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It’s allergy season! Every spring, plants have a yen to reproduce, and that means pollen, lots of it. The clouds of pollen drift through the air, settling on streets, waterbodies, and in the nasal passages. The late winter in northeastern North America has led to pollen overdrive, as blooms that would ordinarily be spaced out are all occurring at once.

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For the unlucky, all this pollen provokes sneezing, itching, discomfort, and even dangerous breathing disorders such as asthma. What causes this reaction? Why do some people have such an extreme reaction to pollen?

Blame it on proteins. Some aspects of the immune system react to surface membrane proteins in pathogenic organisms, sometimes resulting in inflammation as the immune system ramps up to deal with the threat. For seasonal allergy sufferers, the immune system reacts with the proteins on the surface of pollen as if the pollen were a dangerous invader.

Cells involved in the immune response begin to accumulate in the airway or mucus membranes, activating additional cells within the membrane that accumulate as well. (The complete list of cells involved is too long to list, but if you are truly interested check out pages 749 and 761 in this BMJ article). Finally, mediators such as histamine are released. Mediators allow the immune cells easier access to blood vessels, facilitating inflammation and discomfort.

The trouble with pollen is that the immune system overreacts well beyond what is needed to contain a typical threat. Otherwise the described sequence of events is pretty typical for an immune response. Why does the reaction run amok specifically with pollen? Evidence suggests that pollen primes the immune system ahead of time, making an unnecessarily severe immune reaction more likely.

However, pollen itself also contains NADPH oxidases, enzymes which are also found in the immune system and are involved in starting a reaction. When these are introduced directly by pollen, free radicals called ROSs (atoms that are missing one electron in their outer shell, making them highly reactive) are generated in the mucus membranes. These ROSs begin over-stimulating the immune system even before the body initiates its own response, setting in motion the immuno-overkill that leads to hay fever hell.

So that’s the pathway that leads to an allergic reaction, and provides a mechanism for treatment. Antihistamines and other allergy remedies block the mediators, disrupting the chain reaction that leads to discomfort or worse. However, bad news! According to multiple studies in JSTOR, the incidence of allergies is on the rise. One study links the increase to climate change, and predicts worse to come. Time to buy stock in a handkerchief company.


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BMJ: British Medical Journal, Vol. 316, No. 7133 (Mar. 7, 1998), pp. 758-761
Environmental Health Perspectives, Vol. 113, No. 8 (Aug., 2005), pp. 915-919
The National Institute of Environmental Health Sciences (NIEHS)
Environmental Health Perspectives, Vol. 113, No. 8 (Aug., 2005), pp. 915-919
The National Institute of Environmental Health Sciences (NIEHS)