Each spring, summer, and fall, tiny particles are released from trees, weeds, and grasses. These particles, known as pollen, hitch rides on currents of air. Although their mission is to fertilize parts of other plants, many never reach their targets. Instead, they enter human noses and throats, triggering a type of seasonal allergic rhinitis called pollen allergy, which many people know as hay fever or rose fever (depending on the season in which the symptoms occur). Of all the things that can cause an allergy, pollen is one of the most widespread. Many of the foods, drugs, or animals that cause allergies can be avoided to a great extent; even insects and household dust are escapable. Short of staying indoors when the pollen count is high–and even that may not help-there is no easy way to evade windborne pollen.


Plants produce microscopic round or oval pollen grains to reproduce. In some species, the plant uses the pollen from its own flowers to fertilize itself. Other types must be cross-pollinated; that is, in order for fertilization to take place and seeds to form, pollen must be transferred from the flower of one plant to that of another plant of the same species. Insects do this job for certain flowering plants, while other plants rely on wind transport.

The types of pollen that most commonly cause allergic reactions are produced by the plain-looking plants (trees, grasses, and weeds) that do not have showy flowers. These plants manufacture small, light, dry pollen granules that are custom-made for wind transport. Samples of ragweed pollen have been collected 400 miles out at sea and two miles high in the air. Because airborne pollen is carried for long distances, it does little good to rid an area of an offending plant–the pollen can drift in from many miles away. In addition, most allergenic pollen comes from plants that produce it in huge quantities. A single ragweed plant can generate a million grains of pollen a day.

The chemical makeup of pollen is the basic factor that determines whether it is likely to cause hay fever. For example, pine tree pollen is produced in large amounts by a common tree, which would make it a good candidate for causing allergy. The chemical composition of pine pollen, however, appears to make it less allergenic than other types. Because pine pollen is heavy, it tends to fall straight down and does not scatter. Therefore, it rarely reaches human noses.

It is common to hear people say that they are allergic to colorful or scented flowers like roses. In fact, only florists, gardeners, and others who have prolonged, close contact with flowers are likely to become sensitized to pollen from these plants. Most people have little contact with the large, heavy, waxy pollen grains of many flowering plants because this type of pollen is not carried by wind but by insects such as butterflies and bees.

What is an allergy?

An allergy is a sensitivity to a normally harmless substance, one that does not bother most people. The allergen (the foreign substance that provokes a reaction) can be a food, dust particles, a drug, insect venom, or mold spores, as well as pollen. Allergic people often have a sensitivity to more than one substance.

Why are some people allergic to these substances while others are not?

Scientists think that people inherit a tendency to be allergic, although not to any specific allergen. Children of allergic parents are much more likely to develop allergies than other children. Even if only one parent has allergies, a child has a one in four chance of being allergic. Another factor in the development of allergies seems to be exposure to allergens at certain times when the body’s defenses are lowered or weakened such as after a viral infection, during puberty, or during pregnancy. (However, some women find that during pregnancy their hay fever symptoms diminish.)

People with pollen allergies often develop sensitivities to other troublemakers that are present all year such as dust and mold. Year-round allergens like these cause perennial allergic rhinitis, as distinguished from seasonal allergic rhinitis, or hay fever.

What is a pollen allergy?

The signs and symptoms of pollen allergy are familiar to many:

  • Sneezing, the most common, may be accompanied by a runny or clogged nose
  • Itching eyes, nose, and throat
  • Allergic shiners (dark circles under the eyes caused by restricted blood flow near the sinuses)
  • The “allergic salute” (in a child, persistent upward rubbing of the nose that causes a crease mark on the nose)
  • Watering eyes
  • Conjunctivitis (an inflammation of the membrane that lines the eyelids, causing red-rimmed eyes).

In people who are not allergic to pollen, the mucus in the nasal passages simply moves these foreign particles to the throat, where they are swallowed or coughed out. But something different happens to a pollen-sensitive person.

As soon as the allergy-causing pollen lands on the mucous membranes of the nose, a chain reaction occurs that leads the mast cells in these tissues to release histamine. This powerful chemical dilates the many small blood vessels in the nose. Fluids escape through these expanded vessel walls, which causes the nasal passages to swell and results in nasal congestion.

Histamine can also cause itching, irritation, and excess mucus production. Other chemicals, including prostaglandins and leukotrienes, also contribute to allergic symptoms.

Some people with pollen allergy develop asthma, a serious respiratory condition. While asthma may recur each year during pollen season, it can eventually become chronic. The symptoms of asthma include coughing, wheezing, shortness of breath due to a narrowing of the bronchial passages, and excess mucus production. Asthma can be disabling and can sometimes be fatal. If wheezing an shortness of breath accompany the hay fever symptoms, it is a signal that the bronchial tubes also have become, involved indicating the need for medical attention.

What is hay fever?

Hay fever (seasonal allergic rhinitis) is an allergic reaction to pollen and spores. The symptoms include sneezing, runny eyes and nose, itching nose and sometimes headache and tiredness.

Which pollen types cause hay fever?

Grass pollen is the most frequent cause of hay fever but other types are also important including pollen from trees such as alder, hazel, birch and horse chestnut, and from weeds including plantains, mugwort and docks. A person suffering from hay fever may be allergic to one, several or many types of pollen. The relative importance of the kinds of pollen that can cause hay fever varies between different climatic and vegetation zones. For example in Scandinavia , Birch pollen is very prolific and is the most important type whereas in parts of southern Spain Olive pollen ranks as the main cause of hay fever. In Britain about 90 % of hay fever sufferers are allergic to grass pollen and about 25 % are allergic to birch pollen.

Is hay fever hereditary ?

All sorts of allergies tend to run in families. Children of parents with allergies are very likely to develop allergies themselves .If one parent has an allergy there is a one in three chance that any child will inherit it. However the allergy may not be in the same form. A parent with eczema or asthma may have a child who suffers from hay fever and vice versa. Modern research has been able to show that some members of a family may have an underlying tendency to allergy but are symptom free whereas other members have hay fever, asthma or eczema. The reasons for this are not entirely understood but it seems that exposure to any allergy producing substances , such as pollen , in infancy may increase the risk of developing an allergy in later life.

Sources: Pollen UK, Medicine Net

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