Allergic Rhinitis

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.

Pollen allergy:

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:

Pollen Calendar

No blossoming


Middle blossoming

Main blossoming

Type Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Acacia, black
Alder, common
Alfalfa, lucerne
Amaranth, coarse-haired
Ash, common
Beech, common
Bermuda grass
Birch, weeping
Blue grass, Kentucky
Chestnut, sweet
Cob, hazel
Common bent
Couch grass
Creeping bent
Cypress, Italian
Daisy, moon
Dock, curly, sorrel, garden, sheep’s
Elder, common
Elm, Scotch
False oat grass
Geranium, bedding
Golden wattle, Sydney
Hornbeam, common
Johnson grass (Sudan grass)
Lent lily
Lilac, common
Meadow fescue
Meadow foxtail
Nettle, stinging
Oak, English
Oats, common
Pellitory of the wall
Pine, black, Scots and Eastern white
Plane, oriental
Plantain, English
Poplar, white
Privet, common
Rape, oilseed
Red Fescue
Rose, Japanese
Ryegrass, perennial
Spruce, common
Sweet mock orange
Vernal grass, scented
Wheat, common
Wild rye
Yorkshire fog

Copyright 2024. All Rights Reserved