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 2 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.

Among North American plants, weeds are the most prolific producers of
allergenic pollen. Ragweed is the major culprit, but others of
importance 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. These include timothy grass,
Kentucky bluegrass, Johnson grass, Bermuda grass, redtop grass, orchard
grass, and sweet vernal grass. Trees that produce allergenic pollen
include oak, ash, elm, hickory, pecan, box elder, and mountain cedar.

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.

References and Sources: Medline, Pubmed, National Institutes of Health.

last update: March 2006

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