What is the mechanism of an allergic reaction?
Histamine is a substance produced by the body as part of its defence mechanisms. It is stored in cells called mast cells, in almost all tissues of the body. When the body reacts to a foreign substance (known as an allergen, e.g. flower pollen), the mast cells stimulated by the allergen release their stores of histamine.
The released histamine then binds to its receptors (H1 receptors), causing a chain reaction that results in allergic symptoms. It causes an increase in blood flow to the area of the allergy, and the release of other chemicals that add to the allergic response. This results in the symptoms of an allergic reaction.
In allergic rhinitis, histamine causes inflammation of the nose and eyes and results in itchy, watery eyes, a runny nose, sneezing and nasal congestion.
Antihistamines works by blocking histamine H1 receptors but it does not prevent the actual release of histamine from mast cells, but prevents it binding to its receptors. This in turn prevents the release of other allergy chemicals and reduces the blood supply to the area, providing relief from the typical symptoms of allergic rhinitis.
Antihistamines can also be used to relieve the symptoms of an allergic skin rash called urticaria. This is an itchy rash with hives, also known as nettle rash. Blocking the actions of histamine relieves the itching and reduces the rash associated with this condition.
Birch pollen, one of the most common causes of hay fever in Britain. Birch trees release their pollen between March and May, and hay fever sufferers are likely to experience the worst symptoms during April:
|Blue grass, Kentucky|
|Dock, curly, sorrel, garden, sheep’s|
|False oat grass|
|Golden wattle, Sydney|
|Johnson grass (Sudan grass)|
|Pellitory of the wall|
|Pine, black, Scots and Eastern white|
|Sweet mock orange|
|Vernal grass, scented|