The Biology of Hayfever (allergic rhinitis)
Allergic Rhinitis (Hayfever)
The image above shows the inflammation caused by allergic rhinitis (hayfever).

Hay fever (also known as hayfever and more accurately as allergic rhinitis) is generally caused by pollens of specific seasonal plants in people who are allergic to these substances. When these symptoms are caused by pollens, the allergic rhinitis is commonly known as "hay fever," after the fact that it is most prevalent during haying season.

Causes

Most commonly hay fever involves an allergic reaction to pollen. Though an almost identical allergic reaction can also occur to mold, animal dander, dust and similar inhaled allergens. Particulate matter in polluted air and chemicals such as chlorine and detergents, which can normally be tolerated, can greatly aggravate the condition.

It should be noted that the pollens that cause hay fever vary from person to person and from region to region; generally speaking, the tiny, hardly visible pollens of wind-pollinated plants are the predominant culprits. Pollens of insect-pollinated plants are too large to remain airborne and pose no risk. Examples of plants commonly responsible for hay fever include:

  • Trees: such as birch (Betula), alder (Alnus), hazel (Corylus), hornbeam (Carpinus), horse chestnut (Aesculus), willow (Salix), poplar (Populus), plane (Platanus), linden/lime (Tilia) and olive (Olea). In northern latitudes birch is considered to be the most important allergenic tree pollen, with an estimated 15-20% of hay fever sufferers sensitive to birch pollen grains. Olive pollen is most predominant in Mediterranean regions.
  • Grasses (Family Poaceae): especially rye (Lolium sp.) and timothy (Phleum pratense). An estimated 90% of hay fever sufferers are allergic to grass pollen.
  • Weeds:ragweed (Ambrosia), plantain (Plantago), nettle/parietaria (Urticaceae), mugwort (Artemisia), Fat hen (Chenopodium) and sorrel/dock (Rumex)

As hay fever sufferers are affected by different pollens from different plants the time of year at which hay fever symptoms manifest themselves in any given sufferer varies greatly.

Biology of Hay Fever

Hay fever is an allergic reaction to airborne substances such as pollen, skin flakes, dust mites that get into the nose, throat and also the eyes. The allergen gets into the nose and causes the body to react to the substance as a foreign object. As a result the body produces antibodies to combat the percieved threat. These antibodies mostly bind to mast cells, which contain histamine which is then released. Histamine is a derivative of the amino acid histidine that is an important component in the immune reaction. However, it also acts to irritate the upper respiratory passages making them swell and it's this swelling (as shown in the image above) that produces the typical symptoms of hay fever. Recently Drs Jo Bright and John Hancock of the University of the West of England have found evidence that nitric oxide (NO) and nitrite is released by pollen grains, and they suggest that this could be what triggers the allergic response in the nose.

Symptoms of hay fever include: coughing, headache, runny nose, sneezing, streaming eyes, conjunctivitis, wheezing, impaired sense of smell, itching nose, throat, eyes, skin (or any area exposed to the allergen).

Tests and Prevention

One part of alleviating hay fever is determining which specific antigen(s) trigger the reaction. This is generally done by allergy testing or which skin testing is the most common method. Tests can include tradermal, scratch, patch, or other tests. Less commonly, the suspected allergen is dissolved and dropped onto the lower eyelid as a means of testing for allergies. In individuals who cannot undergo skin testing the RAST blood test may be helpful in determining specific allergen sensitivity.

Because allergens are simply proteins covering the surface of the substance that causes the allergic reaction sufferers may find that cross-reactivity occurs. For example, someone allergic to birch pollen may also find that they have an allergic reaction to the skin of apples or potatoes. This is becase the surface proteins of birch pollen are also found in the skins of these foods.

The only sure means of preventing hay fever is to stay away from the substance that's causing the allergic reaction. Essentially for a pollen sufferer this means staying indoors with windows and doors closed, especially when pollen levels are highest (in the morning and evening). If you suffer badly then wear a face mask designed to filter-out pollen grains. You should also avoid garden chores such as mowing the lawn and avoid large areas of grassland such as parks and fields. Regular hand and face washing may help remove pollen grains from sensitive areas.

Treatment

Possible treatments are many and varied and the choice will depend on the type and severity of symptoms. Treatments include:

  • The main treatment includes fast-acting strong antihistimines such as drugs including chlorphenamine which reduce the histamine loading in the body and thus alleviate the symptoms caused by allergies. These are generally supplied as prescription medicines.
  • It is possible to obtain short-acting antihistimines over the counter which will generally relieve mild to moderate symptoms though a common side-effect is drowsiness.
  • New leukotriene receptor antagonists, such as montelukast (Singulair) and zafirlukast (Accolate) have proven very effective in dealing with allergic rhinitis, without the common side-effects of the first-generation antihistimines, such as drowsiness. These medicines are also long-acting, and are recommended to be taken once-daily.
  • Corticosteroid nasal sprays are also efficacious and may be effective without oral antihistamines
  • Topical decongestants can be useful in alleviating symptoms but should not be used for a prolongued period as as stopping them after protracted use can lead to Rhinitis medicamentosa (nasal congestion rebound).
  • "Allergy shots" (Hyposensibilization, immunotherapy) are occasionally recommended if the allergen cannot be avoided and if symptoms are hard to control. This includes regular injections of the allergen, given in increasing doses (each dose is slightly larger than the previous dose) that may help the body adjust to the antigen.
  • Cromolyn sodium (or cromoglycate) is a drug that stabilizes mast cells and prevent their degranulation and subsequent release of histamine. It is available as a nasal spray (Nasalcrom) for treating hay fever.