Why the hay fever drugs don't always work
Battling the body’s biochemical reaction to intruders is a complicated business, writes DICK AHLSTROM, Science Editor, and helps us understand just why hay fever sufferers find little respite through medication
DOES YOURS start with the itchy eyes or the burning nose? Or maybe a burst of sneezes – three, four, five, six in rapid succession?
This will sound all too familiar to those of us who suffer from hay fever. May, June and early July are the peak months for this affliction that affects as many as one in three of us to varying degrees.
“You can’t avoid exposure. There is no protective gear you can wear,” says Anthony Staines, Dublin City University’s professor of health systems within the school of nursing.
Common allergies such as hay fever affect about one in three people across Europe including Ireland, Staines says. “The dominant kind is highly seasonal and is triggered by pollen.”
The condition’s official name, seasonal allergic rhinitis, doesn’t exactly roll lightly off the tongue, but it’s more common name is misleading. It does not cause a fever and is not dependent on the presence of hay.
There is no confusion over the symptoms once you have hay fever or indeed common allergies to other substances such as cat dander or house dust mite – more specifically the mite’s excrement.
The symptoms triggered by exposure to these substances in turn arise because of a very specific series of biochemical reactions that take place where the allergy-causing substance or allergen encounters our tissues, explains Prof Kingston Mills of Trinity College Dublin.
“Hay fever would be just one of the diseases under the general term allergy,” says the professor of immunology and head of the school of biochemistry and immunology at Trinity.
It doesn’t matter whether the culprit is tree or grass pollen, mites or pets, he says. The allergic reaction is caused by “an inappropriate immune response” to molecules inside these allergens.
Although harmless, we can become sensitised to these substances, something that causes our immune systems to misinterpret the threat, he says. Instead of ignoring pollen the immune system launches an all-out protective defence as if it had encountered the Black Death.
Pollen grains drifting on the wind and breathed in set the whole process in train. The moist surfaces inside the nasal airways trap the pollen but also help break open its tough outer shell.
This releases a selection of molecules and these are what upsets the immune system, causing it to go into overdrive. If you are sensitive to pollen, it means your body has produced antibodies against it and will react whenever it encounters pollen.
Immune system cells, called T helper [Th] cells, drive the whole thing, explains Mills. They are involved in activating and directing other immune cells and Th2 and Th17 are two forms that seem particularly involved when pollen is encountered.
“These are pathogenic T cells because they cause disease,” he says. “Everybody has Th2 and Th17 cells but they are regulated to control them.”
Unfortunately this regulatory control is not there for those allergic to pollen. Instead the Th cells call in the cavalry, encouraging other immune cells to join the fray.
Invoking an all-out defence in turn leads to the release of histamines and histamine-like substances, something that makes the nearby tissues react as though they had just been stung with nettles, he says. “That is what causes the itchy eyes and runny nose, the histamine-like compounds.”
Hay fever sufferers know how this story finishes. Your eyes and nose stream and sneezing can affect you at any stage. The standard response is to take medicines that halt the stinging nettle response by blocking the histamines, a family of drugs known as anti-histamines.
“There are no good treatments really,” says Mills. The drugs act on the symptoms rather than the cause of the affliction.
Other ways to beat hay fever are either available now or are under study. One involves a method called desensitisation, explains DCU’s Staines. If the target allergen is grass pollen then you give the person the very thing that causes their reaction, more grass pollen.
Initially the person will react strongly to the pollen, delivering all the usual symptoms. The dose is increased and increased again, in a process that continues until the symptoms suddenly stop. The person has become desensitised to pollen and hopefully won’t be affected by it again.
The approach only works in some people, says Staines, an epidemiologist and former paediatrician, with children often responding well to it.
Another approach is to vaccinate against hay fever, Mills says. You don’t use the pollen itself to produce the vaccine, you use one of the specific allergens it carries.
Once given to the person it is meant to increase production of the regulatory substances that hold Th2 and Th17 in check. “This approach is currently in clinical trial,” he says. One of the great ironies about allergies such as hay fever and dust mite is that the way we live our lives today may be adding to the numbers affected.
“Our modern lifestyles are actually increasing our susceptibility to allergy because we are too clean,” explains Mills.
It seems that attempts to sterilise our environment and reduce challenges to our immune systems only serve to make them weaker. There is accumulating evidence that exposure to allergens and infections boosts the immune response, he says.
People who work on farms and handle a lot of animals get less hay fever and asthma. This has also been seen among farmers in Africa who face more parasitic disease.
POLLEN IS THE villain when it comes to the main cause of hay fever or “seasonal allergic rhinitis”. Produced in spring and summer first by trees then grasses and finally weeds, it is carried on the wind.
The problem pollens are wind-borne, not those transferred from plant to plant by bees and birds, says Hazel Proctor, a postgraduate student at Trinity College Dublin, studying birch pollen.
The individual round or oval grains are microscopic, measuring less than a tenth of a millimetre across, but as hay fever suffers can tell you, pollen packs a tremendous punch despite its size.
It is not the grain itself but what it contains that causes all the trouble, she says. Pollen carries the male sex cell in plants that form seeds. Its tough outer coat breaks open if the pollen lands on the pistil or female part of the plant, but unfortunately it also readily does so on the moist surfaces in the nasal airways. These substances trigger the cascade of symptoms hay fever suffers dread.
Each season brings a new mix of pollen grains with alder and hazel trees and later birch releasing pollen early, she says. Grass pollen follows in May and June, a time when symptoms are at a peak.
Proctor wants to see whether global warming has changed the time when pollen is released by birch trees. She reveals, however, that there is a darker side to this work. “Since starting this project 18 months ago I have developed an allergy to birch pollen.”