Until recently, hay fever was limited to spring, summer, and early fall. That's logical, because those are the periods in which plants release their pollen for the purpose of sexual reproduction.
But nowadays, autumn allergies are becoming more common. This phenomenon, called Autumn Sneezing Syndrome, is said to be caused by changing pollen patterns as well as an invasive family of plants called ragweeds (Ambrosia). A single plant may produce about a billion grains of pollen per season. Ragweed pollen can remain airborne for days and travel great distances, and can even be carried over 500 kilometers.
Ragweeds, native to the Americas, have been introduced to Europe in the nineteenth century and especially during World War I, and have spread rapidly since the 1950s.
'Usually you get hay fever starting with the tree pollen in March and April and then grass pollen from May to August, then that was the end of it for most people,' said Dr Adrian Morris, principal allergist at Surrey Allergy Clinic (UK).
Dr Morris said the number of people affected was increasing, in part, due to allergic reactions caused by ragweed, which sheds its pollen later in the year than most plants.
The major allergenic compound in the pollen has been identified as Amb a 1, a 38 kDa nonglycosylated protein composed of two subunits. It also contains other allergenic components, such as profilin and calcium-binding proteins[1].
However, Dr Morris said the main reason why such a large amount of people were continuing to experience allergies long after the summer months was still unclear, emphasising it was a 'reasonably new phenomenon'.
[1] Wopfner et al: The spectrum of allergens in ragweed and mugwort pollen in International Archives of Allergy and Immunology – 2005
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