The responses that cause the symptoms of hay fever are nearly identical to the body鈥檚 mechanism for destroying and expelling parasites. How these reactions come to be triggered by innocuous substances like pollen is not well understood.
What is known is that the sensitisation process generally starts months or years before you notice any symptoms. The end result is a network of immune cells primed to react to specific allergens when you encounter them again in the future.
For some reason, more and more people are being sensitised. Allergic rhinitis affects people across the world but the highest recorded incidence among adults is 30 per cent in the UK, . The US, Australia, New Zealand and other Western countries have experienced similar growth. The global average is about 16 per cent, meaning that more than a billion people have hay fever.
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Some figures suggest it is more common among teenagers, and hay fever often first raises its head during the teen years. But symptoms can suddenly start at any time of life, even in old age. Quite why it can take so long to come to a head isn鈥檛 known. 鈥淚t could be due to changing hormone levels, or it may simply take time for sensitisation to develop,鈥� says Durham. Drinking alcohol and smoking also increase the risk of developing hay fever and make symptoms worse.
What鈥檚 behind this epidemic? Increased awareness could be part of it, but that鈥檚 not the whole story 鈥� allergies of all kinds have seemingly become .
The leading explanation is the hygiene hypothesis 鈥� the idea that decreased exposure to bacterial infections and parasites during early childhood disrupts the normal development of the immune system, causing it to pick fights with harmless substances. Growing up on a farm seems to protect children against hay fever and asthma, as does drinking unpasteurised milk.
But the hygiene hypothesis is not a complete explanation either. Certain countries such as Japan are extremely Westernised yet have very low rates of allergy 鈥� possibly because of underlying genetic factors that influence susceptibility. If you have one close family member with hay fever, you have a greater than 50 per cent chance of developing it yourself.
One recent twist is the suggestion that reduced contact with natural environments might lower the diversity of microorganisms living in and on us, and that this might towards an allergy-prone state. Ilkka Hanski at the University of Helsinki in Finland recently discovered that, compared with healthy individuals, people who are predisposed to developing allergies are more likely to live in built-up areas, and have less-diverse bacteria living on their skin.
One group of bacteria called Acinetobacter looks especially interesting because it seems to encourage immune cells to produce an anti-inflammatory substance called IL-10. 鈥淐hildren growing up in homes that are surrounded by more forest and agricultural land tend to have more of these bacteria on their skin, and lower rates of allergy,鈥� Hanski says.
Other possible factors that increase susceptibility include greater use of antibiotics during childhood, lower levels of vitamin D or exposure to certain chemicals. For now, nobody really knows.
Read more: 鈥�Hay fever: Why it鈥檚 not to be sneezed at鈥�
This article appeared in print under the headline 鈥淲hat causes the immune system to go haywire?鈥�
