Allergy Cases: Pollen Role in Allergy and Asthma - Case-based Curriculum of Allergy and Immunology

Author: V. Dimov, M.D., Allergist/Immunologist and Assistant Professor at University of Chicago
Reviewer: S. Randhawa, M.D., Allergist/Immunologist and Assistant Professor at LSU (Shreveport) Department of Allergy and Immunology

What is Pollen

Pollen is a fine to coarse powder consisting of microgametophytes (pollen grains, male). In plants, a gametophyte is the multicellular haploid structure containing a single set of chromosomes. In other words, pollens are small, egg-shaped male cells found in flowering plants.

The female gametophyte (ovule) is called a megagametophyte and the male gametophyte (pollen) is called a microgametophyte. The transfer of pollen grains to the female reproductive structure is called pollination.

Pollen is an airborne outdoor allergen.

Sources of Pollen

Mnemonic for the pollen calendar: TGR MI

Tree pollen - spring
Grass pollen - summer
Ragweed and weeds - fall

Mold
Indoor allergens are year-round

Tree pollen in the spring

Ash
Elm
Birch
Maple
Oak
Cypress
Hickory
Poplar
Sycamore
Western red cedar

Elm tree

There are approximately 30 to 40 species of elm.

Ash tree

The tree's common English name goes back to the Old English æsc, a word also routinely used in Old English documents to refer to spears made of ash wood. The wood is hard (a hardwood), tough and very strong but elastic, extensively used for making bows, tool handles, and baseball bats.

Birch

These are generally small to medium-size trees, mostly of northern temperate climates. The bark of all birches is characteristically marked with long horizontal lenticels, and often separates into thin papery plates.

The common name birch is derived from an old Germanic root, birka, with the Proto-Indo-European root *bherəg, "white, bright; to shine."

In Belarus, Russia, the Baltic States, Finland, Sweden and parts of northern China, birch sap is consumed as a refreshing beer. The birch is considered a national tree of Russia and is also New Hampshire's state tree.

Grass pollen in the summer

Bermuda grass
Timothy grass
Bluegrass
Orchard grass
Red top grass
Sweet vernal grass

Bermuda grass

Bermuda grass (Cynodon, Greek "Dog-tooth") is a genus of nine species of grasses, native to warm temperate to tropical regions of the Old World. The genus as a whole is known as Bermuda Grass or Dog's Tooth Grass.

It is difficult to pull out because the rhizomes and stolons break readily, and then re-grow. It is also noted for its common use on the surface of greens on golf courses, as well as football and baseball playing fields.

Timothy grass

Timothy-grass (Phleum pratense), is an abundant perennial grass native to most of Europe except for the Mediterranean region. It grows to 50–150 cm tall/ The flowerhead is 7–15 cm long, with densely packed spikelets.

Timothy-grass can be confused with Meadow Foxtail.

Timothy-grass was unintentionally introduced to North America by early settlers, and was first described in 1711 by John Herd from plants growing in New Hampshire. Herd named the grass "herd grass" but a farmer named Timothy Hanson began to promote cultivation of it as a hay about 1720, and the grass has been known by its present name since then.

It is commonly grown for cattle feed and, in particular, as hay for horses. It is considered part of the standard mix for grass hay and provides quality nutrition for horses. Timothy Hay is a staple food for domestic pet rabbits, guinea pigs, chinchillas, and degus, often making up the bulk of their diet.

It has been used as SLIT under the name Grazax.

Ragweed and weed pollen in the fall

Ragweed
Pigweed
Tumbleweed, Russian thistle
Cockleweeds
Sagebrush

Ragweed

Ragweeds (Ambrosia), also called bitterweeds and bloodweeds, are a genus of flowering plants from the sunflower family.

The scientific name of this genus is sometimes claimed to be derived from the Ancient Greek term for the perfumed nourishment of the gods, ambrosia which would be ironic since the genus is best known for one fact: its pollen is a common allergen.

Ambrosia produces separate male and female flower heads on the same plant.

After wind pollination, the female flowers develops into a prickly, ovoid burr with 9-18 straight spines. This burr gets dispersed by clinging to the fur or feathers of animals passing by.

Each plant is able to produce millions of grains of pollen over a season, and the plant is wind-pollinated. It is highly allergenic, and generally considered the greatest allergen of all pollens, and one of the primary causes of allergic rhinitis in North America.

Ragweed is a plant of concern in global warming because higher levels of carbon dioxide may increase pollen production. On dry windy days, the pollen may travel many miles.

No area in the United States is ragweed pollen free. Ragweed was accidentally introduced to Europe during World War I; they thrived and have greatly spread since the 1950s.

Total eradication of ragweed is considered impossible, owing to the plant's tenacity and tremendous seed-producing capability.

Tumbleweed (Salsola)

Salsola (also known as Tumbleweed, Saltwort or Russian thistle) is a genus of herbs, subshrubs, shrubs and small trees.

"Tumbleweed" plants break away from their roots in the autumn, and are driven by the wind as a light, rolling mass, scattering seed far and wide. The seeds are produced in large numbers. The deep, ineradicable taproot survives to grow again the following season.

A wind-blown tumbleweed caught against a fence. Image source: Wikipedia, public domain.

The leaves and shoots of many species are edible and some are grown as vegetables, often used for salads.

In film and television shows, a tumbleweed blowing across a scene can be used to indicate a deserted location or to emphasize an awkward silence.

Goldenrod

The goldenrod is a yellow flowering plant in the Family Asteraceae. Probably due to their bright, golden yellow flower heads blooming in late summer, the goldenrod is often blamed for causing hay fever in humans. The pollen causing these allergy problems is mainly produced by Ragweed (Ambrosia sp.), blooming at the same time as the goldenrod, but is wind-pollinated. Goldenrod pollen is too heavy and sticky to be blown far from the flowers, and is thus mainly pollinated by insects.

The goldenrod is the state flower of the U.S. states of Kentucky and Nebraska.

References

Clinical review: ABC of allergies, Summer hay fever. BMJ 1998;316:843, figure.
Pollen Calendar for North America.

Interactive Allergy Map by Greer Labs. Click your state to find region-specific, common airborne allergens there.
Pollen, from Wikipedia, the free encyclopedia.
Tips to Remember: Outdoor allergens. AAAAI.
Modified text and links about different pollinating plants are from Wikipedia, public domain.

Images