Each spring, summer, and fall, tiny pollen grains are
released from trees, weeds, and grasses. These grains
hitch rides on currents of air. Although the mission of
pollen is to fertilize parts of other plants, many never
reach their targets. Instead, pollen enters human noses
and throats, triggering a type of seasonal allergic rhinitis
called pollen allergy. Many people know this as hay fever.
Of all the things that can cause an allergy, pollen is one
of the most common. Many of the foods, medicines, or
animals that cause allergies can be avoided to a great
extent. Even insects and household dust are escapable.
But short of staying indoors, with the windows closed,
when the pollen count is high-and even that may not
help-there is no easy way to avoid airborne pollen.
What is pollen?
Plants produce pollen too tiny to see with the naked eye-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. Cross-pollination means that for fertilization to take place and seeds to form, pollen must be transferred from the flower of one plant to that of another of the same species. Insects do this job for certain flowering plants, while other plants rely on wind for 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 make small, light, dry pollen
grains that are custom-made for wind
Amazingly, scientists have collected samples
of ragweed pollen 400 miles out at sea and
2 miles high in the air. Because airborne
pollen can drift for many miles, it does little
good to rid an area of an offending plant.
In addition, most allergenic pollen comes
from plants that produce it in huge quantities. For
example, a single ragweed plant can generate a million
grains of pollen a day.
The type of allergens in the pollen is the main factor that
determines whether the pollen 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. It is, however, a relatively rare cause
of allergy because the type of allergens in pine pollen
appear to make it less allergenic.
Among North American plants, weeds are the most
prolific producers of allergenic pollen. Ragweed is the
major culprit, but other important sources are sagebrush,
redroot pigweed, lamb's quarters, Russian thistle
(tumbleweed), and English plantain.
Grasses and trees, too, are important sources of
allergenic pollens. Although more than 1,000 species
of grass grow in North America, only a few produce
highly allergenic pollen.
It is common to hear people say 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 be sensitive to pollen
from these plants. Most people have little contact with
the large, heavy, waxy pollen grains of such flowering
plants because this type of pollen is not carried by wind
but by insects such as butterflies and bees.
Some grasses that produce pollen
Sweet vernal grass
Some trees that produce pollen
When do plants make pollen?
One of the most obvious features of pollen allergy is its
seasonal nature-people have symptoms only when the
pollen grains to which they are allergic are in the air.
Each plant has a pollinating period that is more or less
the same from year to year. Exactly when a plant starts
to pollinate seems to depend on the relative length of
night and day-and therefore on geographical location-
rather than on the weather. On the other hand, weather
conditions during pollination can affect the amount of
pollen produced and distributed in a specific year. Thus,
in the Northern Hemisphere, the farther north you go,
the later the start of the pollinating period and the later
the start of the allergy season.
A pollen count, familiar to many people from local
weather reports, is a measure of how much pollen is
in the air. This count represents the concentration of
all the pollen (or of one particular type, like ragweed)
in the air in a certain area at a specific time. It is shown
in grains of pollen per square meter of air collected
over 24 hours. Pollen counts tend to be the highest early
in the morning on warm, dry, breezy days and lowest
during chilly, wet periods. Although the pollen count is
an approximate measure that changes, it is useful as
a general guide for when it may be wise to stay indoors
and avoid contact with the pollen.
Courtesy: National Institute of Allergy and Infectious Diseases
|| San Antonio, Texas
Introduction to Allergies
What is an allergy?
What is allergic rhinitis? (Hay Fever)
Why are some people allergic?
What is an allergic reaction?
What Is Food Allergy?
Introduction to Asthma
What are Hives?