The onslaught of allergies

Unless you suffer from hay fever, there’s no reason why this relatively inconspicuous looking plant should strike fear into your heart. But if you are allergic to pollen, Ambrosia artemisiifolia, otherwise known as common ragweed, is just about as close to vegetal hell as you’re going to get.

Ragweed is found across the US and central Europe and is notoriously difficult to deal with. “It’s a really potent aero-allergen,” says Matthew Smith, a graduate researcher at the UK’s National Pollen and Aerobiology Research Unit in Worcester. “The seeds can stay in the soil for up to 40 years. Each plant can produce millions of pollen grains and people can get symptoms at less than five grains per cubic metre.”

The strain that ragweed and other aero-allergens put on health systems is colossal; there are as many hay fever sufferers in the US as there are Californians. Globally, the cost of hay fever (seasonal allergic rhinitis) and perennial allergic rhinitis, which occurs all year round, comes in at a staggering US$20 billion – more than many African countries’ entire economies are valued at.

But it’s not just hay fever that health authorities have to contend with. Allergic diseases, including asthma and food allergies, have been on the rise for decades. Estimates from 2001 suggest asthma maintains a tight stranglehold on the population’s breathing passages, killing over 4,000 people every year in the U.S. alone.

Clifford Bassett, a clinical immunologist with the American Academy of Allergy Asthma and Immunology, says that improved treatments may be starting to have an impact on the scourge. “Better access to respiratory specialists and newer asthma medications have helped to reduce the number of asthma deaths.” However, he says, “Any technology that would help to alleviate the triggers and day-to-day symptoms of asthma would certainly be welcome." Yet even now, science still does not have a definitive picture of the cellular and molecular mechanisms of allergy.

What triggers an allergic reaction?

Allergic reactions are an example of our bodies’ immune systems working against our best interests. They occur when certain types of cells, the main culprits being mast cells, are activated due to the presence of ‘antigens’, which may be pollen grains, cat hairs or nut proteins, depending on the allergy. Mast cells are found close to the blood vessels in most of the body’s tissues. Although their contents vary according to the tissue, all are tightly packed with granules that contain a host of different chemicals. These can have diverse and powerful effects on the body. For example, when a stray pollen grain finds its way into a hay fever sufferer’s airways, their mast cells overreact, spilling their contents in a process known as ‘degranulation’. The various chemicals that are released in this process are responsible for what we see as the streaming sinuses and weepy eyes.

The symptoms of hay fever are irritating and uncomfortable but other allergies can manifest themselves in even more unpleasant and sometimes terrifying ways. When swallowed, some antigens can result in violent vomiting and diarrhoea or huge visible swellings on the face, and in susceptible individuals certain insect stings can result in anaphylactic shocks - severe and potentially lethal reactions where the blood pressure plunges to extreme lows.

In hay fever sufferers, mast cells are activated by grass pollen or ragweed, where as in asthma, the trigger is not necessarily a physical agent of allergy; attacks can be caused by cold or exercise. In a recent study of 107 college athletes in Ohio, more than a third showed signs of asthma after training sessions. This exercise-induced asthma, along with other types which do not involve a specific allergen, is often referred to as ‘non-allergic asthma’. However, the basic process of airway constriction due to degranulation appears to resemble that of allergic patients, whose asthma can be activated by dust mites and even mould. Researchers at Cardiff University in the UK recently announced that they had recorded marked differences between levels of asthma in clean, well-ventilated homes and those purposely left to go mouldy. Sufferers living in “mould-free” homes said they experienced less sneezing and didn’t need to use their inhalers as often.

Who gets allergies?