot - the classic signs of inflammation. The pain is due to pressure from the swelling and irritation of nerve endings by mediators such as histamine.


Inflammation is usually a beneficial process. Without it, injuries would not heal and trivial infections could lead to death from blood poisoning. It does have drawbacks, however, the most important of which is temporary loss of normal function. An inflamed, infected finger, for example, can put the hand out of action, partly because of the pain and partly because the inflamed finger is swollen. And, when the lining of the nose is inflamed by hay fever, it pours out extra mucus as a defence mechanism and the sense of smell is lost for a while.


These negative effects of inflammation are an acceptable price to pay when there is a real threat to survival such as bacterial invasion. But, when it is caused by reaction to generally harmless substances such as pollen, nickel-plated jewellery or a particular food (as it can be in allergic individuals), then inflammation is a serious problem.


The Role of Histamine in Allergy

Histamine is believed to be one of the most important of the mediators of inflammation. It is released from damaged normal cells as well as from specialised cells like the mast cells, which contain many other mediators. The function of histamine is to stimulate the metabolism of affected tissues, helping the processes of defence and repair. Its main effect is vasodilation, causing increased local blood flow.


Histamine is also released in response to contact with an allergen in hypersensitive people. It is directly responsible for most of the local symptoms of allergy. These include itching and redness in the eye, nose and skin and constriction of the airways due to swelling and inflammation of the lining in the nasal cavities and the lungs.


The inflammatory response that histamine helps to trigger has other local effects. In the eye, profuse watering can occur, due to hypersecretion of tear fluid. This is a normal response to irritation, such as when grit or dust enters the eye and is intended to wash out the offending substance. In allergic individuals the 'irritant' is a harmless substance like pollen. In the nose and the lungs, histamine and the inflammatory response cause hypersecretion of mucus. Again, this is intended to help remove or dilute the irritating substance involved.