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250px Pollen grains from a variety of common plants can cause hay fever.

Allergic rhinitis, also called pollinosis, hay fever or nasal allergies, is a collection of symptoms, predominantly in the nose and eyes, that occur after exposure to airborne particles of dust, dander , or the pollens of certain 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 it is most prevalent during haying.

1 Causes

Allergies are caused by an oversensitive immune system, leading to a misdirected immune response. The immune system normally protects the body against harmful substances such as bacteria and viruses. Allergy occurs when the immune system reacts to substances ( allergens) that are generally harmless and in most people do not cause an immune response.

As noted above, hay fever involves an allergic reaction to pollen. A virtually identical reaction occurs with allergy to mold, animal dander, dust, and similar inhaled allergens. This is often exaggerated in populated areas (cities,) where the male to female ratio of plants can frequently reach 9|1 or 10|1 (because male plants typically do not produce the clutter (fruit,) that is consequent in female plants.) Stuff such as pollution and manmade chemicals, (chlorine, detergents, etc.) which can normally be tolerated can greatly aggrevate the condition.

The pollens that cause hay fever vary from person to person and from region to region, but generally speaking it's the tiny, hardly visible pollens of wind-pollinated plants that are the 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, alder, horse chestnut, hazel, willow, poplar, lindenLinden is one of three English names for a genus of trees, Tilia also known as lime and basswood . Linden is also widely used as a place name, very often being a place where linden trees grow: In Belgium: Linden is a town in the municipality of Lubbeek. (birch is the most common culprit: 15-20% of people suffering from hay fever are allergic to birch pollen)
• Grasses ( Gramineae), especially ryeReferences: 2002-09-22 Rye Secale cereale is a grass grown extensively as a grain and forage crop. It is a member of the wheat tribe and is closely related to barley and wheat, although it is the singular species in the genus Secale . Rye grain is used to and timothyTimothy (died AD 80) was a first century Christian bishop; he is venerated as a saint by most Christians that venerate saints. Timothy was Paul's companion in many of his journeyings. His mother, Eunice, and his grandmother, Lois, are mentioned as eminent
• Wind-pollinated flowers (e.g. daisiesSee also Daisy, Arkansas, United States and Daisy, Georgia, United States. See Gerbera for Transvaal Daisy or Barberston Daisy True daisy Scientific classification Kingdom: Plantae Division: Magnoliophyta Class: Magnoliopsida Order: Asterales Family: Aste)
• Weeds (e.g. ragweedSee text Ragweeds Ambrosia is a mostly American genus of mostly shrubs and subshrubs from the Sunflower family Asteraceae. They are noxious weeds that grow in disturbed soil in fallow fields, along roadsides, in vacant lots, and along river banks. There a, plantainsPlantain is the common name for two very different plants. The term plantain can refer to plants of the genus Plantago''. Its leaves can be eaten as a potherb or in salads. To many of us it is nothing more than a common lawn weed like the dandelion or but, nettlesNettle Scientific classification Kingdom: Plantae Division: Magnoliophyta Class: Magnoliopsida Order: Urticales Family: Urticaceae Genus Urtica Species see text Nettles are members of the genus Urtica in the family Urticaceae. The most prominent member of, mugwort, sorrel)

In addition to individual sensitivity and geographic differences in local plant populations, the amount of pollen in the air can be a factor in whether hay fever symptoms develop. Hot, dry, windy days are more likely to have increased amounts of pollen in the air than cool, damp, rainy days when most pollen is washed to the ground.

When an allergen such as pollen or dust is inhaled by a person with a sensitized immune system, it triggers antibody production. These antibodies bind to cells that contain histamine. When the antibodies are stimulated by pollen and dust, histamine (and other chemicals) are released. This causes itching, swelling, and mucus production. Symptoms vary in severity from person to person. Very sensitive individuals can experience hives or other rashes.

Some disorders may be associated with allergies. These include eczema and asthma, among others.

Allergies are common. Heredity and environmental exposures may contribute to a predisposition to allergies.