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Before we consider their role in allergies, there are many fascinating things we should know about pollen grains. First, pollen, like sheep, is a collective noun, so we never say or write "pollens", although a surprising number of professional people make that mistake.
All of the plants that are grouped together as Flowering Plants or Angiosperms produce pollen as part of their reproductive process. Pollen grains are tiny, often roughly spherical structures that contain and transport male sex cells of flowering plants. The familiar flowers that decorate your table or garden all have a similar structure with bright colours and showy petals. These features of flowers have been designed by nature to attract insects that aid in the transfer of the male component of reproduction (pollen) to the respective female organ (stigma).
But not all flowering plants depend on insect visitors; in fact the primary culprits of pollen allergies are the best examples of wind pollination. All of the trees and shrubs that cause spring allergy are wind-pollinated. Their flowers have been stripped down to the bare minimum and are often grouped together in long dangling structures (anthers) that expose the pollen grains to the wind. During pollination, the wind blows pollen off the anthers and carries it for various distances eventually to land on some surface (soil, lakes, nose and eyes of humans), but only a very few will find their way to a receptive female stigma.
Wind pollinated species compensate for this less precise transfer of sex cells by sending clouds of pollen into the air, and because of this, individuals are more often allergic to these species. Individuals can have allergic reactions to insect pollinating species, however similar symptoms to allergy (sneezing or wheezing) can occur in response to the aroma of a flower or plant.
Can you see a pollen grain?
Yes and no. With the aid of a compound microscope, the pollen grains of different plant types can be differentiated allowing scientists to study the number and types of pollen grains released into the air. Masses of pollen are visible to the naked eye on the end of a stamen of a tulip or other flowers. But the naked eye cannot distinguish an individual pollen grain; it is far too small.
How many pollen grains float through the air?
That depends on the type. For example, the white birch is one of the most allergenic taxa. Their flowers, called catkins, are long, dark, pendulous, worm-like structures on the ends of the branches. Each catkin can produce roughly 2 to 5 million pollen grains hence a typical tree will produce and release to the air roughly 2,000,000,000 pollen grains per season. In a typical residential area roughly 30 years old, the frequency of white birch is 45 trees for every kilometre of street. Therefore it is not surprising to find concentration of white birch pollen as high as 4000 grains per cubic metre of air at the height of its flowering period. Other trees such as oak and pine can also reach concentration of 1000 to 2000 at their peak, however grass concentrations are generally lower, reaching roughly 200 grains per cubic metre of air.
How long is the flowering season?
Trees flower in the springtime when temperature is increasing. For each type of tree, the flowering period is defined by specific conditions, which usually occur at approximately the same time each year, lasting roughly 2 weeks and the peak pollinating period (time when there are the maximum concentration of pollen in the air) lasting only a few days. The tree season in New Zealand is relatively short compared with Europe, where the birch season is several months long. Here it is a month or less.
Tree pollen is, therefore, less of an issue compared with grass pollen. Grass allergy is a severe problem because its season goes from August/September through to March. This makes New Zealand’s pollen season a nine-month nasal marathon!
Many people allergic to grass are allergic to more than one species creating a long protracted suffering period.
As a further complication, pollen concentrations in any flowering period vary on a daily basis in response to the various weather conditions. Pollen release is favourable on warm, dry, windy days whereas rain washes the air clean of pollen. Due to the biology of the plants, pollination usually occurs in the morning. Pollen concentrations are typically lowest at roughly 6 am increasing to the peak at 12 noon and decreasing through the afternoon and evening. See our Pollen Calendar for specific high risk times for different species of trees, grasses and weeds.
So what is it about pollen grains that make them allergenic?
Pollen grains carry on their exterior coat 30-40 different proteins that are required by the female parts of the flower to identify which pollen grains are a suitable match for pollination. When pollen grains are breathed into the nasal passages or contact the membranes of the eye, they release these proteins to the mucous membranes just as they would onto the surface of the receptive female stigma. This exposes the proteins to the immune system in the blood vessels of the mucous membranes.
The immune system is designed to rid the body of "foreign" proteins, and this usually occurs on a daily basis without any notice at all. However, for some people, for reasons that are still undiscovered, the immune system does not discard some of these pollen proteins through the usual route, but instead produces a special class of antibodies, IgE antibodies. The IgE antibodies bind to specialised cells called mast cells and upon contact with the pollen protein, signal the mast cell to release its contents.
One of the chemicals released in this process is histamine, which is responsible for producing the symptoms of allergy, e.g. swelling, redness, itchiness and secretion of mucous. All of these symptoms can occur when the immune system recognises one or more of these pollen proteins and produces IgE antibody to it. Some proteins are more likely to become allergenic than others, and some pollen types carry proteins that are more allergenic. For example, pine is a prolific pollen producer, but very few people are allergic to the pollen proteins, whereas ragweed, which produces less pollen, has proteins that are very allergenic.
How can pollen be avoided?
We know that pollen concentrations vary in both space and time. Learn to identify the plants that you are allergic to (there are many books to help you), find out where they like to live and know at what time of year they are pollinating, then STAY AWAY. The highest concentrations of pollen are within 10 metres of the plant and concentrations drop quickly as you move farther away, so you can significantly reduce your exposure to pollen by removing yourself physically from the plant when it is pollinating. Also, keep your windows closed during this time and stay indoors, especially in the morning hours. Before and after the pollination period, the plant should pose no harm to you (unless you have a contact type of plant allergy), so you can take walks in the woods at these times.Pollination is one of nature's wonders - learning about it helps us to cope with the bad luck of being allergic.
Based on information supplied by Christine Rogers
Department of Botany University of Toronto